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| author | upstream source tree <ports@midipix.org> | 2015-03-15 20:14:05 -0400 |
|---|---|---|
| committer | upstream source tree <ports@midipix.org> | 2015-03-15 20:14:05 -0400 |
| commit | 554fd8c5195424bdbcabf5de30fdc183aba391bd (patch) | |
| tree | 976dc5ab7fddf506dadce60ae936f43f58787092 /libjava/classpath/external/jsr166/java/util/concurrent/locks | |
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diff --git a/libjava/classpath/external/jsr166/java/util/concurrent/locks/AbstractOwnableSynchronizer.java b/libjava/classpath/external/jsr166/java/util/concurrent/locks/AbstractOwnableSynchronizer.java new file mode 100644 index 000000000..f3780e5a6 --- /dev/null +++ b/libjava/classpath/external/jsr166/java/util/concurrent/locks/AbstractOwnableSynchronizer.java @@ -0,0 +1,57 @@ +/* + * Written by Doug Lea with assistance from members of JCP JSR-166 + * Expert Group and released to the public domain, as explained at + * http://creativecommons.org/licenses/publicdomain + */ + +package java.util.concurrent.locks; + +/** + * A synchronizer that may be exclusively owned by a thread. This + * class provides a basis for creating locks and related synchronizers + * that may entail a notion of ownership. The + * <tt>AbstractOwnableSynchronizer</tt> class itself does not manage or + * use this information. However, subclasses and tools may use + * appropriately maintained values to help control and monitor access + * and provide diagnostics. + * + * @since 1.6 + * @author Doug Lea + */ +public abstract class AbstractOwnableSynchronizer + implements java.io.Serializable { + + /** Use serial ID even though all fields transient. */ + private static final long serialVersionUID = 3737899427754241961L; + + /** + * Empty constructor for use by subclasses. + */ + protected AbstractOwnableSynchronizer() { } + + /** + * The current owner of exclusive mode synchronization. + */ + private transient Thread exclusiveOwnerThread; + + /** + * Sets the thread that currently owns exclusive access. A + * <tt>null</tt> argument indicates that no thread owns access. + * This method does not otherwise impose any synchronization or + * <tt>volatile</tt> field accesses. + */ + protected final void setExclusiveOwnerThread(Thread t) { + exclusiveOwnerThread = t; + } + + /** + * Returns the thread last set by + * <tt>setExclusiveOwnerThread</tt>, or <tt>null</tt> if never + * set. This method does not otherwise impose any synchronization + * or <tt>volatile</tt> field accesses. + * @return the owner thread + */ + protected final Thread getExclusiveOwnerThread() { + return exclusiveOwnerThread; + } +} diff --git a/libjava/classpath/external/jsr166/java/util/concurrent/locks/AbstractQueuedLongSynchronizer.java b/libjava/classpath/external/jsr166/java/util/concurrent/locks/AbstractQueuedLongSynchronizer.java new file mode 100644 index 000000000..45d744bb8 --- /dev/null +++ b/libjava/classpath/external/jsr166/java/util/concurrent/locks/AbstractQueuedLongSynchronizer.java @@ -0,0 +1,1934 @@ +/* + * Written by Doug Lea with assistance from members of JCP JSR-166 + * Expert Group and released to the public domain, as explained at + * http://creativecommons.org/licenses/publicdomain + */ + +package java.util.concurrent.locks; +import java.util.*; +import java.util.concurrent.*; +import java.util.concurrent.atomic.*; +import sun.misc.Unsafe; + +/** + * A version of {@link AbstractQueuedSynchronizer} in + * which synchronization state is maintained as a <tt>long</tt>. + * This class has exactly the same structure, properties, and methods + * as <tt>AbstractQueuedSynchronizer</tt> with the exception + * that all state-related parameters and results are defined + * as <tt>long</tt> rather than <tt>int</tt>. This class + * may be useful when creating synchronizers such as + * multilevel locks and barriers that require + * 64 bits of state. + * + * <p>See {@link AbstractQueuedSynchronizer} for usage + * notes and examples. + * + * @since 1.6 + * @author Doug Lea + */ +public abstract class AbstractQueuedLongSynchronizer + extends AbstractOwnableSynchronizer + implements java.io.Serializable { + + private static final long serialVersionUID = 7373984972572414692L; + + /* + To keep sources in sync, the remainder of this source file is + exactly cloned from AbstractQueuedSynchronizer, replacing class + name and changing ints related with sync state to longs. Please + keep it that way. + */ + + /** + * Creates a new <tt>AbstractQueuedLongSynchronizer</tt> instance + * with initial synchronization state of zero. + */ + protected AbstractQueuedLongSynchronizer() { } + + /** + * Wait queue node class. + * + * <p>The wait queue is a variant of a "CLH" (Craig, Landin, and + * Hagersten) lock queue. CLH locks are normally used for + * spinlocks. We instead use them for blocking synchronizers, but + * use the same basic tactic of holding some of the control + * information about a thread in the predecessor of its node. A + * "status" field in each node keeps track of whether a thread + * should block. A node is signalled when its predecessor + * releases. Each node of the queue otherwise serves as a + * specific-notification-style monitor holding a single waiting + * thread. The status field does NOT control whether threads are + * granted locks etc though. A thread may try to acquire if it is + * first in the queue. But being first does not guarantee success; + * it only gives the right to contend. So the currently released + * contender thread may need to rewait. + * + * <p>To enqueue into a CLH lock, you atomically splice it in as new + * tail. To dequeue, you just set the head field. + * <pre> + * +------+ prev +-----+ +-----+ + * head | | <---- | | <---- | | tail + * +------+ +-----+ +-----+ + * </pre> + * + * <p>Insertion into a CLH queue requires only a single atomic + * operation on "tail", so there is a simple atomic point of + * demarcation from unqueued to queued. Similarly, dequeing + * involves only updating the "head". However, it takes a bit + * more work for nodes to determine who their successors are, + * in part to deal with possible cancellation due to timeouts + * and interrupts. + * + * <p>The "prev" links (not used in original CLH locks), are mainly + * needed to handle cancellation. If a node is cancelled, its + * successor is (normally) relinked to a non-cancelled + * predecessor. For explanation of similar mechanics in the case + * of spin locks, see the papers by Scott and Scherer at + * http://www.cs.rochester.edu/u/scott/synchronization/ + * + * <p>We also use "next" links to implement blocking mechanics. + * The thread id for each node is kept in its own node, so a + * predecessor signals the next node to wake up by traversing + * next link to determine which thread it is. Determination of + * successor must avoid races with newly queued nodes to set + * the "next" fields of their predecessors. This is solved + * when necessary by checking backwards from the atomically + * updated "tail" when a node's successor appears to be null. + * (Or, said differently, the next-links are an optimization + * so that we don't usually need a backward scan.) + * + * <p>Cancellation introduces some conservatism to the basic + * algorithms. Since we must poll for cancellation of other + * nodes, we can miss noticing whether a cancelled node is + * ahead or behind us. This is dealt with by always unparking + * successors upon cancellation, allowing them to stabilize on + * a new predecessor. + * + * <p>CLH queues need a dummy header node to get started. But + * we don't create them on construction, because it would be wasted + * effort if there is never contention. Instead, the node + * is constructed and head and tail pointers are set upon first + * contention. + * + * <p>Threads waiting on Conditions use the same nodes, but + * use an additional link. Conditions only need to link nodes + * in simple (non-concurrent) linked queues because they are + * only accessed when exclusively held. Upon await, a node is + * inserted into a condition queue. Upon signal, the node is + * transferred to the main queue. A special value of status + * field is used to mark which queue a node is on. + * + * <p>Thanks go to Dave Dice, Mark Moir, Victor Luchangco, Bill + * Scherer and Michael Scott, along with members of JSR-166 + * expert group, for helpful ideas, discussions, and critiques + * on the design of this class. + */ + static final class Node { + /** waitStatus value to indicate thread has cancelled */ + static final int CANCELLED = 1; + /** waitStatus value to indicate successor's thread needs unparking */ + static final int SIGNAL = -1; + /** waitStatus value to indicate thread is waiting on condition */ + static final int CONDITION = -2; + /** Marker to indicate a node is waiting in shared mode */ + static final Node SHARED = new Node(); + /** Marker to indicate a node is waiting in exclusive mode */ + static final Node EXCLUSIVE = null; + + /** + * Status field, taking on only the values: + * SIGNAL: The successor of this node is (or will soon be) + * blocked (via park), so the current node must + * unpark its successor when it releases or + * cancels. To avoid races, acquire methods must + * first indicate they need a signal, + * then retry the atomic acquire, and then, + * on failure, block. + * CANCELLED: This node is cancelled due to timeout or interrupt. + * Nodes never leave this state. In particular, + * a thread with cancelled node never again blocks. + * CONDITION: This node is currently on a condition queue. + * It will not be used as a sync queue node until + * transferred. (Use of this value here + * has nothing to do with the other uses + * of the field, but simplifies mechanics.) + * 0: None of the above + * + * The values are arranged numerically to simplify use. + * Non-negative values mean that a node doesn't need to + * signal. So, most code doesn't need to check for particular + * values, just for sign. + * + * The field is initialized to 0 for normal sync nodes, and + * CONDITION for condition nodes. It is modified only using + * CAS. + */ + volatile int waitStatus; + + /** + * Link to predecessor node that current node/thread relies on + * for checking waitStatus. Assigned during enqueing, and nulled + * out (for sake of GC) only upon dequeuing. Also, upon + * cancellation of a predecessor, we short-circuit while + * finding a non-cancelled one, which will always exist + * because the head node is never cancelled: A node becomes + * head only as a result of successful acquire. A + * cancelled thread never succeeds in acquiring, and a thread only + * cancels itself, not any other node. + */ + volatile Node prev; + + /** + * Link to the successor node that the current node/thread + * unparks upon release. Assigned once during enqueuing, and + * nulled out (for sake of GC) when no longer needed. Upon + * cancellation, we cannot adjust this field, but can notice + * status and bypass the node if cancelled. The enq operation + * does not assign next field of a predecessor until after + * attachment, so seeing a null next field does not + * necessarily mean that node is at end of queue. However, if + * a next field appears to be null, we can scan prev's from + * the tail to double-check. + */ + volatile Node next; + + /** + * The thread that enqueued this node. Initialized on + * construction and nulled out after use. + */ + volatile Thread thread; + + /** + * Link to next node waiting on condition, or the special + * value SHARED. Because condition queues are accessed only + * when holding in exclusive mode, we just need a simple + * linked queue to hold nodes while they are waiting on + * conditions. They are then transferred to the queue to + * re-acquire. And because conditions can only be exclusive, + * we save a field by using special value to indicate shared + * mode. + */ + Node nextWaiter; + + /** + * Returns true if node is waiting in shared mode + */ + final boolean isShared() { + return nextWaiter == SHARED; + } + + /** + * Returns previous node, or throws NullPointerException if + * null. Use when predecessor cannot be null. + * @return the predecessor of this node + */ + final Node predecessor() throws NullPointerException { + Node p = prev; + if (p == null) + throw new NullPointerException(); + else + return p; + } + + Node() { // Used to establish initial head or SHARED marker + } + + Node(Thread thread, Node mode) { // Used by addWaiter + this.nextWaiter = mode; + this.thread = thread; + } + + Node(Thread thread, int waitStatus) { // Used by Condition + this.waitStatus = waitStatus; + this.thread = thread; + } + } + + /** + * Head of the wait queue, lazily initialized. Except for + * initialization, it is modified only via method setHead. Note: + * If head exists, its waitStatus is guaranteed not to be + * CANCELLED. + */ + private transient volatile Node head; + + /** + * Tail of the wait queue, lazily initialized. Modified only via + * method enq to add new wait node. + */ + private transient volatile Node tail; + + /** + * The synchronization state. + */ + private volatile long state; + + /** + * Returns the current value of synchronization state. + * This operation has memory semantics of a <tt>volatile</tt> read. + * @return current state value + */ + protected final long getState() { + return state; + } + + /** + * Sets the value of synchronization state. + * This operation has memory semantics of a <tt>volatile</tt> write. + * @param newState the new state value + */ + protected final void setState(long newState) { + state = newState; + } + + /** + * Atomically sets synchronization state to the given updated + * value if the current state value equals the expected value. + * This operation has memory semantics of a <tt>volatile</tt> read + * and write. + * + * @param expect the expected value + * @param update the new value + * @return true if successful. False return indicates that the actual + * value was not equal to the expected value. + */ + protected final boolean compareAndSetState(long expect, long update) { + // See below for intrinsics setup to support this + return unsafe.compareAndSwapLong(this, stateOffset, expect, update); + } + + // Queuing utilities + + /** + * The number of nanoseconds for which it is faster to spin + * rather than to use timed park. A rough estimate suffices + * to improve responsiveness with very short timeouts. + */ + static final long spinForTimeoutThreshold = 1000L; + + /** + * Inserts node into queue, initializing if necessary. See picture above. + * @param node the node to insert + * @return node's predecessor + */ + private Node enq(final Node node) { + for (;;) { + Node t = tail; + if (t == null) { // Must initialize + Node h = new Node(); // Dummy header + h.next = node; + node.prev = h; + if (compareAndSetHead(h)) { + tail = node; + return h; + } + } + else { + node.prev = t; + if (compareAndSetTail(t, node)) { + t.next = node; + return t; + } + } + } + } + + /** + * Creates and enqueues node for given thread and mode. + * + * @param current the thread + * @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared + * @return the new node + */ + private Node addWaiter(Node mode) { + Node node = new Node(Thread.currentThread(), mode); + // Try the fast path of enq; backup to full enq on failure + Node pred = tail; + if (pred != null) { + node.prev = pred; + if (compareAndSetTail(pred, node)) { + pred.next = node; + return node; + } + } + enq(node); + return node; + } + + /** + * Sets head of queue to be node, thus dequeuing. Called only by + * acquire methods. Also nulls out unused fields for sake of GC + * and to suppress unnecessary signals and traversals. + * + * @param node the node + */ + private void setHead(Node node) { + head = node; + node.thread = null; + node.prev = null; + } + + /** + * Wakes up node's successor, if one exists. + * + * @param node the node + */ + private void unparkSuccessor(Node node) { + /* + * Try to clear status in anticipation of signalling. It is + * OK if this fails or if status is changed by waiting thread. + */ + compareAndSetWaitStatus(node, Node.SIGNAL, 0); + + /* + * Thread to unpark is held in successor, which is normally + * just the next node. But if cancelled or apparently null, + * traverse backwards from tail to find the actual + * non-cancelled successor. + */ + Node s = node.next; + if (s == null || s.waitStatus > 0) { + s = null; + for (Node t = tail; t != null && t != node; t = t.prev) + if (t.waitStatus <= 0) + s = t; + } + if (s != null) + LockSupport.unpark(s.thread); + } + + /** + * Sets head of queue, and checks if successor may be waiting + * in shared mode, if so propagating if propagate > 0. + * + * @param pred the node holding waitStatus for node + * @param node the node + * @param propagate the return value from a tryAcquireShared + */ + private void setHeadAndPropagate(Node node, long propagate) { + setHead(node); + if (propagate > 0 && node.waitStatus != 0) { + /* + * Don't bother fully figuring out successor. If it + * looks null, call unparkSuccessor anyway to be safe. + */ + Node s = node.next; + if (s == null || s.isShared()) + unparkSuccessor(node); + } + } + + // Utilities for various versions of acquire + + /** + * Cancels an ongoing attempt to acquire. + * + * @param node the node + */ + private void cancelAcquire(Node node) { + if (node != null) { // Ignore if node doesn't exist + node.thread = null; + // Can use unconditional write instead of CAS here + node.waitStatus = Node.CANCELLED; + unparkSuccessor(node); + } + } + + /** + * Checks and updates status for a node that failed to acquire. + * Returns true if thread should block. This is the main signal + * control in all acquire loops. Requires that pred == node.prev + * + * @param pred node's predecessor holding status + * @param node the node + * @return {@code true} if thread should block + */ + private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) { + int s = pred.waitStatus; + if (s < 0) + /* + * This node has already set status asking a release + * to signal it, so it can safely park + */ + return true; + if (s > 0) + /* + * Predecessor was cancelled. Move up to its predecessor + * and indicate retry. + */ + node.prev = pred.prev; + else + /* + * Indicate that we need a signal, but don't park yet. Caller + * will need to retry to make sure it cannot acquire before + * parking. + */ + compareAndSetWaitStatus(pred, 0, Node.SIGNAL); + return false; + } + + /** + * Convenience method to interrupt current thread. + */ + private static void selfInterrupt() { + Thread.currentThread().interrupt(); + } + + /** + * Convenience method to park and then check if interrupted + * + * @return {@code true} if interrupted + */ + private final boolean parkAndCheckInterrupt() { + LockSupport.park(this); + return Thread.interrupted(); + } + + /* + * Various flavors of acquire, varying in exclusive/shared and + * control modes. Each is mostly the same, but annoyingly + * different. Only a little bit of factoring is possible due to + * interactions of exception mechanics (including ensuring that we + * cancel if tryAcquire throws exception) and other control, at + * least not without hurting performance too much. + */ + + /** + * Acquires in exclusive uninterruptible mode for thread already in + * queue. Used by condition wait methods as well as acquire. + * + * @param node the node + * @param arg the acquire argument + * @return {@code true} if interrupted while waiting + */ + final boolean acquireQueued(final Node node, long arg) { + try { + boolean interrupted = false; + for (;;) { + final Node p = node.predecessor(); + if (p == head && tryAcquire(arg)) { + setHead(node); + p.next = null; // help GC + return interrupted; + } + if (shouldParkAfterFailedAcquire(p, node) && + parkAndCheckInterrupt()) + interrupted = true; + } + } catch (RuntimeException ex) { + cancelAcquire(node); + throw ex; + } + } + + /** + * Acquires in exclusive interruptible mode. + * @param arg the acquire argument + */ + private void doAcquireInterruptibly(long arg) + throws InterruptedException { + final Node node = addWaiter(Node.EXCLUSIVE); + try { + for (;;) { + final Node p = node.predecessor(); + if (p == head && tryAcquire(arg)) { + setHead(node); + p.next = null; // help GC + return; + } + if (shouldParkAfterFailedAcquire(p, node) && + parkAndCheckInterrupt()) + break; + } + } catch (RuntimeException ex) { + cancelAcquire(node); + throw ex; + } + // Arrive here only if interrupted + cancelAcquire(node); + throw new InterruptedException(); + } + + /** + * Acquires in exclusive timed mode. + * + * @param arg the acquire argument + * @param nanosTimeout max wait time + * @return {@code true} if acquired + */ + private boolean doAcquireNanos(long arg, long nanosTimeout) + throws InterruptedException { + long lastTime = System.nanoTime(); + final Node node = addWaiter(Node.EXCLUSIVE); + try { + for (;;) { + final Node p = node.predecessor(); + if (p == head && tryAcquire(arg)) { + setHead(node); + p.next = null; // help GC + return true; + } + if (nanosTimeout <= 0) { + cancelAcquire(node); + return false; + } + if (nanosTimeout > spinForTimeoutThreshold && + shouldParkAfterFailedAcquire(p, node)) + LockSupport.parkNanos(this, nanosTimeout); + long now = System.nanoTime(); + nanosTimeout -= now - lastTime; + lastTime = now; + if (Thread.interrupted()) + break; + } + } catch (RuntimeException ex) { + cancelAcquire(node); + throw ex; + } + // Arrive here only if interrupted + cancelAcquire(node); + throw new InterruptedException(); + } + + /** + * Acquires in shared uninterruptible mode. + * @param arg the acquire argument + */ + private void doAcquireShared(long arg) { + final Node node = addWaiter(Node.SHARED); + try { + boolean interrupted = false; + for (;;) { + final Node p = node.predecessor(); + if (p == head) { + long r = tryAcquireShared(arg); + if (r >= 0) { + setHeadAndPropagate(node, r); + p.next = null; // help GC + if (interrupted) + selfInterrupt(); + return; + } + } + if (shouldParkAfterFailedAcquire(p, node) && + parkAndCheckInterrupt()) + interrupted = true; + } + } catch (RuntimeException ex) { + cancelAcquire(node); + throw ex; + } + } + + /** + * Acquires in shared interruptible mode. + * @param arg the acquire argument + */ + private void doAcquireSharedInterruptibly(long arg) + throws InterruptedException { + final Node node = addWaiter(Node.SHARED); + try { + for (;;) { + final Node p = node.predecessor(); + if (p == head) { + long r = tryAcquireShared(arg); + if (r >= 0) { + setHeadAndPropagate(node, r); + p.next = null; // help GC + return; + } + } + if (shouldParkAfterFailedAcquire(p, node) && + parkAndCheckInterrupt()) + break; + } + } catch (RuntimeException ex) { + cancelAcquire(node); + throw ex; + } + // Arrive here only if interrupted + cancelAcquire(node); + throw new InterruptedException(); + } + + /** + * Acquires in shared timed mode. + * + * @param arg the acquire argument + * @param nanosTimeout max wait time + * @return {@code true} if acquired + */ + private boolean doAcquireSharedNanos(long arg, long nanosTimeout) + throws InterruptedException { + + long lastTime = System.nanoTime(); + final Node node = addWaiter(Node.SHARED); + try { + for (;;) { + final Node p = node.predecessor(); + if (p == head) { + long r = tryAcquireShared(arg); + if (r >= 0) { + setHeadAndPropagate(node, r); + p.next = null; // help GC + return true; + } + } + if (nanosTimeout <= 0) { + cancelAcquire(node); + return false; + } + if (nanosTimeout > spinForTimeoutThreshold && + shouldParkAfterFailedAcquire(p, node)) + LockSupport.parkNanos(this, nanosTimeout); + long now = System.nanoTime(); + nanosTimeout -= now - lastTime; + lastTime = now; + if (Thread.interrupted()) + break; + } + } catch (RuntimeException ex) { + cancelAcquire(node); + throw ex; + } + // Arrive here only if interrupted + cancelAcquire(node); + throw new InterruptedException(); + } + + // Main exported methods + + /** + * Attempts to acquire in exclusive mode. This method should query + * if the state of the object permits it to be acquired in the + * exclusive mode, and if so to acquire it. + * + * <p>This method is always invoked by the thread performing + * acquire. If this method reports failure, the acquire method + * may queue the thread, if it is not already queued, until it is + * signalled by a release from some other thread. This can be used + * to implement method {@link Lock#tryLock()}. + * + * <p>The default + * implementation throws {@link UnsupportedOperationException}. + * + * @param arg the acquire argument. This value is always the one + * passed to an acquire method, or is the value saved on entry + * to a condition wait. The value is otherwise uninterpreted + * and can represent anything you like. + * @return {@code true} if successful. Upon success, this object has + * been acquired. + * @throws IllegalMonitorStateException if acquiring would place this + * synchronizer in an illegal state. This exception must be + * thrown in a consistent fashion for synchronization to work + * correctly. + * @throws UnsupportedOperationException if exclusive mode is not supported + */ + protected boolean tryAcquire(long arg) { + throw new UnsupportedOperationException(); + } + + /** + * Attempts to set the state to reflect a release in exclusive + * mode. + * + * <p>This method is always invoked by the thread performing release. + * + * <p>The default implementation throws + * {@link UnsupportedOperationException}. + * + * @param arg the release argument. This value is always the one + * passed to a release method, or the current state value upon + * entry to a condition wait. The value is otherwise + * uninterpreted and can represent anything you like. + * @return {@code true} if this object is now in a fully released + * state, so that any waiting threads may attempt to acquire; + * and {@code false} otherwise. + * @throws IllegalMonitorStateException if releasing would place this + * synchronizer in an illegal state. This exception must be + * thrown in a consistent fashion for synchronization to work + * correctly. + * @throws UnsupportedOperationException if exclusive mode is not supported + */ + protected boolean tryRelease(long arg) { + throw new UnsupportedOperationException(); + } + + /** + * Attempts to acquire in shared mode. This method should query if + * the state of the object permits it to be acquired in the shared + * mode, and if so to acquire it. + * + * <p>This method is always invoked by the thread performing + * acquire. If this method reports failure, the acquire method + * may queue the thread, if it is not already queued, until it is + * signalled by a release from some other thread. + * + * <p>The default implementation throws {@link + * UnsupportedOperationException}. + * + * @param arg the acquire argument. This value is always the one + * passed to an acquire method, or is the value saved on entry + * to a condition wait. The value is otherwise uninterpreted + * and can represent anything you like. + * @return a negative value on failure; zero if acquisition in shared + * mode succeeded but no subsequent shared-mode acquire can + * succeed; and a positive value if acquisition in shared + * mode succeeded and subsequent shared-mode acquires might + * also succeed, in which case a subsequent waiting thread + * must check availability. (Support for three different + * return values enables this method to be used in contexts + * where acquires only sometimes act exclusively.) Upon + * success, this object has been acquired. + * @throws IllegalMonitorStateException if acquiring would place this + * synchronizer in an illegal state. This exception must be + * thrown in a consistent fashion for synchronization to work + * correctly. + * @throws UnsupportedOperationException if shared mode is not supported + */ + protected long tryAcquireShared(long arg) { + throw new UnsupportedOperationException(); + } + + /** + * Attempts to set the state to reflect a release in shared mode. + * + * <p>This method is always invoked by the thread performing release. + * + * <p>The default implementation throws + * {@link UnsupportedOperationException}. + * + * @param arg the release argument. This value is always the one + * passed to a release method, or the current state value upon + * entry to a condition wait. The value is otherwise + * uninterpreted and can represent anything you like. + * @return {@code true} if this release of shared mode may permit a + * waiting acquire (shared or exclusive) to succeed; and + * {@code false} otherwise + * @throws IllegalMonitorStateException if releasing would place this + * synchronizer in an illegal state. This exception must be + * thrown in a consistent fashion for synchronization to work + * correctly. + * @throws UnsupportedOperationException if shared mode is not supported + */ + protected boolean tryReleaseShared(long arg) { + throw new UnsupportedOperationException(); + } + + /** + * Returns {@code true} if synchronization is held exclusively with + * respect to the current (calling) thread. This method is invoked + * upon each call to a non-waiting {@link ConditionObject} method. + * (Waiting methods instead invoke {@link #release}.) + * + * <p>The default implementation throws {@link + * UnsupportedOperationException}. This method is invoked + * internally only within {@link ConditionObject} methods, so need + * not be defined if conditions are not used. + * + * @return {@code true} if synchronization is held exclusively; + * {@code false} otherwise + * @throws UnsupportedOperationException if conditions are not supported + */ + protected boolean isHeldExclusively() { + throw new UnsupportedOperationException(); + } + + /** + * Acquires in exclusive mode, ignoring interrupts. Implemented + * by invoking at least once {@link #tryAcquire}, + * returning on success. Otherwise the thread is queued, possibly + * repeatedly blocking and unblocking, invoking {@link + * #tryAcquire} until success. This method can be used + * to implement method {@link Lock#lock}. + * + * @param arg the acquire argument. This value is conveyed to + * {@link #tryAcquire} but is otherwise uninterpreted and + * can represent anything you like. + */ + public final void acquire(long arg) { + if (!tryAcquire(arg) && + acquireQueued(addWaiter(Node.EXCLUSIVE), arg)) + selfInterrupt(); + } + + /** + * Acquires in exclusive mode, aborting if interrupted. + * Implemented by first checking interrupt status, then invoking + * at least once {@link #tryAcquire}, returning on + * success. Otherwise the thread is queued, possibly repeatedly + * blocking and unblocking, invoking {@link #tryAcquire} + * until success or the thread is interrupted. This method can be + * used to implement method {@link Lock#lockInterruptibly}. + * + * @param arg the acquire argument. This value is conveyed to + * {@link #tryAcquire} but is otherwise uninterpreted and + * can represent anything you like. + * @throws InterruptedException if the current thread is interrupted + */ + public final void acquireInterruptibly(long arg) throws InterruptedException { + if (Thread.interrupted()) + throw new InterruptedException(); + if (!tryAcquire(arg)) + doAcquireInterruptibly(arg); + } + + /** + * Attempts to acquire in exclusive mode, aborting if interrupted, + * and failing if the given timeout elapses. Implemented by first + * checking interrupt status, then invoking at least once {@link + * #tryAcquire}, returning on success. Otherwise, the thread is + * queued, possibly repeatedly blocking and unblocking, invoking + * {@link #tryAcquire} until success or the thread is interrupted + * or the timeout elapses. This method can be used to implement + * method {@link Lock#tryLock(long, TimeUnit)}. + * + * @param arg the acquire argument. This value is conveyed to + * {@link #tryAcquire} but is otherwise uninterpreted and + * can represent anything you like. + * @param nanosTimeout the maximum number of nanoseconds to wait + * @return {@code true} if acquired; {@code false} if timed out + * @throws InterruptedException if the current thread is interrupted + */ + public final boolean tryAcquireNanos(long arg, long nanosTimeout) throws InterruptedException { + if (Thread.interrupted()) + throw new InterruptedException(); + return tryAcquire(arg) || + doAcquireNanos(arg, nanosTimeout); + } + + /** + * Releases in exclusive mode. Implemented by unblocking one or + * more threads if {@link #tryRelease} returns true. + * This method can be used to implement method {@link Lock#unlock}. + * + * @param arg the release argument. This value is conveyed to + * {@link #tryRelease} but is otherwise uninterpreted and + * can represent anything you like. + * @return the value returned from {@link #tryRelease} + */ + public final boolean release(long arg) { + if (tryRelease(arg)) { + Node h = head; + if (h != null && h.waitStatus != 0) + unparkSuccessor(h); + return true; + } + return false; + } + + /** + * Acquires in shared mode, ignoring interrupts. Implemented by + * first invoking at least once {@link #tryAcquireShared}, + * returning on success. Otherwise the thread is queued, possibly + * repeatedly blocking and unblocking, invoking {@link + * #tryAcquireShared} until success. + * + * @param arg the acquire argument. This value is conveyed to + * {@link #tryAcquireShared} but is otherwise uninterpreted + * and can represent anything you like. + */ + public final void acquireShared(long arg) { + if (tryAcquireShared(arg) < 0) + doAcquireShared(arg); + } + + /** + * Acquires in shared mode, aborting if interrupted. Implemented + * by first checking interrupt status, then invoking at least once + * {@link #tryAcquireShared}, returning on success. Otherwise the + * thread is queued, possibly repeatedly blocking and unblocking, + * invoking {@link #tryAcquireShared} until success or the thread + * is interrupted. + * @param arg the acquire argument. + * This value is conveyed to {@link #tryAcquireShared} but is + * otherwise uninterpreted and can represent anything + * you like. + * @throws InterruptedException if the current thread is interrupted + */ + public final void acquireSharedInterruptibly(long arg) throws InterruptedException { + if (Thread.interrupted()) + throw new InterruptedException(); + if (tryAcquireShared(arg) < 0) + doAcquireSharedInterruptibly(arg); + } + + /** + * Attempts to acquire in shared mode, aborting if interrupted, and + * failing if the given timeout elapses. Implemented by first + * checking interrupt status, then invoking at least once {@link + * #tryAcquireShared}, returning on success. Otherwise, the + * thread is queued, possibly repeatedly blocking and unblocking, + * invoking {@link #tryAcquireShared} until success or the thread + * is interrupted or the timeout elapses. + * + * @param arg the acquire argument. This value is conveyed to + * {@link #tryAcquireShared} but is otherwise uninterpreted + * and can represent anything you like. + * @param nanosTimeout the maximum number of nanoseconds to wait + * @return {@code true} if acquired; {@code false} if timed out + * @throws InterruptedException if the current thread is interrupted + */ + public final boolean tryAcquireSharedNanos(long arg, long nanosTimeout) throws InterruptedException { + if (Thread.interrupted()) + throw new InterruptedException(); + return tryAcquireShared(arg) >= 0 || + doAcquireSharedNanos(arg, nanosTimeout); + } + + /** + * Releases in shared mode. Implemented by unblocking one or more + * threads if {@link #tryReleaseShared} returns true. + * + * @param arg the release argument. This value is conveyed to + * {@link #tryReleaseShared} but is otherwise uninterpreted + * and can represent anything you like. + * @return the value returned from {@link #tryReleaseShared} + */ + public final boolean releaseShared(long arg) { + if (tryReleaseShared(arg)) { + Node h = head; + if (h != null && h.waitStatus != 0) + unparkSuccessor(h); + return true; + } + return false; + } + + // Queue inspection methods + + /** + * Queries whether any threads are waiting to acquire. Note that + * because cancellations due to interrupts and timeouts may occur + * at any time, a {@code true} return does not guarantee that any + * other thread will ever acquire. + * + * <p>In this implementation, this operation returns in + * constant time. + * + * @return {@code true} if there may be other threads waiting to acquire + */ + public final boolean hasQueuedThreads() { + return head != tail; + } + + /** + * Queries whether any threads have ever contended to acquire this + * synchronizer; that is if an acquire method has ever blocked. + * + * <p>In this implementation, this operation returns in + * constant time. + * + * @return {@code true} if there has ever been contention + */ + public final boolean hasContended() { + return head != null; + } + + /** + * Returns the first (longest-waiting) thread in the queue, or + * {@code null} if no threads are currently queued. + * + * <p>In this implementation, this operation normally returns in + * constant time, but may iterate upon contention if other threads are + * concurrently modifying the queue. + * + * @return the first (longest-waiting) thread in the queue, or + * {@code null} if no threads are currently queued + */ + public final Thread getFirstQueuedThread() { + // handle only fast path, else relay + return (head == tail)? null : fullGetFirstQueuedThread(); + } + + /** + * Version of getFirstQueuedThread called when fastpath fails + */ + private Thread fullGetFirstQueuedThread() { + /* + * The first node is normally h.next. Try to get its + * thread field, ensuring consistent reads: If thread + * field is nulled out or s.prev is no longer head, then + * some other thread(s) concurrently performed setHead in + * between some of our reads. We try this twice before + * resorting to traversal. + */ + Node h, s; + Thread st; + if (((h = head) != null && (s = h.next) != null && + s.prev == head && (st = s.thread) != null) || + ((h = head) != null && (s = h.next) != null && + s.prev == head && (st = s.thread) != null)) + return st; + + /* + * Head's next field might not have been set yet, or may have + * been unset after setHead. So we must check to see if tail + * is actually first node. If not, we continue on, safely + * traversing from tail back to head to find first, + * guaranteeing termination. + */ + + Node t = tail; + Thread firstThread = null; + while (t != null && t != head) { + Thread tt = t.thread; + if (tt != null) + firstThread = tt; + t = t.prev; + } + return firstThread; + } + + /** + * Returns true if the given thread is currently queued. + * + * <p>This implementation traverses the queue to determine + * presence of the given thread. + * + * @param thread the thread + * @return {@code true} if the given thread is on the queue + * @throws NullPointerException if the thread is null + */ + public final boolean isQueued(Thread thread) { + if (thread == null) + throw new NullPointerException(); + for (Node p = tail; p != null; p = p.prev) + if (p.thread == thread) + return true; + return false; + } + + /** + * Return {@code true} if the apparent first queued thread, if one + * exists, is not waiting in exclusive mode. Used only as a heuristic + * in ReentrantReadWriteLock. + */ + final boolean apparentlyFirstQueuedIsExclusive() { + Node h, s; + return ((h = head) != null && (s = h.next) != null && + s.nextWaiter != Node.SHARED); + } + + /** + * Return {@code true} if the queue is empty or if the given thread + * is at the head of the queue. This is reliable only if + * <tt>current</tt> is actually Thread.currentThread() of caller. + */ + final boolean isFirst(Thread current) { + Node h, s; + return ((h = head) == null || + ((s = h.next) != null && s.thread == current) || + fullIsFirst(current)); + } + + final boolean fullIsFirst(Thread current) { + // same idea as fullGetFirstQueuedThread + Node h, s; + Thread firstThread = null; + if (((h = head) != null && (s = h.next) != null && + s.prev == head && (firstThread = s.thread) != null)) + return firstThread == current; + Node t = tail; + while (t != null && t != head) { + Thread tt = t.thread; + if (tt != null) + firstThread = tt; + t = t.prev; + } + return firstThread == current || firstThread == null; + } + + + // Instrumentation and monitoring methods + + /** + * Returns an estimate of the number of threads waiting to + * acquire. The value is only an estimate because the number of + * threads may change dynamically while this method traverses + * internal data structures. This method is designed for use in + * monitoring system state, not for synchronization + * control. + * + * @return the estimated number of threads waiting to acquire + */ + public final int getQueueLength() { + int n = 0; + for (Node p = tail; p != null; p = p.prev) { + if (p.thread != null) + ++n; + } + return n; + } + + /** + * Returns a collection containing threads that may be waiting to + * acquire. Because the actual set of threads may change + * dynamically while constructing this result, the returned + * collection is only a best-effort estimate. The elements of the + * returned collection are in no particular order. This method is + * designed to facilitate construction of subclasses that provide + * more extensive monitoring facilities. + * + * @return the collection of threads + */ + public final Collection<Thread> getQueuedThreads() { + ArrayList<Thread> list = new ArrayList<Thread>(); + for (Node p = tail; p != null; p = p.prev) { + Thread t = p.thread; + if (t != null) + list.add(t); + } + return list; + } + + /** + * Returns a collection containing threads that may be waiting to + * acquire in exclusive mode. This has the same properties + * as {@link #getQueuedThreads} except that it only returns + * those threads waiting due to an exclusive acquire. + * + * @return the collection of threads + */ + public final Collection<Thread> getExclusiveQueuedThreads() { + ArrayList<Thread> list = new ArrayList<Thread>(); + for (Node p = tail; p != null; p = p.prev) { + if (!p.isShared()) { + Thread t = p.thread; + if (t != null) + list.add(t); + } + } + return list; + } + + /** + * Returns a collection containing threads that may be waiting to + * acquire in shared mode. This has the same properties + * as {@link #getQueuedThreads} except that it only returns + * those threads waiting due to a shared acquire. + * + * @return the collection of threads + */ + public final Collection<Thread> getSharedQueuedThreads() { + ArrayList<Thread> list = new ArrayList<Thread>(); + for (Node p = tail; p != null; p = p.prev) { + if (p.isShared()) { + Thread t = p.thread; + if (t != null) + list.add(t); + } + } + return list; + } + + /** + * Returns a string identifying this synchronizer, as well as its state. + * The state, in brackets, includes the String {@code "State ="} + * followed by the current value of {@link #getState}, and either + * {@code "nonempty"} or {@code "empty"} depending on whether the + * queue is empty. + * + * @return a string identifying this synchronizer, as well as its state + */ + public String toString() { + long s = getState(); + String q = hasQueuedThreads()? "non" : ""; + return super.toString() + + "[State = " + s + ", " + q + "empty queue]"; + } + + + // Internal support methods for Conditions + + /** + * Returns true if a node, always one that was initially placed on + * a condition queue, is now waiting to reacquire on sync queue. + * @param node the node + * @return true if is reacquiring + */ + final boolean isOnSyncQueue(Node node) { + if (node.waitStatus == Node.CONDITION || node.prev == null) + return false; + if (node.next != null) // If has successor, it must be on queue + return true; + /* + * node.prev can be non-null, but not yet on queue because + * the CAS to place it on queue can fail. So we have to + * traverse from tail to make sure it actually made it. It + * will always be near the tail in calls to this method, and + * unless the CAS failed (which is unlikely), it will be + * there, so we hardly ever traverse much. + */ + return findNodeFromTail(node); + } + + /** + * Returns true if node is on sync queue by searching backwards from tail. + * Called only when needed by isOnSyncQueue. + * @return true if present + */ + private boolean findNodeFromTail(Node node) { + Node t = tail; + for (;;) { + if (t == node) + return true; + if (t == null) + return false; + t = t.prev; + } + } + + /** + * Transfers a node from a condition queue onto sync queue. + * Returns true if successful. + * @param node the node + * @return true if successfully transferred (else the node was + * cancelled before signal). + */ + final boolean transferForSignal(Node node) { + /* + * If cannot change waitStatus, the node has been cancelled. + */ + if (!compareAndSetWaitStatus(node, Node.CONDITION, 0)) + return false; + + /* + * Splice onto queue and try to set waitStatus of predecessor to + * indicate that thread is (probably) waiting. If cancelled or + * attempt to set waitStatus fails, wake up to resync (in which + * case the waitStatus can be transiently and harmlessly wrong). + */ + Node p = enq(node); + int c = p.waitStatus; + if (c > 0 || !compareAndSetWaitStatus(p, c, Node.SIGNAL)) + LockSupport.unpark(node.thread); + return true; + } + + /** + * Transfers node, if necessary, to sync queue after a cancelled + * wait. Returns true if thread was cancelled before being + * signalled. + * @param current the waiting thread + * @param node its node + * @return true if cancelled before the node was signalled. + */ + final boolean transferAfterCancelledWait(Node node) { + if (compareAndSetWaitStatus(node, Node.CONDITION, 0)) { + enq(node); + return true; + } + /* + * If we lost out to a signal(), then we can't proceed + * until it finishes its enq(). Cancelling during an + * incomplete transfer is both rare and transient, so just + * spin. + */ + while (!isOnSyncQueue(node)) + Thread.yield(); + return false; + } + + /** + * Invokes release with current state value; returns saved state. + * Cancels node and throws exception on failure. + * @param node the condition node for this wait + * @return previous sync state + */ + final long fullyRelease(Node node) { + try { + long savedState = getState(); + if (release(savedState)) + return savedState; + } catch (RuntimeException ex) { + node.waitStatus = Node.CANCELLED; + throw ex; + } + // reach here if release fails + node.waitStatus = Node.CANCELLED; + throw new IllegalMonitorStateException(); + } + + // Instrumentation methods for conditions + + /** + * Queries whether the given ConditionObject + * uses this synchronizer as its lock. + * + * @param condition the condition + * @return <tt>true</tt> if owned + * @throws NullPointerException if the condition is null + */ + public final boolean owns(ConditionObject condition) { + if (condition == null) + throw new NullPointerException(); + return condition.isOwnedBy(this); + } + + /** + * Queries whether any threads are waiting on the given condition + * associated with this synchronizer. Note that because timeouts + * and interrupts may occur at any time, a <tt>true</tt> return + * does not guarantee that a future <tt>signal</tt> will awaken + * any threads. This method is designed primarily for use in + * monitoring of the system state. + * + * @param condition the condition + * @return <tt>true</tt> if there are any waiting threads + * @throws IllegalMonitorStateException if exclusive synchronization + * is not held + * @throws IllegalArgumentException if the given condition is + * not associated with this synchronizer + * @throws NullPointerException if the condition is null + */ + public final boolean hasWaiters(ConditionObject condition) { + if (!owns(condition)) + throw new IllegalArgumentException("Not owner"); + return condition.hasWaiters(); + } + + /** + * Returns an estimate of the number of threads waiting on the + * given condition associated with this synchronizer. Note that + * because timeouts and interrupts may occur at any time, the + * estimate serves only as an upper bound on the actual number of + * waiters. This method is designed for use in monitoring of the + * system state, not for synchronization control. + * + * @param condition the condition + * @return the estimated number of waiting threads + * @throws IllegalMonitorStateException if exclusive synchronization + * is not held + * @throws IllegalArgumentException if the given condition is + * not associated with this synchronizer + * @throws NullPointerException if the condition is null + */ + public final int getWaitQueueLength(ConditionObject condition) { + if (!owns(condition)) + throw new IllegalArgumentException("Not owner"); + return condition.getWaitQueueLength(); + } + + /** + * Returns a collection containing those threads that may be + * waiting on the given condition associated with this + * synchronizer. Because the actual set of threads may change + * dynamically while constructing this result, the returned + * collection is only a best-effort estimate. The elements of the + * returned collection are in no particular order. + * + * @param condition the condition + * @return the collection of threads + * @throws IllegalMonitorStateException if exclusive synchronization + * is not held + * @throws IllegalArgumentException if the given condition is + * not associated with this synchronizer + * @throws NullPointerException if the condition is null + */ + public final Collection<Thread> getWaitingThreads(ConditionObject condition) { + if (!owns(condition)) + throw new IllegalArgumentException("Not owner"); + return condition.getWaitingThreads(); + } + + /** + * Condition implementation for a {@link + * AbstractQueuedLongSynchronizer} serving as the basis of a {@link + * Lock} implementation. + * + * <p>Method documentation for this class describes mechanics, + * not behavioral specifications from the point of view of Lock + * and Condition users. Exported versions of this class will in + * general need to be accompanied by documentation describing + * condition semantics that rely on those of the associated + * <tt>AbstractQueuedLongSynchronizer</tt>. + * + * <p>This class is Serializable, but all fields are transient, + * so deserialized conditions have no waiters. + * + * @since 1.6 + */ + public class ConditionObject implements Condition, java.io.Serializable { + private static final long serialVersionUID = 1173984872572414699L; + /** First node of condition queue. */ + private transient Node firstWaiter; + /** Last node of condition queue. */ + private transient Node lastWaiter; + + /** + * Creates a new <tt>ConditionObject</tt> instance. + */ + public ConditionObject() { } + + // Internal methods + + /** + * Adds a new waiter to wait queue. + * @return its new wait node + */ + private Node addConditionWaiter() { + Node node = new Node(Thread.currentThread(), Node.CONDITION); + Node t = lastWaiter; + if (t == null) + firstWaiter = node; + else + t.nextWaiter = node; + lastWaiter = node; + return node; + } + + /** + * Removes and transfers nodes until hit non-cancelled one or + * null. Split out from signal in part to encourage compilers + * to inline the case of no waiters. + * @param first (non-null) the first node on condition queue + */ + private void doSignal(Node first) { + do { + if ( (firstWaiter = first.nextWaiter) == null) + lastWaiter = null; + first.nextWaiter = null; + } while (!transferForSignal(first) && + (first = firstWaiter) != null); + } + + /** + * Removes and transfers all nodes. + * @param first (non-null) the first node on condition queue + */ + private void doSignalAll(Node first) { + lastWaiter = firstWaiter = null; + do { + Node next = first.nextWaiter; + first.nextWaiter = null; + transferForSignal(first); + first = next; + } while (first != null); + } + + /** + * Returns true if given node is on this condition queue. + * Call only when holding lock. + */ + private boolean isOnConditionQueue(Node node) { + return node.next != null || node == lastWaiter; + } + + /** + * Unlinks a cancelled waiter node from condition queue. This + * is called when cancellation occurred during condition wait, + * not lock wait, and is called only after lock has been + * re-acquired by a cancelled waiter and the node is not known + * to already have been dequeued. It is needed to avoid + * garbage retention in the absence of signals. So even though + * it may require a full traversal, it comes into play only + * when timeouts or cancellations occur in the absence of + * signals. + */ + private void unlinkCancelledWaiter(Node node) { + Node t = firstWaiter; + Node trail = null; + while (t != null) { + if (t == node) { + Node next = t.nextWaiter; + if (trail == null) + firstWaiter = next; + else + trail.nextWaiter = next; + if (lastWaiter == node) + lastWaiter = trail; + break; + } + trail = t; + t = t.nextWaiter; + } + } + + // public methods + + /** + * Moves the longest-waiting thread, if one exists, from the + * wait queue for this condition to the wait queue for the + * owning lock. + * + * @throws IllegalMonitorStateException if {@link #isHeldExclusively} + * returns {@code false} + */ + public final void signal() { + if (!isHeldExclusively()) + throw new IllegalMonitorStateException(); + Node first = firstWaiter; + if (first != null) + doSignal(first); + } + + /** + * Moves all threads from the wait queue for this condition to + * the wait queue for the owning lock. + * + * @throws IllegalMonitorStateException if {@link #isHeldExclusively} + * returns {@code false} + */ + public final void signalAll() { + if (!isHeldExclusively()) + throw new IllegalMonitorStateException(); + Node first = firstWaiter; + if (first != null) + doSignalAll(first); + } + + /** + * Implements uninterruptible condition wait. + * <ol> + * <li> Save lock state returned by {@link #getState} + * <li> Invoke {@link #release} with + * saved state as argument, throwing + * IllegalMonitorStateException if it fails. + * <li> Block until signalled + * <li> Reacquire by invoking specialized version of + * {@link #acquire} with saved state as argument. + * </ol> + */ + public final void awaitUninterruptibly() { + Node node = addConditionWaiter(); + long savedState = fullyRelease(node); + boolean interrupted = false; + while (!isOnSyncQueue(node)) { + LockSupport.park(this); + if (Thread.interrupted()) + interrupted = true; + } + if (acquireQueued(node, savedState) || interrupted) + selfInterrupt(); + } + + /* + * For interruptible waits, we need to track whether to throw + * InterruptedException, if interrupted while blocked on + * condition, versus reinterrupt current thread, if + * interrupted while blocked waiting to re-acquire. + */ + + /** Mode meaning to reinterrupt on exit from wait */ + private static final int REINTERRUPT = 1; + /** Mode meaning to throw InterruptedException on exit from wait */ + private static final int THROW_IE = -1; + + /** + * Checks for interrupt, returning THROW_IE if interrupted + * before signalled, REINTERRUPT if after signalled, or + * 0 if not interrupted. + */ + private int checkInterruptWhileWaiting(Node node) { + return (Thread.interrupted()) ? + ((transferAfterCancelledWait(node))? THROW_IE : REINTERRUPT) : + 0; + } + + /** + * Throws InterruptedException, reinterrupts current thread, or + * does nothing, depending on mode. + */ + private void reportInterruptAfterWait(int interruptMode) + throws InterruptedException { + if (interruptMode == THROW_IE) + throw new InterruptedException(); + else if (interruptMode == REINTERRUPT) + selfInterrupt(); + } + + /** + * Implements interruptible condition wait. + * <ol> + * <li> If current thread is interrupted, throw InterruptedException + * <li> Save lock state returned by {@link #getState} + * <li> Invoke {@link #release} with + * saved state as argument, throwing + * IllegalMonitorStateException if it fails. + * <li> Block until signalled or interrupted + * <li> Reacquire by invoking specialized version of + * {@link #acquire} with saved state as argument. + * <li> If interrupted while blocked in step 4, throw exception + * </ol> + */ + public final void await() throws InterruptedException { + if (Thread.interrupted()) + throw new InterruptedException(); + Node node = addConditionWaiter(); + long savedState = fullyRelease(node); + int interruptMode = 0; + while (!isOnSyncQueue(node)) { + LockSupport.park(this); + if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) + break; + } + if (acquireQueued(node, savedState) && interruptMode != THROW_IE) + interruptMode = REINTERRUPT; + if (isOnConditionQueue(node)) + unlinkCancelledWaiter(node); + if (interruptMode != 0) + reportInterruptAfterWait(interruptMode); + } + + /** + * Implements timed condition wait. + * <ol> + * <li> If current thread is interrupted, throw InterruptedException + * <li> Save lock state returned by {@link #getState} + * <li> Invoke {@link #release} with + * saved state as argument, throwing + * IllegalMonitorStateException if it fails. + * <li> Block until signalled, interrupted, or timed out + * <li> Reacquire by invoking specialized version of + * {@link #acquire} with saved state as argument. + * <li> If interrupted while blocked in step 4, throw InterruptedException + * </ol> + */ + public final long awaitNanos(long nanosTimeout) throws InterruptedException { + if (Thread.interrupted()) + throw new InterruptedException(); + Node node = addConditionWaiter(); + long savedState = fullyRelease(node); + long lastTime = System.nanoTime(); + int interruptMode = 0; + while (!isOnSyncQueue(node)) { + if (nanosTimeout <= 0L) { + transferAfterCancelledWait(node); + break; + } + LockSupport.parkNanos(this, nanosTimeout); + if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) + break; + + long now = System.nanoTime(); + nanosTimeout -= now - lastTime; + lastTime = now; + } + if (acquireQueued(node, savedState) && interruptMode != THROW_IE) + interruptMode = REINTERRUPT; + if (isOnConditionQueue(node)) + unlinkCancelledWaiter(node); + if (interruptMode != 0) + reportInterruptAfterWait(interruptMode); + return nanosTimeout - (System.nanoTime() - lastTime); + } + + /** + * Implements absolute timed condition wait. + * <ol> + * <li> If current thread is interrupted, throw InterruptedException + * <li> Save lock state returned by {@link #getState} + * <li> Invoke {@link #release} with + * saved state as argument, throwing + * IllegalMonitorStateException if it fails. + * <li> Block until signalled, interrupted, or timed out + * <li> Reacquire by invoking specialized version of + * {@link #acquire} with saved state as argument. + * <li> If interrupted while blocked in step 4, throw InterruptedException + * <li> If timed out while blocked in step 4, return false, else true + * </ol> + */ + public final boolean awaitUntil(Date deadline) throws InterruptedException { + if (deadline == null) + throw new NullPointerException(); + long abstime = deadline.getTime(); + if (Thread.interrupted()) + throw new InterruptedException(); + Node node = addConditionWaiter(); + long savedState = fullyRelease(node); + boolean timedout = false; + int interruptMode = 0; + while (!isOnSyncQueue(node)) { + if (System.currentTimeMillis() > abstime) { + timedout = transferAfterCancelledWait(node); + break; + } + LockSupport.parkUntil(this, abstime); + if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) + break; + } + if (acquireQueued(node, savedState) && interruptMode != THROW_IE) + interruptMode = REINTERRUPT; + if (isOnConditionQueue(node)) + unlinkCancelledWaiter(node); + if (interruptMode != 0) + reportInterruptAfterWait(interruptMode); + return !timedout; + } + + /** + * Implements timed condition wait. + * <ol> + * <li> If current thread is interrupted, throw InterruptedException + * <li> Save lock state returned by {@link #getState} + * <li> Invoke {@link #release} with + * saved state as argument, throwing + * IllegalMonitorStateException if it fails. + * <li> Block until signalled, interrupted, or timed out + * <li> Reacquire by invoking specialized version of + * {@link #acquire} with saved state as argument. + * <li> If interrupted while blocked in step 4, throw InterruptedException + * <li> If timed out while blocked in step 4, return false, else true + * </ol> + */ + public final boolean await(long time, TimeUnit unit) throws InterruptedException { + if (unit == null) + throw new NullPointerException(); + long nanosTimeout = unit.toNanos(time); + if (Thread.interrupted()) + throw new InterruptedException(); + Node node = addConditionWaiter(); + long savedState = fullyRelease(node); + long lastTime = System.nanoTime(); + boolean timedout = false; + int interruptMode = 0; + while (!isOnSyncQueue(node)) { + if (nanosTimeout <= 0L) { + timedout = transferAfterCancelledWait(node); + break; + } + LockSupport.parkNanos(this, nanosTimeout); + if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) + break; + long now = System.nanoTime(); + nanosTimeout -= now - lastTime; + lastTime = now; + } + if (acquireQueued(node, savedState) && interruptMode != THROW_IE) + interruptMode = REINTERRUPT; + if (isOnConditionQueue(node)) + unlinkCancelledWaiter(node); + if (interruptMode != 0) + reportInterruptAfterWait(interruptMode); + return !timedout; + } + + // support for instrumentation + + /** + * Returns true if this condition was created by the given + * synchronization object. + * + * @return {@code true} if owned + */ + final boolean isOwnedBy(AbstractQueuedLongSynchronizer sync) { + return sync == AbstractQueuedLongSynchronizer.this; + } + + /** + * Queries whether any threads are waiting on this condition. + * Implements {@link AbstractQueuedLongSynchronizer#hasWaiters}. + * + * @return {@code true} if there are any waiting threads + * @throws IllegalMonitorStateException if {@link #isHeldExclusively} + * returns {@code false} + */ + protected final boolean hasWaiters() { + if (!isHeldExclusively()) + throw new IllegalMonitorStateException(); + for (Node w = firstWaiter; w != null; w = w.nextWaiter) { + if (w.waitStatus == Node.CONDITION) + return true; + } + return false; + } + + /** + * Returns an estimate of the number of threads waiting on + * this condition. + * Implements {@link AbstractQueuedLongSynchronizer#getWaitQueueLength}. + * + * @return the estimated number of waiting threads + * @throws IllegalMonitorStateException if {@link #isHeldExclusively} + * returns {@code false} + */ + protected final int getWaitQueueLength() { + if (!isHeldExclusively()) + throw new IllegalMonitorStateException(); + int n = 0; + for (Node w = firstWaiter; w != null; w = w.nextWaiter) { + if (w.waitStatus == Node.CONDITION) + ++n; + } + return n; + } + + /** + * Returns a collection containing those threads that may be + * waiting on this Condition. + * Implements {@link AbstractQueuedLongSynchronizer#getWaitingThreads}. + * + * @return the collection of threads + * @throws IllegalMonitorStateException if {@link #isHeldExclusively} + * returns {@code false} + */ + protected final Collection<Thread> getWaitingThreads() { + if (!isHeldExclusively()) + throw new IllegalMonitorStateException(); + ArrayList<Thread> list = new ArrayList<Thread>(); + for (Node w = firstWaiter; w != null; w = w.nextWaiter) { + if (w.waitStatus == Node.CONDITION) { + Thread t = w.thread; + if (t != null) + list.add(t); + } + } + return list; + } + } + + /** + * Setup to support compareAndSet. We need to natively implement + * this here: For the sake of permitting future enhancements, we + * cannot explicitly subclass AtomicLong, which would be + * efficient and useful otherwise. So, as the lesser of evils, we + * natively implement using hotspot intrinsics API. And while we + * are at it, we do the same for other CASable fields (which could + * otherwise be done with atomic field updaters). + */ + private static final Unsafe unsafe = Unsafe.getUnsafe(); + private static final long stateOffset; + private static final long headOffset; + private static final long tailOffset; + private static final long waitStatusOffset; + + static { + try { + stateOffset = unsafe.objectFieldOffset + (AbstractQueuedLongSynchronizer.class.getDeclaredField("state")); + headOffset = unsafe.objectFieldOffset + (AbstractQueuedLongSynchronizer.class.getDeclaredField("head")); + tailOffset = unsafe.objectFieldOffset + (AbstractQueuedLongSynchronizer.class.getDeclaredField("tail")); + waitStatusOffset = unsafe.objectFieldOffset + (Node.class.getDeclaredField("waitStatus")); + + } catch (Exception ex) { throw new Error(ex); } + } + + /** + * CAS head field. Used only by enq + */ + private final boolean compareAndSetHead(Node update) { + return unsafe.compareAndSwapObject(this, headOffset, null, update); + } + + /** + * CAS tail field. Used only by enq + */ + private final boolean compareAndSetTail(Node expect, Node update) { + return unsafe.compareAndSwapObject(this, tailOffset, expect, update); + } + + /** + * CAS waitStatus field of a node. + */ + private final static boolean compareAndSetWaitStatus(Node node, + int expect, + int update) { + return unsafe.compareAndSwapInt(node, waitStatusOffset, + expect, update); + } +} diff --git a/libjava/classpath/external/jsr166/java/util/concurrent/locks/AbstractQueuedSynchronizer.java b/libjava/classpath/external/jsr166/java/util/concurrent/locks/AbstractQueuedSynchronizer.java new file mode 100644 index 000000000..647f4fcbc --- /dev/null +++ b/libjava/classpath/external/jsr166/java/util/concurrent/locks/AbstractQueuedSynchronizer.java @@ -0,0 +1,2159 @@ +/* + * Written by Doug Lea with assistance from members of JCP JSR-166 + * Expert Group and released to the public domain, as explained at + * http://creativecommons.org/licenses/publicdomain + */ + +package java.util.concurrent.locks; +import java.util.*; +import java.util.concurrent.*; +import java.util.concurrent.atomic.*; +import sun.misc.Unsafe; + +/** + * Provides a framework for implementing blocking locks and related + * synchronizers (semaphores, events, etc) that rely on + * first-in-first-out (FIFO) wait queues. This class is designed to + * be a useful basis for most kinds of synchronizers that rely on a + * single atomic <tt>int</tt> value to represent state. Subclasses + * must define the protected methods that change this state, and which + * define what that state means in terms of this object being acquired + * or released. Given these, the other methods in this class carry + * out all queuing and blocking mechanics. Subclasses can maintain + * other state fields, but only the atomically updated <tt>int</tt> + * value manipulated using methods {@link #getState}, {@link + * #setState} and {@link #compareAndSetState} is tracked with respect + * to synchronization. + * + * <p>Subclasses should be defined as non-public internal helper + * classes that are used to implement the synchronization properties + * of their enclosing class. Class + * <tt>AbstractQueuedSynchronizer</tt> does not implement any + * synchronization interface. Instead it defines methods such as + * {@link #acquireInterruptibly} that can be invoked as + * appropriate by concrete locks and related synchronizers to + * implement their public methods. + * + * <p>This class supports either or both a default <em>exclusive</em> + * mode and a <em>shared</em> mode. When acquired in exclusive mode, + * attempted acquires by other threads cannot succeed. Shared mode + * acquires by multiple threads may (but need not) succeed. This class + * does not "understand" these differences except in the + * mechanical sense that when a shared mode acquire succeeds, the next + * waiting thread (if one exists) must also determine whether it can + * acquire as well. Threads waiting in the different modes share the + * same FIFO queue. Usually, implementation subclasses support only + * one of these modes, but both can come into play for example in a + * {@link ReadWriteLock}. Subclasses that support only exclusive or + * only shared modes need not define the methods supporting the unused mode. + * + * <p>This class defines a nested {@link ConditionObject} class that + * can be used as a {@link Condition} implementation by subclasses + * supporting exclusive mode for which method {@link + * #isHeldExclusively} reports whether synchronization is exclusively + * held with respect to the current thread, method {@link #release} + * invoked with the current {@link #getState} value fully releases + * this object, and {@link #acquire}, given this saved state value, + * eventually restores this object to its previous acquired state. No + * <tt>AbstractQueuedSynchronizer</tt> method otherwise creates such a + * condition, so if this constraint cannot be met, do not use it. The + * behavior of {@link ConditionObject} depends of course on the + * semantics of its synchronizer implementation. + * + * <p>This class provides inspection, instrumentation, and monitoring + * methods for the internal queue, as well as similar methods for + * condition objects. These can be exported as desired into classes + * using an <tt>AbstractQueuedSynchronizer</tt> for their + * synchronization mechanics. + * + * <p>Serialization of this class stores only the underlying atomic + * integer maintaining state, so deserialized objects have empty + * thread queues. Typical subclasses requiring serializability will + * define a <tt>readObject</tt> method that restores this to a known + * initial state upon deserialization. + * + * <h3>Usage</h3> + * + * <p>To use this class as the basis of a synchronizer, redefine the + * following methods, as applicable, by inspecting and/or modifying + * the synchronization state using {@link #getState}, {@link + * #setState} and/or {@link #compareAndSetState}: + * + * <ul> + * <li> {@link #tryAcquire} + * <li> {@link #tryRelease} + * <li> {@link #tryAcquireShared} + * <li> {@link #tryReleaseShared} + * <li> {@link #isHeldExclusively} + *</ul> + * + * Each of these methods by default throws {@link + * UnsupportedOperationException}. Implementations of these methods + * must be internally thread-safe, and should in general be short and + * not block. Defining these methods is the <em>only</em> supported + * means of using this class. All other methods are declared + * <tt>final</tt> because they cannot be independently varied. + * + * <p>You may also find the inherited methods from {@link + * AbstractOwnableSynchronizer} useful to keep track of the thread + * owning an exclusive synchronizer. You are encouraged to use them + * -- this enables monitoring and diagnostic tools to assist users in + * determining which threads hold locks. + * + * <p>Even though this class is based on an internal FIFO queue, it + * does not automatically enforce FIFO acquisition policies. The core + * of exclusive synchronization takes the form: + * + * <pre> + * Acquire: + * while (!tryAcquire(arg)) { + * <em>enqueue thread if it is not already queued</em>; + * <em>possibly block current thread</em>; + * } + * + * Release: + * if (tryRelease(arg)) + * <em>unblock the first queued thread</em>; + * </pre> + * + * (Shared mode is similar but may involve cascading signals.) + * + * <p>Because checks in acquire are invoked before enqueuing, a newly + * acquiring thread may <em>barge</em> ahead of others that are + * blocked and queued. However, you can, if desired, define + * <tt>tryAcquire</tt> and/or <tt>tryAcquireShared</tt> to disable + * barging by internally invoking one or more of the inspection + * methods. In particular, a strict FIFO lock can define + * <tt>tryAcquire</tt> to immediately return <tt>false</tt> if {@link + * #getFirstQueuedThread} does not return the current thread. A + * normally preferable non-strict fair version can immediately return + * <tt>false</tt> only if {@link #hasQueuedThreads} returns + * <tt>true</tt> and <tt>getFirstQueuedThread</tt> is not the current + * thread; or equivalently, that <tt>getFirstQueuedThread</tt> is both + * non-null and not the current thread. Further variations are + * possible. + * + * <p>Throughput and scalability are generally highest for the + * default barging (also known as <em>greedy</em>, + * <em>renouncement</em>, and <em>convoy-avoidance</em>) strategy. + * While this is not guaranteed to be fair or starvation-free, earlier + * queued threads are allowed to recontend before later queued + * threads, and each recontention has an unbiased chance to succeed + * against incoming threads. Also, while acquires do not + * "spin" in the usual sense, they may perform multiple + * invocations of <tt>tryAcquire</tt> interspersed with other + * computations before blocking. This gives most of the benefits of + * spins when exclusive synchronization is only briefly held, without + * most of the liabilities when it isn't. If so desired, you can + * augment this by preceding calls to acquire methods with + * "fast-path" checks, possibly prechecking {@link #hasContended} + * and/or {@link #hasQueuedThreads} to only do so if the synchronizer + * is likely not to be contended. + * + * <p>This class provides an efficient and scalable basis for + * synchronization in part by specializing its range of use to + * synchronizers that can rely on <tt>int</tt> state, acquire, and + * release parameters, and an internal FIFO wait queue. When this does + * not suffice, you can build synchronizers from a lower level using + * {@link java.util.concurrent.atomic atomic} classes, your own custom + * {@link java.util.Queue} classes, and {@link LockSupport} blocking + * support. + * + * <h3>Usage Examples</h3> + * + * <p>Here is a non-reentrant mutual exclusion lock class that uses + * the value zero to represent the unlocked state, and one to + * represent the locked state. While a non-reentrant lock + * does not strictly require recording of the current owner + * thread, this class does so anyway to make usage easier to monitor. + * It also supports conditions and exposes + * one of the instrumentation methods: + * + * <pre> + * class Mutex implements Lock, java.io.Serializable { + * + * // Our internal helper class + * private static class Sync extends AbstractQueuedSynchronizer { + * // Report whether in locked state + * protected boolean isHeldExclusively() { + * return getState() == 1; + * } + * + * // Acquire the lock if state is zero + * public boolean tryAcquire(int acquires) { + * assert acquires == 1; // Otherwise unused + * if (compareAndSetState(0, 1)) { + * setExclusiveOwnerThread(Thread.currentThread()); + * return true; + * } + * return false; + * } + * + * // Release the lock by setting state to zero + * protected boolean tryRelease(int releases) { + * assert releases == 1; // Otherwise unused + * if (getState() == 0) throw new IllegalMonitorStateException(); + * setExclusiveOwnerThread(null); + * setState(0); + * return true; + * } + * + * // Provide a Condition + * Condition newCondition() { return new ConditionObject(); } + * + * // Deserialize properly + * private void readObject(ObjectInputStream s) + * throws IOException, ClassNotFoundException { + * s.defaultReadObject(); + * setState(0); // reset to unlocked state + * } + * } + * + * // The sync object does all the hard work. We just forward to it. + * private final Sync sync = new Sync(); + * + * public void lock() { sync.acquire(1); } + * public boolean tryLock() { return sync.tryAcquire(1); } + * public void unlock() { sync.release(1); } + * public Condition newCondition() { return sync.newCondition(); } + * public boolean isLocked() { return sync.isHeldExclusively(); } + * public boolean hasQueuedThreads() { return sync.hasQueuedThreads(); } + * public void lockInterruptibly() throws InterruptedException { + * sync.acquireInterruptibly(1); + * } + * public boolean tryLock(long timeout, TimeUnit unit) + * throws InterruptedException { + * return sync.tryAcquireNanos(1, unit.toNanos(timeout)); + * } + * } + * </pre> + * + * <p>Here is a latch class that is like a {@link CountDownLatch} + * except that it only requires a single <tt>signal</tt> to + * fire. Because a latch is non-exclusive, it uses the <tt>shared</tt> + * acquire and release methods. + * + * <pre> + * class BooleanLatch { + * + * private static class Sync extends AbstractQueuedSynchronizer { + * boolean isSignalled() { return getState() != 0; } + * + * protected int tryAcquireShared(int ignore) { + * return isSignalled()? 1 : -1; + * } + * + * protected boolean tryReleaseShared(int ignore) { + * setState(1); + * return true; + * } + * } + * + * private final Sync sync = new Sync(); + * public boolean isSignalled() { return sync.isSignalled(); } + * public void signal() { sync.releaseShared(1); } + * public void await() throws InterruptedException { + * sync.acquireSharedInterruptibly(1); + * } + * } + * </pre> + * + * @since 1.5 + * @author Doug Lea + */ +public abstract class AbstractQueuedSynchronizer + extends AbstractOwnableSynchronizer + implements java.io.Serializable { + + private static final long serialVersionUID = 7373984972572414691L; + + /** + * Creates a new <tt>AbstractQueuedSynchronizer</tt> instance + * with initial synchronization state of zero. + */ + protected AbstractQueuedSynchronizer() { } + + /** + * Wait queue node class. + * + * <p>The wait queue is a variant of a "CLH" (Craig, Landin, and + * Hagersten) lock queue. CLH locks are normally used for + * spinlocks. We instead use them for blocking synchronizers, but + * use the same basic tactic of holding some of the control + * information about a thread in the predecessor of its node. A + * "status" field in each node keeps track of whether a thread + * should block. A node is signalled when its predecessor + * releases. Each node of the queue otherwise serves as a + * specific-notification-style monitor holding a single waiting + * thread. The status field does NOT control whether threads are + * granted locks etc though. A thread may try to acquire if it is + * first in the queue. But being first does not guarantee success; + * it only gives the right to contend. So the currently released + * contender thread may need to rewait. + * + * <p>To enqueue into a CLH lock, you atomically splice it in as new + * tail. To dequeue, you just set the head field. + * <pre> + * +------+ prev +-----+ +-----+ + * head | | <---- | | <---- | | tail + * +------+ +-----+ +-----+ + * </pre> + * + * <p>Insertion into a CLH queue requires only a single atomic + * operation on "tail", so there is a simple atomic point of + * demarcation from unqueued to queued. Similarly, dequeing + * involves only updating the "head". However, it takes a bit + * more work for nodes to determine who their successors are, + * in part to deal with possible cancellation due to timeouts + * and interrupts. + * + * <p>The "prev" links (not used in original CLH locks), are mainly + * needed to handle cancellation. If a node is cancelled, its + * successor is (normally) relinked to a non-cancelled + * predecessor. For explanation of similar mechanics in the case + * of spin locks, see the papers by Scott and Scherer at + * http://www.cs.rochester.edu/u/scott/synchronization/ + * + * <p>We also use "next" links to implement blocking mechanics. + * The thread id for each node is kept in its own node, so a + * predecessor signals the next node to wake up by traversing + * next link to determine which thread it is. Determination of + * successor must avoid races with newly queued nodes to set + * the "next" fields of their predecessors. This is solved + * when necessary by checking backwards from the atomically + * updated "tail" when a node's successor appears to be null. + * (Or, said differently, the next-links are an optimization + * so that we don't usually need a backward scan.) + * + * <p>Cancellation introduces some conservatism to the basic + * algorithms. Since we must poll for cancellation of other + * nodes, we can miss noticing whether a cancelled node is + * ahead or behind us. This is dealt with by always unparking + * successors upon cancellation, allowing them to stabilize on + * a new predecessor. + * + * <p>CLH queues need a dummy header node to get started. But + * we don't create them on construction, because it would be wasted + * effort if there is never contention. Instead, the node + * is constructed and head and tail pointers are set upon first + * contention. + * + * <p>Threads waiting on Conditions use the same nodes, but + * use an additional link. Conditions only need to link nodes + * in simple (non-concurrent) linked queues because they are + * only accessed when exclusively held. Upon await, a node is + * inserted into a condition queue. Upon signal, the node is + * transferred to the main queue. A special value of status + * field is used to mark which queue a node is on. + * + * <p>Thanks go to Dave Dice, Mark Moir, Victor Luchangco, Bill + * Scherer and Michael Scott, along with members of JSR-166 + * expert group, for helpful ideas, discussions, and critiques + * on the design of this class. + */ + static final class Node { + /** waitStatus value to indicate thread has cancelled */ + static final int CANCELLED = 1; + /** waitStatus value to indicate successor's thread needs unparking */ + static final int SIGNAL = -1; + /** waitStatus value to indicate thread is waiting on condition */ + static final int CONDITION = -2; + /** Marker to indicate a node is waiting in shared mode */ + static final Node SHARED = new Node(); + /** Marker to indicate a node is waiting in exclusive mode */ + static final Node EXCLUSIVE = null; + + /** + * Status field, taking on only the values: + * SIGNAL: The successor of this node is (or will soon be) + * blocked (via park), so the current node must + * unpark its successor when it releases or + * cancels. To avoid races, acquire methods must + * first indicate they need a signal, + * then retry the atomic acquire, and then, + * on failure, block. + * CANCELLED: This node is cancelled due to timeout or interrupt. + * Nodes never leave this state. In particular, + * a thread with cancelled node never again blocks. + * CONDITION: This node is currently on a condition queue. + * It will not be used as a sync queue node until + * transferred. (Use of this value here + * has nothing to do with the other uses + * of the field, but simplifies mechanics.) + * 0: None of the above + * + * The values are arranged numerically to simplify use. + * Non-negative values mean that a node doesn't need to + * signal. So, most code doesn't need to check for particular + * values, just for sign. + * + * The field is initialized to 0 for normal sync nodes, and + * CONDITION for condition nodes. It is modified only using + * CAS. + */ + volatile int waitStatus; + + /** + * Link to predecessor node that current node/thread relies on + * for checking waitStatus. Assigned during enqueing, and nulled + * out (for sake of GC) only upon dequeuing. Also, upon + * cancellation of a predecessor, we short-circuit while + * finding a non-cancelled one, which will always exist + * because the head node is never cancelled: A node becomes + * head only as a result of successful acquire. A + * cancelled thread never succeeds in acquiring, and a thread only + * cancels itself, not any other node. + */ + volatile Node prev; + + /** + * Link to the successor node that the current node/thread + * unparks upon release. Assigned once during enqueuing, and + * nulled out (for sake of GC) when no longer needed. Upon + * cancellation, we cannot adjust this field, but can notice + * status and bypass the node if cancelled. The enq operation + * does not assign next field of a predecessor until after + * attachment, so seeing a null next field does not + * necessarily mean that node is at end of queue. However, if + * a next field appears to be null, we can scan prev's from + * the tail to double-check. + */ + volatile Node next; + + /** + * The thread that enqueued this node. Initialized on + * construction and nulled out after use. + */ + volatile Thread thread; + + /** + * Link to next node waiting on condition, or the special + * value SHARED. Because condition queues are accessed only + * when holding in exclusive mode, we just need a simple + * linked queue to hold nodes while they are waiting on + * conditions. They are then transferred to the queue to + * re-acquire. And because conditions can only be exclusive, + * we save a field by using special value to indicate shared + * mode. + */ + Node nextWaiter; + + /** + * Returns true if node is waiting in shared mode + */ + final boolean isShared() { + return nextWaiter == SHARED; + } + + /** + * Returns previous node, or throws NullPointerException if + * null. Use when predecessor cannot be null. + * @return the predecessor of this node + */ + final Node predecessor() throws NullPointerException { + Node p = prev; + if (p == null) + throw new NullPointerException(); + else + return p; + } + + Node() { // Used to establish initial head or SHARED marker + } + + Node(Thread thread, Node mode) { // Used by addWaiter + this.nextWaiter = mode; + this.thread = thread; + } + + Node(Thread thread, int waitStatus) { // Used by Condition + this.waitStatus = waitStatus; + this.thread = thread; + } + } + + /** + * Head of the wait queue, lazily initialized. Except for + * initialization, it is modified only via method setHead. Note: + * If head exists, its waitStatus is guaranteed not to be + * CANCELLED. + */ + private transient volatile Node head; + + /** + * Tail of the wait queue, lazily initialized. Modified only via + * method enq to add new wait node. + */ + private transient volatile Node tail; + + /** + * The synchronization state. + */ + private volatile int state; + + /** + * Returns the current value of synchronization state. + * This operation has memory semantics of a <tt>volatile</tt> read. + * @return current state value + */ + protected final int getState() { + return state; + } + + /** + * Sets the value of synchronization state. + * This operation has memory semantics of a <tt>volatile</tt> write. + * @param newState the new state value + */ + protected final void setState(int newState) { + state = newState; + } + + /** + * Atomically sets synchronization state to the given updated + * value if the current state value equals the expected value. + * This operation has memory semantics of a <tt>volatile</tt> read + * and write. + * + * @param expect the expected value + * @param update the new value + * @return true if successful. False return indicates that the actual + * value was not equal to the expected value. + */ + protected final boolean compareAndSetState(int expect, int update) { + // See below for intrinsics setup to support this + return unsafe.compareAndSwapInt(this, stateOffset, expect, update); + } + + // Queuing utilities + + /** + * The number of nanoseconds for which it is faster to spin + * rather than to use timed park. A rough estimate suffices + * to improve responsiveness with very short timeouts. + */ + static final long spinForTimeoutThreshold = 1000L; + + /** + * Inserts node into queue, initializing if necessary. See picture above. + * @param node the node to insert + * @return node's predecessor + */ + private Node enq(final Node node) { + for (;;) { + Node t = tail; + if (t == null) { // Must initialize + Node h = new Node(); // Dummy header + h.next = node; + node.prev = h; + if (compareAndSetHead(h)) { + tail = node; + return h; + } + } + else { + node.prev = t; + if (compareAndSetTail(t, node)) { + t.next = node; + return t; + } + } + } + } + + /** + * Creates and enqueues node for given thread and mode. + * + * @param current the thread + * @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared + * @return the new node + */ + private Node addWaiter(Node mode) { + Node node = new Node(Thread.currentThread(), mode); + // Try the fast path of enq; backup to full enq on failure + Node pred = tail; + if (pred != null) { + node.prev = pred; + if (compareAndSetTail(pred, node)) { + pred.next = node; + return node; + } + } + enq(node); + return node; + } + + /** + * Sets head of queue to be node, thus dequeuing. Called only by + * acquire methods. Also nulls out unused fields for sake of GC + * and to suppress unnecessary signals and traversals. + * + * @param node the node + */ + private void setHead(Node node) { + head = node; + node.thread = null; + node.prev = null; + } + + /** + * Wakes up node's successor, if one exists. + * + * @param node the node + */ + private void unparkSuccessor(Node node) { + /* + * Try to clear status in anticipation of signalling. It is + * OK if this fails or if status is changed by waiting thread. + */ + compareAndSetWaitStatus(node, Node.SIGNAL, 0); + + /* + * Thread to unpark is held in successor, which is normally + * just the next node. But if cancelled or apparently null, + * traverse backwards from tail to find the actual + * non-cancelled successor. + */ + Node s = node.next; + if (s == null || s.waitStatus > 0) { + s = null; + for (Node t = tail; t != null && t != node; t = t.prev) + if (t.waitStatus <= 0) + s = t; + } + if (s != null) + LockSupport.unpark(s.thread); + } + + /** + * Sets head of queue, and checks if successor may be waiting + * in shared mode, if so propagating if propagate > 0. + * + * @param pred the node holding waitStatus for node + * @param node the node + * @param propagate the return value from a tryAcquireShared + */ + private void setHeadAndPropagate(Node node, int propagate) { + setHead(node); + if (propagate > 0 && node.waitStatus != 0) { + /* + * Don't bother fully figuring out successor. If it + * looks null, call unparkSuccessor anyway to be safe. + */ + Node s = node.next; + if (s == null || s.isShared()) + unparkSuccessor(node); + } + } + + // Utilities for various versions of acquire + + /** + * Cancels an ongoing attempt to acquire. + * + * @param node the node + */ + private void cancelAcquire(Node node) { + if (node != null) { // Ignore if node doesn't exist + node.thread = null; + // Can use unconditional write instead of CAS here + node.waitStatus = Node.CANCELLED; + unparkSuccessor(node); + } + } + + /** + * Checks and updates status for a node that failed to acquire. + * Returns true if thread should block. This is the main signal + * control in all acquire loops. Requires that pred == node.prev + * + * @param pred node's predecessor holding status + * @param node the node + * @return {@code true} if thread should block + */ + private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) { + int s = pred.waitStatus; + if (s < 0) + /* + * This node has already set status asking a release + * to signal it, so it can safely park + */ + return true; + if (s > 0) + /* + * Predecessor was cancelled. Move up to its predecessor + * and indicate retry. + */ + node.prev = pred.prev; + else + /* + * Indicate that we need a signal, but don't park yet. Caller + * will need to retry to make sure it cannot acquire before + * parking. + */ + compareAndSetWaitStatus(pred, 0, Node.SIGNAL); + return false; + } + + /** + * Convenience method to interrupt current thread. + */ + private static void selfInterrupt() { + Thread.currentThread().interrupt(); + } + + /** + * Convenience method to park and then check if interrupted + * + * @return {@code true} if interrupted + */ + private final boolean parkAndCheckInterrupt() { + LockSupport.park(this); + return Thread.interrupted(); + } + + /* + * Various flavors of acquire, varying in exclusive/shared and + * control modes. Each is mostly the same, but annoyingly + * different. Only a little bit of factoring is possible due to + * interactions of exception mechanics (including ensuring that we + * cancel if tryAcquire throws exception) and other control, at + * least not without hurting performance too much. + */ + + /** + * Acquires in exclusive uninterruptible mode for thread already in + * queue. Used by condition wait methods as well as acquire. + * + * @param node the node + * @param arg the acquire argument + * @return {@code true} if interrupted while waiting + */ + final boolean acquireQueued(final Node node, int arg) { + try { + boolean interrupted = false; + for (;;) { + final Node p = node.predecessor(); + if (p == head && tryAcquire(arg)) { + setHead(node); + p.next = null; // help GC + return interrupted; + } + if (shouldParkAfterFailedAcquire(p, node) && + parkAndCheckInterrupt()) + interrupted = true; + } + } catch (RuntimeException ex) { + cancelAcquire(node); + throw ex; + } + } + + /** + * Acquires in exclusive interruptible mode. + * @param arg the acquire argument + */ + private void doAcquireInterruptibly(int arg) + throws InterruptedException { + final Node node = addWaiter(Node.EXCLUSIVE); + try { + for (;;) { + final Node p = node.predecessor(); + if (p == head && tryAcquire(arg)) { + setHead(node); + p.next = null; // help GC + return; + } + if (shouldParkAfterFailedAcquire(p, node) && + parkAndCheckInterrupt()) + break; + } + } catch (RuntimeException ex) { + cancelAcquire(node); + throw ex; + } + // Arrive here only if interrupted + cancelAcquire(node); + throw new InterruptedException(); + } + + /** + * Acquires in exclusive timed mode. + * + * @param arg the acquire argument + * @param nanosTimeout max wait time + * @return {@code true} if acquired + */ + private boolean doAcquireNanos(int arg, long nanosTimeout) + throws InterruptedException { + long lastTime = System.nanoTime(); + final Node node = addWaiter(Node.EXCLUSIVE); + try { + for (;;) { + final Node p = node.predecessor(); + if (p == head && tryAcquire(arg)) { + setHead(node); + p.next = null; // help GC + return true; + } + if (nanosTimeout <= 0) { + cancelAcquire(node); + return false; + } + if (nanosTimeout > spinForTimeoutThreshold && + shouldParkAfterFailedAcquire(p, node)) + LockSupport.parkNanos(this, nanosTimeout); + long now = System.nanoTime(); + nanosTimeout -= now - lastTime; + lastTime = now; + if (Thread.interrupted()) + break; + } + } catch (RuntimeException ex) { + cancelAcquire(node); + throw ex; + } + // Arrive here only if interrupted + cancelAcquire(node); + throw new InterruptedException(); + } + + /** + * Acquires in shared uninterruptible mode. + * @param arg the acquire argument + */ + private void doAcquireShared(int arg) { + final Node node = addWaiter(Node.SHARED); + try { + boolean interrupted = false; + for (;;) { + final Node p = node.predecessor(); + if (p == head) { + int r = tryAcquireShared(arg); + if (r >= 0) { + setHeadAndPropagate(node, r); + p.next = null; // help GC + if (interrupted) + selfInterrupt(); + return; + } + } + if (shouldParkAfterFailedAcquire(p, node) && + parkAndCheckInterrupt()) + interrupted = true; + } + } catch (RuntimeException ex) { + cancelAcquire(node); + throw ex; + } + } + + /** + * Acquires in shared interruptible mode. + * @param arg the acquire argument + */ + private void doAcquireSharedInterruptibly(int arg) + throws InterruptedException { + final Node node = addWaiter(Node.SHARED); + try { + for (;;) { + final Node p = node.predecessor(); + if (p == head) { + int r = tryAcquireShared(arg); + if (r >= 0) { + setHeadAndPropagate(node, r); + p.next = null; // help GC + return; + } + } + if (shouldParkAfterFailedAcquire(p, node) && + parkAndCheckInterrupt()) + break; + } + } catch (RuntimeException ex) { + cancelAcquire(node); + throw ex; + } + // Arrive here only if interrupted + cancelAcquire(node); + throw new InterruptedException(); + } + + /** + * Acquires in shared timed mode. + * + * @param arg the acquire argument + * @param nanosTimeout max wait time + * @return {@code true} if acquired + */ + private boolean doAcquireSharedNanos(int arg, long nanosTimeout) + throws InterruptedException { + + long lastTime = System.nanoTime(); + final Node node = addWaiter(Node.SHARED); + try { + for (;;) { + final Node p = node.predecessor(); + if (p == head) { + int r = tryAcquireShared(arg); + if (r >= 0) { + setHeadAndPropagate(node, r); + p.next = null; // help GC + return true; + } + } + if (nanosTimeout <= 0) { + cancelAcquire(node); + return false; + } + if (nanosTimeout > spinForTimeoutThreshold && + shouldParkAfterFailedAcquire(p, node)) + LockSupport.parkNanos(this, nanosTimeout); + long now = System.nanoTime(); + nanosTimeout -= now - lastTime; + lastTime = now; + if (Thread.interrupted()) + break; + } + } catch (RuntimeException ex) { + cancelAcquire(node); + throw ex; + } + // Arrive here only if interrupted + cancelAcquire(node); + throw new InterruptedException(); + } + + // Main exported methods + + /** + * Attempts to acquire in exclusive mode. This method should query + * if the state of the object permits it to be acquired in the + * exclusive mode, and if so to acquire it. + * + * <p>This method is always invoked by the thread performing + * acquire. If this method reports failure, the acquire method + * may queue the thread, if it is not already queued, until it is + * signalled by a release from some other thread. This can be used + * to implement method {@link Lock#tryLock()}. + * + * <p>The default + * implementation throws {@link UnsupportedOperationException}. + * + * @param arg the acquire argument. This value is always the one + * passed to an acquire method, or is the value saved on entry + * to a condition wait. The value is otherwise uninterpreted + * and can represent anything you like. + * @return {@code true} if successful. Upon success, this object has + * been acquired. + * @throws IllegalMonitorStateException if acquiring would place this + * synchronizer in an illegal state. This exception must be + * thrown in a consistent fashion for synchronization to work + * correctly. + * @throws UnsupportedOperationException if exclusive mode is not supported + */ + protected boolean tryAcquire(int arg) { + throw new UnsupportedOperationException(); + } + + /** + * Attempts to set the state to reflect a release in exclusive + * mode. + * + * <p>This method is always invoked by the thread performing release. + * + * <p>The default implementation throws + * {@link UnsupportedOperationException}. + * + * @param arg the release argument. This value is always the one + * passed to a release method, or the current state value upon + * entry to a condition wait. The value is otherwise + * uninterpreted and can represent anything you like. + * @return {@code true} if this object is now in a fully released + * state, so that any waiting threads may attempt to acquire; + * and {@code false} otherwise. + * @throws IllegalMonitorStateException if releasing would place this + * synchronizer in an illegal state. This exception must be + * thrown in a consistent fashion for synchronization to work + * correctly. + * @throws UnsupportedOperationException if exclusive mode is not supported + */ + protected boolean tryRelease(int arg) { + throw new UnsupportedOperationException(); + } + + /** + * Attempts to acquire in shared mode. This method should query if + * the state of the object permits it to be acquired in the shared + * mode, and if so to acquire it. + * + * <p>This method is always invoked by the thread performing + * acquire. If this method reports failure, the acquire method + * may queue the thread, if it is not already queued, until it is + * signalled by a release from some other thread. + * + * <p>The default implementation throws {@link + * UnsupportedOperationException}. + * + * @param arg the acquire argument. This value is always the one + * passed to an acquire method, or is the value saved on entry + * to a condition wait. The value is otherwise uninterpreted + * and can represent anything you like. + * @return a negative value on failure; zero if acquisition in shared + * mode succeeded but no subsequent shared-mode acquire can + * succeed; and a positive value if acquisition in shared + * mode succeeded and subsequent shared-mode acquires might + * also succeed, in which case a subsequent waiting thread + * must check availability. (Support for three different + * return values enables this method to be used in contexts + * where acquires only sometimes act exclusively.) Upon + * success, this object has been acquired. + * @throws IllegalMonitorStateException if acquiring would place this + * synchronizer in an illegal state. This exception must be + * thrown in a consistent fashion for synchronization to work + * correctly. + * @throws UnsupportedOperationException if shared mode is not supported + */ + protected int tryAcquireShared(int arg) { + throw new UnsupportedOperationException(); + } + + /** + * Attempts to set the state to reflect a release in shared mode. + * + * <p>This method is always invoked by the thread performing release. + * + * <p>The default implementation throws + * {@link UnsupportedOperationException}. + * + * @param arg the release argument. This value is always the one + * passed to a release method, or the current state value upon + * entry to a condition wait. The value is otherwise + * uninterpreted and can represent anything you like. + * @return {@code true} if this release of shared mode may permit a + * waiting acquire (shared or exclusive) to succeed; and + * {@code false} otherwise + * @throws IllegalMonitorStateException if releasing would place this + * synchronizer in an illegal state. This exception must be + * thrown in a consistent fashion for synchronization to work + * correctly. + * @throws UnsupportedOperationException if shared mode is not supported + */ + protected boolean tryReleaseShared(int arg) { + throw new UnsupportedOperationException(); + } + + /** + * Returns {@code true} if synchronization is held exclusively with + * respect to the current (calling) thread. This method is invoked + * upon each call to a non-waiting {@link ConditionObject} method. + * (Waiting methods instead invoke {@link #release}.) + * + * <p>The default implementation throws {@link + * UnsupportedOperationException}. This method is invoked + * internally only within {@link ConditionObject} methods, so need + * not be defined if conditions are not used. + * + * @return {@code true} if synchronization is held exclusively; + * {@code false} otherwise + * @throws UnsupportedOperationException if conditions are not supported + */ + protected boolean isHeldExclusively() { + throw new UnsupportedOperationException(); + } + + /** + * Acquires in exclusive mode, ignoring interrupts. Implemented + * by invoking at least once {@link #tryAcquire}, + * returning on success. Otherwise the thread is queued, possibly + * repeatedly blocking and unblocking, invoking {@link + * #tryAcquire} until success. This method can be used + * to implement method {@link Lock#lock}. + * + * @param arg the acquire argument. This value is conveyed to + * {@link #tryAcquire} but is otherwise uninterpreted and + * can represent anything you like. + */ + public final void acquire(int arg) { + if (!tryAcquire(arg) && + acquireQueued(addWaiter(Node.EXCLUSIVE), arg)) + selfInterrupt(); + } + + /** + * Acquires in exclusive mode, aborting if interrupted. + * Implemented by first checking interrupt status, then invoking + * at least once {@link #tryAcquire}, returning on + * success. Otherwise the thread is queued, possibly repeatedly + * blocking and unblocking, invoking {@link #tryAcquire} + * until success or the thread is interrupted. This method can be + * used to implement method {@link Lock#lockInterruptibly}. + * + * @param arg the acquire argument. This value is conveyed to + * {@link #tryAcquire} but is otherwise uninterpreted and + * can represent anything you like. + * @throws InterruptedException if the current thread is interrupted + */ + public final void acquireInterruptibly(int arg) throws InterruptedException { + if (Thread.interrupted()) + throw new InterruptedException(); + if (!tryAcquire(arg)) + doAcquireInterruptibly(arg); + } + + /** + * Attempts to acquire in exclusive mode, aborting if interrupted, + * and failing if the given timeout elapses. Implemented by first + * checking interrupt status, then invoking at least once {@link + * #tryAcquire}, returning on success. Otherwise, the thread is + * queued, possibly repeatedly blocking and unblocking, invoking + * {@link #tryAcquire} until success or the thread is interrupted + * or the timeout elapses. This method can be used to implement + * method {@link Lock#tryLock(long, TimeUnit)}. + * + * @param arg the acquire argument. This value is conveyed to + * {@link #tryAcquire} but is otherwise uninterpreted and + * can represent anything you like. + * @param nanosTimeout the maximum number of nanoseconds to wait + * @return {@code true} if acquired; {@code false} if timed out + * @throws InterruptedException if the current thread is interrupted + */ + public final boolean tryAcquireNanos(int arg, long nanosTimeout) throws InterruptedException { + if (Thread.interrupted()) + throw new InterruptedException(); + return tryAcquire(arg) || + doAcquireNanos(arg, nanosTimeout); + } + + /** + * Releases in exclusive mode. Implemented by unblocking one or + * more threads if {@link #tryRelease} returns true. + * This method can be used to implement method {@link Lock#unlock}. + * + * @param arg the release argument. This value is conveyed to + * {@link #tryRelease} but is otherwise uninterpreted and + * can represent anything you like. + * @return the value returned from {@link #tryRelease} + */ + public final boolean release(int arg) { + if (tryRelease(arg)) { + Node h = head; + if (h != null && h.waitStatus != 0) + unparkSuccessor(h); + return true; + } + return false; + } + + /** + * Acquires in shared mode, ignoring interrupts. Implemented by + * first invoking at least once {@link #tryAcquireShared}, + * returning on success. Otherwise the thread is queued, possibly + * repeatedly blocking and unblocking, invoking {@link + * #tryAcquireShared} until success. + * + * @param arg the acquire argument. This value is conveyed to + * {@link #tryAcquireShared} but is otherwise uninterpreted + * and can represent anything you like. + */ + public final void acquireShared(int arg) { + if (tryAcquireShared(arg) < 0) + doAcquireShared(arg); + } + + /** + * Acquires in shared mode, aborting if interrupted. Implemented + * by first checking interrupt status, then invoking at least once + * {@link #tryAcquireShared}, returning on success. Otherwise the + * thread is queued, possibly repeatedly blocking and unblocking, + * invoking {@link #tryAcquireShared} until success or the thread + * is interrupted. + * @param arg the acquire argument. + * This value is conveyed to {@link #tryAcquireShared} but is + * otherwise uninterpreted and can represent anything + * you like. + * @throws InterruptedException if the current thread is interrupted + */ + public final void acquireSharedInterruptibly(int arg) throws InterruptedException { + if (Thread.interrupted()) + throw new InterruptedException(); + if (tryAcquireShared(arg) < 0) + doAcquireSharedInterruptibly(arg); + } + + /** + * Attempts to acquire in shared mode, aborting if interrupted, and + * failing if the given timeout elapses. Implemented by first + * checking interrupt status, then invoking at least once {@link + * #tryAcquireShared}, returning on success. Otherwise, the + * thread is queued, possibly repeatedly blocking and unblocking, + * invoking {@link #tryAcquireShared} until success or the thread + * is interrupted or the timeout elapses. + * + * @param arg the acquire argument. This value is conveyed to + * {@link #tryAcquireShared} but is otherwise uninterpreted + * and can represent anything you like. + * @param nanosTimeout the maximum number of nanoseconds to wait + * @return {@code true} if acquired; {@code false} if timed out + * @throws InterruptedException if the current thread is interrupted + */ + public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout) throws InterruptedException { + if (Thread.interrupted()) + throw new InterruptedException(); + return tryAcquireShared(arg) >= 0 || + doAcquireSharedNanos(arg, nanosTimeout); + } + + /** + * Releases in shared mode. Implemented by unblocking one or more + * threads if {@link #tryReleaseShared} returns true. + * + * @param arg the release argument. This value is conveyed to + * {@link #tryReleaseShared} but is otherwise uninterpreted + * and can represent anything you like. + * @return the value returned from {@link #tryReleaseShared} + */ + public final boolean releaseShared(int arg) { + if (tryReleaseShared(arg)) { + Node h = head; + if (h != null && h.waitStatus != 0) + unparkSuccessor(h); + return true; + } + return false; + } + + // Queue inspection methods + + /** + * Queries whether any threads are waiting to acquire. Note that + * because cancellations due to interrupts and timeouts may occur + * at any time, a {@code true} return does not guarantee that any + * other thread will ever acquire. + * + * <p>In this implementation, this operation returns in + * constant time. + * + * @return {@code true} if there may be other threads waiting to acquire + */ + public final boolean hasQueuedThreads() { + return head != tail; + } + + /** + * Queries whether any threads have ever contended to acquire this + * synchronizer; that is if an acquire method has ever blocked. + * + * <p>In this implementation, this operation returns in + * constant time. + * + * @return {@code true} if there has ever been contention + */ + public final boolean hasContended() { + return head != null; + } + + /** + * Returns the first (longest-waiting) thread in the queue, or + * {@code null} if no threads are currently queued. + * + * <p>In this implementation, this operation normally returns in + * constant time, but may iterate upon contention if other threads are + * concurrently modifying the queue. + * + * @return the first (longest-waiting) thread in the queue, or + * {@code null} if no threads are currently queued + */ + public final Thread getFirstQueuedThread() { + // handle only fast path, else relay + return (head == tail)? null : fullGetFirstQueuedThread(); + } + + /** + * Version of getFirstQueuedThread called when fastpath fails + */ + private Thread fullGetFirstQueuedThread() { + /* + * The first node is normally h.next. Try to get its + * thread field, ensuring consistent reads: If thread + * field is nulled out or s.prev is no longer head, then + * some other thread(s) concurrently performed setHead in + * between some of our reads. We try this twice before + * resorting to traversal. + */ + Node h, s; + Thread st; + if (((h = head) != null && (s = h.next) != null && + s.prev == head && (st = s.thread) != null) || + ((h = head) != null && (s = h.next) != null && + s.prev == head && (st = s.thread) != null)) + return st; + + /* + * Head's next field might not have been set yet, or may have + * been unset after setHead. So we must check to see if tail + * is actually first node. If not, we continue on, safely + * traversing from tail back to head to find first, + * guaranteeing termination. + */ + + Node t = tail; + Thread firstThread = null; + while (t != null && t != head) { + Thread tt = t.thread; + if (tt != null) + firstThread = tt; + t = t.prev; + } + return firstThread; + } + + /** + * Returns true if the given thread is currently queued. + * + * <p>This implementation traverses the queue to determine + * presence of the given thread. + * + * @param thread the thread + * @return {@code true} if the given thread is on the queue + * @throws NullPointerException if the thread is null + */ + public final boolean isQueued(Thread thread) { + if (thread == null) + throw new NullPointerException(); + for (Node p = tail; p != null; p = p.prev) + if (p.thread == thread) + return true; + return false; + } + + /** + * Return {@code true} if the apparent first queued thread, if one + * exists, is not waiting in exclusive mode. Used only as a heuristic + * in ReentrantReadWriteLock. + */ + final boolean apparentlyFirstQueuedIsExclusive() { + Node h, s; + return ((h = head) != null && (s = h.next) != null && + s.nextWaiter != Node.SHARED); + } + + /** + * Return {@code true} if the queue is empty or if the given thread + * is at the head of the queue. This is reliable only if + * <tt>current</tt> is actually Thread.currentThread() of caller. + */ + final boolean isFirst(Thread current) { + Node h, s; + return ((h = head) == null || + ((s = h.next) != null && s.thread == current) || + fullIsFirst(current)); + } + + final boolean fullIsFirst(Thread current) { + // same idea as fullGetFirstQueuedThread + Node h, s; + Thread firstThread = null; + if (((h = head) != null && (s = h.next) != null && + s.prev == head && (firstThread = s.thread) != null)) + return firstThread == current; + Node t = tail; + while (t != null && t != head) { + Thread tt = t.thread; + if (tt != null) + firstThread = tt; + t = t.prev; + } + return firstThread == current || firstThread == null; + } + + + // Instrumentation and monitoring methods + + /** + * Returns an estimate of the number of threads waiting to + * acquire. The value is only an estimate because the number of + * threads may change dynamically while this method traverses + * internal data structures. This method is designed for use in + * monitoring system state, not for synchronization + * control. + * + * @return the estimated number of threads waiting to acquire + */ + public final int getQueueLength() { + int n = 0; + for (Node p = tail; p != null; p = p.prev) { + if (p.thread != null) + ++n; + } + return n; + } + + /** + * Returns a collection containing threads that may be waiting to + * acquire. Because the actual set of threads may change + * dynamically while constructing this result, the returned + * collection is only a best-effort estimate. The elements of the + * returned collection are in no particular order. This method is + * designed to facilitate construction of subclasses that provide + * more extensive monitoring facilities. + * + * @return the collection of threads + */ + public final Collection<Thread> getQueuedThreads() { + ArrayList<Thread> list = new ArrayList<Thread>(); + for (Node p = tail; p != null; p = p.prev) { + Thread t = p.thread; + if (t != null) + list.add(t); + } + return list; + } + + /** + * Returns a collection containing threads that may be waiting to + * acquire in exclusive mode. This has the same properties + * as {@link #getQueuedThreads} except that it only returns + * those threads waiting due to an exclusive acquire. + * + * @return the collection of threads + */ + public final Collection<Thread> getExclusiveQueuedThreads() { + ArrayList<Thread> list = new ArrayList<Thread>(); + for (Node p = tail; p != null; p = p.prev) { + if (!p.isShared()) { + Thread t = p.thread; + if (t != null) + list.add(t); + } + } + return list; + } + + /** + * Returns a collection containing threads that may be waiting to + * acquire in shared mode. This has the same properties + * as {@link #getQueuedThreads} except that it only returns + * those threads waiting due to a shared acquire. + * + * @return the collection of threads + */ + public final Collection<Thread> getSharedQueuedThreads() { + ArrayList<Thread> list = new ArrayList<Thread>(); + for (Node p = tail; p != null; p = p.prev) { + if (p.isShared()) { + Thread t = p.thread; + if (t != null) + list.add(t); + } + } + return list; + } + + /** + * Returns a string identifying this synchronizer, as well as its state. + * The state, in brackets, includes the String {@code "State ="} + * followed by the current value of {@link #getState}, and either + * {@code "nonempty"} or {@code "empty"} depending on whether the + * queue is empty. + * + * @return a string identifying this synchronizer, as well as its state + */ + public String toString() { + int s = getState(); + String q = hasQueuedThreads()? "non" : ""; + return super.toString() + + "[State = " + s + ", " + q + "empty queue]"; + } + + + // Internal support methods for Conditions + + /** + * Returns true if a node, always one that was initially placed on + * a condition queue, is now waiting to reacquire on sync queue. + * @param node the node + * @return true if is reacquiring + */ + final boolean isOnSyncQueue(Node node) { + if (node.waitStatus == Node.CONDITION || node.prev == null) + return false; + if (node.next != null) // If has successor, it must be on queue + return true; + /* + * node.prev can be non-null, but not yet on queue because + * the CAS to place it on queue can fail. So we have to + * traverse from tail to make sure it actually made it. It + * will always be near the tail in calls to this method, and + * unless the CAS failed (which is unlikely), it will be + * there, so we hardly ever traverse much. + */ + return findNodeFromTail(node); + } + + /** + * Returns true if node is on sync queue by searching backwards from tail. + * Called only when needed by isOnSyncQueue. + * @return true if present + */ + private boolean findNodeFromTail(Node node) { + Node t = tail; + for (;;) { + if (t == node) + return true; + if (t == null) + return false; + t = t.prev; + } + } + + /** + * Transfers a node from a condition queue onto sync queue. + * Returns true if successful. + * @param node the node + * @return true if successfully transferred (else the node was + * cancelled before signal). + */ + final boolean transferForSignal(Node node) { + /* + * If cannot change waitStatus, the node has been cancelled. + */ + if (!compareAndSetWaitStatus(node, Node.CONDITION, 0)) + return false; + + /* + * Splice onto queue and try to set waitStatus of predecessor to + * indicate that thread is (probably) waiting. If cancelled or + * attempt to set waitStatus fails, wake up to resync (in which + * case the waitStatus can be transiently and harmlessly wrong). + */ + Node p = enq(node); + int c = p.waitStatus; + if (c > 0 || !compareAndSetWaitStatus(p, c, Node.SIGNAL)) + LockSupport.unpark(node.thread); + return true; + } + + /** + * Transfers node, if necessary, to sync queue after a cancelled + * wait. Returns true if thread was cancelled before being + * signalled. + * @param current the waiting thread + * @param node its node + * @return true if cancelled before the node was signalled. + */ + final boolean transferAfterCancelledWait(Node node) { + if (compareAndSetWaitStatus(node, Node.CONDITION, 0)) { + enq(node); + return true; + } + /* + * If we lost out to a signal(), then we can't proceed + * until it finishes its enq(). Cancelling during an + * incomplete transfer is both rare and transient, so just + * spin. + */ + while (!isOnSyncQueue(node)) + Thread.yield(); + return false; + } + + /** + * Invokes release with current state value; returns saved state. + * Cancels node and throws exception on failure. + * @param node the condition node for this wait + * @return previous sync state + */ + final int fullyRelease(Node node) { + try { + int savedState = getState(); + if (release(savedState)) + return savedState; + } catch (RuntimeException ex) { + node.waitStatus = Node.CANCELLED; + throw ex; + } + // reach here if release fails + node.waitStatus = Node.CANCELLED; + throw new IllegalMonitorStateException(); + } + + // Instrumentation methods for conditions + + /** + * Queries whether the given ConditionObject + * uses this synchronizer as its lock. + * + * @param condition the condition + * @return <tt>true</tt> if owned + * @throws NullPointerException if the condition is null + */ + public final boolean owns(ConditionObject condition) { + if (condition == null) + throw new NullPointerException(); + return condition.isOwnedBy(this); + } + + /** + * Queries whether any threads are waiting on the given condition + * associated with this synchronizer. Note that because timeouts + * and interrupts may occur at any time, a <tt>true</tt> return + * does not guarantee that a future <tt>signal</tt> will awaken + * any threads. This method is designed primarily for use in + * monitoring of the system state. + * + * @param condition the condition + * @return <tt>true</tt> if there are any waiting threads + * @throws IllegalMonitorStateException if exclusive synchronization + * is not held + * @throws IllegalArgumentException if the given condition is + * not associated with this synchronizer + * @throws NullPointerException if the condition is null + */ + public final boolean hasWaiters(ConditionObject condition) { + if (!owns(condition)) + throw new IllegalArgumentException("Not owner"); + return condition.hasWaiters(); + } + + /** + * Returns an estimate of the number of threads waiting on the + * given condition associated with this synchronizer. Note that + * because timeouts and interrupts may occur at any time, the + * estimate serves only as an upper bound on the actual number of + * waiters. This method is designed for use in monitoring of the + * system state, not for synchronization control. + * + * @param condition the condition + * @return the estimated number of waiting threads + * @throws IllegalMonitorStateException if exclusive synchronization + * is not held + * @throws IllegalArgumentException if the given condition is + * not associated with this synchronizer + * @throws NullPointerException if the condition is null + */ + public final int getWaitQueueLength(ConditionObject condition) { + if (!owns(condition)) + throw new IllegalArgumentException("Not owner"); + return condition.getWaitQueueLength(); + } + + /** + * Returns a collection containing those threads that may be + * waiting on the given condition associated with this + * synchronizer. Because the actual set of threads may change + * dynamically while constructing this result, the returned + * collection is only a best-effort estimate. The elements of the + * returned collection are in no particular order. + * + * @param condition the condition + * @return the collection of threads + * @throws IllegalMonitorStateException if exclusive synchronization + * is not held + * @throws IllegalArgumentException if the given condition is + * not associated with this synchronizer + * @throws NullPointerException if the condition is null + */ + public final Collection<Thread> getWaitingThreads(ConditionObject condition) { + if (!owns(condition)) + throw new IllegalArgumentException("Not owner"); + return condition.getWaitingThreads(); + } + + /** + * Condition implementation for a {@link + * AbstractQueuedSynchronizer} serving as the basis of a {@link + * Lock} implementation. + * + * <p>Method documentation for this class describes mechanics, + * not behavioral specifications from the point of view of Lock + * and Condition users. Exported versions of this class will in + * general need to be accompanied by documentation describing + * condition semantics that rely on those of the associated + * <tt>AbstractQueuedSynchronizer</tt>. + * + * <p>This class is Serializable, but all fields are transient, + * so deserialized conditions have no waiters. + */ + public class ConditionObject implements Condition, java.io.Serializable { + private static final long serialVersionUID = 1173984872572414699L; + /** First node of condition queue. */ + private transient Node firstWaiter; + /** Last node of condition queue. */ + private transient Node lastWaiter; + + /** + * Creates a new <tt>ConditionObject</tt> instance. + */ + public ConditionObject() { } + + // Internal methods + + /** + * Adds a new waiter to wait queue. + * @return its new wait node + */ + private Node addConditionWaiter() { + Node node = new Node(Thread.currentThread(), Node.CONDITION); + Node t = lastWaiter; + if (t == null) + firstWaiter = node; + else + t.nextWaiter = node; + lastWaiter = node; + return node; + } + + /** + * Removes and transfers nodes until hit non-cancelled one or + * null. Split out from signal in part to encourage compilers + * to inline the case of no waiters. + * @param first (non-null) the first node on condition queue + */ + private void doSignal(Node first) { + do { + if ( (firstWaiter = first.nextWaiter) == null) + lastWaiter = null; + first.nextWaiter = null; + } while (!transferForSignal(first) && + (first = firstWaiter) != null); + } + + /** + * Removes and transfers all nodes. + * @param first (non-null) the first node on condition queue + */ + private void doSignalAll(Node first) { + lastWaiter = firstWaiter = null; + do { + Node next = first.nextWaiter; + first.nextWaiter = null; + transferForSignal(first); + first = next; + } while (first != null); + } + + /** + * Returns true if given node is on this condition queue. + * Call only when holding lock. + */ + private boolean isOnConditionQueue(Node node) { + return node.next != null || node == lastWaiter; + } + + /** + * Unlinks a cancelled waiter node from condition queue. This + * is called when cancellation occurred during condition wait, + * not lock wait, and is called only after lock has been + * re-acquired by a cancelled waiter and the node is not known + * to already have been dequeued. It is needed to avoid + * garbage retention in the absence of signals. So even though + * it may require a full traversal, it comes into play only + * when timeouts or cancellations occur in the absence of + * signals. + */ + private void unlinkCancelledWaiter(Node node) { + Node t = firstWaiter; + Node trail = null; + while (t != null) { + if (t == node) { + Node next = t.nextWaiter; + if (trail == null) + firstWaiter = next; + else + trail.nextWaiter = next; + if (lastWaiter == node) + lastWaiter = trail; + break; + } + trail = t; + t = t.nextWaiter; + } + } + + // public methods + + /** + * Moves the longest-waiting thread, if one exists, from the + * wait queue for this condition to the wait queue for the + * owning lock. + * + * @throws IllegalMonitorStateException if {@link #isHeldExclusively} + * returns {@code false} + */ + public final void signal() { + if (!isHeldExclusively()) + throw new IllegalMonitorStateException(); + Node first = firstWaiter; + if (first != null) + doSignal(first); + } + + /** + * Moves all threads from the wait queue for this condition to + * the wait queue for the owning lock. + * + * @throws IllegalMonitorStateException if {@link #isHeldExclusively} + * returns {@code false} + */ + public final void signalAll() { + if (!isHeldExclusively()) + throw new IllegalMonitorStateException(); + Node first = firstWaiter; + if (first != null) + doSignalAll(first); + } + + /** + * Implements uninterruptible condition wait. + * <ol> + * <li> Save lock state returned by {@link #getState} + * <li> Invoke {@link #release} with + * saved state as argument, throwing + * IllegalMonitorStateException if it fails. + * <li> Block until signalled + * <li> Reacquire by invoking specialized version of + * {@link #acquire} with saved state as argument. + * </ol> + */ + public final void awaitUninterruptibly() { + Node node = addConditionWaiter(); + int savedState = fullyRelease(node); + boolean interrupted = false; + while (!isOnSyncQueue(node)) { + LockSupport.park(this); + if (Thread.interrupted()) + interrupted = true; + } + if (acquireQueued(node, savedState) || interrupted) + selfInterrupt(); + } + + /* + * For interruptible waits, we need to track whether to throw + * InterruptedException, if interrupted while blocked on + * condition, versus reinterrupt current thread, if + * interrupted while blocked waiting to re-acquire. + */ + + /** Mode meaning to reinterrupt on exit from wait */ + private static final int REINTERRUPT = 1; + /** Mode meaning to throw InterruptedException on exit from wait */ + private static final int THROW_IE = -1; + + /** + * Checks for interrupt, returning THROW_IE if interrupted + * before signalled, REINTERRUPT if after signalled, or + * 0 if not interrupted. + */ + private int checkInterruptWhileWaiting(Node node) { + return (Thread.interrupted()) ? + ((transferAfterCancelledWait(node))? THROW_IE : REINTERRUPT) : + 0; + } + + /** + * Throws InterruptedException, reinterrupts current thread, or + * does nothing, depending on mode. + */ + private void reportInterruptAfterWait(int interruptMode) + throws InterruptedException { + if (interruptMode == THROW_IE) + throw new InterruptedException(); + else if (interruptMode == REINTERRUPT) + selfInterrupt(); + } + + /** + * Implements interruptible condition wait. + * <ol> + * <li> If current thread is interrupted, throw InterruptedException + * <li> Save lock state returned by {@link #getState} + * <li> Invoke {@link #release} with + * saved state as argument, throwing + * IllegalMonitorStateException if it fails. + * <li> Block until signalled or interrupted + * <li> Reacquire by invoking specialized version of + * {@link #acquire} with saved state as argument. + * <li> If interrupted while blocked in step 4, throw exception + * </ol> + */ + public final void await() throws InterruptedException { + if (Thread.interrupted()) + throw new InterruptedException(); + Node node = addConditionWaiter(); + int savedState = fullyRelease(node); + int interruptMode = 0; + while (!isOnSyncQueue(node)) { + LockSupport.park(this); + if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) + break; + } + if (acquireQueued(node, savedState) && interruptMode != THROW_IE) + interruptMode = REINTERRUPT; + if (isOnConditionQueue(node)) + unlinkCancelledWaiter(node); + if (interruptMode != 0) + reportInterruptAfterWait(interruptMode); + } + + /** + * Implements timed condition wait. + * <ol> + * <li> If current thread is interrupted, throw InterruptedException + * <li> Save lock state returned by {@link #getState} + * <li> Invoke {@link #release} with + * saved state as argument, throwing + * IllegalMonitorStateException if it fails. + * <li> Block until signalled, interrupted, or timed out + * <li> Reacquire by invoking specialized version of + * {@link #acquire} with saved state as argument. + * <li> If interrupted while blocked in step 4, throw InterruptedException + * </ol> + */ + public final long awaitNanos(long nanosTimeout) throws InterruptedException { + if (Thread.interrupted()) + throw new InterruptedException(); + Node node = addConditionWaiter(); + int savedState = fullyRelease(node); + long lastTime = System.nanoTime(); + int interruptMode = 0; + while (!isOnSyncQueue(node)) { + if (nanosTimeout <= 0L) { + transferAfterCancelledWait(node); + break; + } + LockSupport.parkNanos(this, nanosTimeout); + if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) + break; + + long now = System.nanoTime(); + nanosTimeout -= now - lastTime; + lastTime = now; + } + if (acquireQueued(node, savedState) && interruptMode != THROW_IE) + interruptMode = REINTERRUPT; + if (isOnConditionQueue(node)) + unlinkCancelledWaiter(node); + if (interruptMode != 0) + reportInterruptAfterWait(interruptMode); + return nanosTimeout - (System.nanoTime() - lastTime); + } + + /** + * Implements absolute timed condition wait. + * <ol> + * <li> If current thread is interrupted, throw InterruptedException + * <li> Save lock state returned by {@link #getState} + * <li> Invoke {@link #release} with + * saved state as argument, throwing + * IllegalMonitorStateException if it fails. + * <li> Block until signalled, interrupted, or timed out + * <li> Reacquire by invoking specialized version of + * {@link #acquire} with saved state as argument. + * <li> If interrupted while blocked in step 4, throw InterruptedException + * <li> If timed out while blocked in step 4, return false, else true + * </ol> + */ + public final boolean awaitUntil(Date deadline) throws InterruptedException { + if (deadline == null) + throw new NullPointerException(); + long abstime = deadline.getTime(); + if (Thread.interrupted()) + throw new InterruptedException(); + Node node = addConditionWaiter(); + int savedState = fullyRelease(node); + boolean timedout = false; + int interruptMode = 0; + while (!isOnSyncQueue(node)) { + if (System.currentTimeMillis() > abstime) { + timedout = transferAfterCancelledWait(node); + break; + } + LockSupport.parkUntil(this, abstime); + if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) + break; + } + if (acquireQueued(node, savedState) && interruptMode != THROW_IE) + interruptMode = REINTERRUPT; + if (isOnConditionQueue(node)) + unlinkCancelledWaiter(node); + if (interruptMode != 0) + reportInterruptAfterWait(interruptMode); + return !timedout; + } + + /** + * Implements timed condition wait. + * <ol> + * <li> If current thread is interrupted, throw InterruptedException + * <li> Save lock state returned by {@link #getState} + * <li> Invoke {@link #release} with + * saved state as argument, throwing + * IllegalMonitorStateException if it fails. + * <li> Block until signalled, interrupted, or timed out + * <li> Reacquire by invoking specialized version of + * {@link #acquire} with saved state as argument. + * <li> If interrupted while blocked in step 4, throw InterruptedException + * <li> If timed out while blocked in step 4, return false, else true + * </ol> + */ + public final boolean await(long time, TimeUnit unit) throws InterruptedException { + if (unit == null) + throw new NullPointerException(); + long nanosTimeout = unit.toNanos(time); + if (Thread.interrupted()) + throw new InterruptedException(); + Node node = addConditionWaiter(); + int savedState = fullyRelease(node); + long lastTime = System.nanoTime(); + boolean timedout = false; + int interruptMode = 0; + while (!isOnSyncQueue(node)) { + if (nanosTimeout <= 0L) { + timedout = transferAfterCancelledWait(node); + break; + } + LockSupport.parkNanos(this, nanosTimeout); + if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) + break; + long now = System.nanoTime(); + nanosTimeout -= now - lastTime; + lastTime = now; + } + if (acquireQueued(node, savedState) && interruptMode != THROW_IE) + interruptMode = REINTERRUPT; + if (isOnConditionQueue(node)) + unlinkCancelledWaiter(node); + if (interruptMode != 0) + reportInterruptAfterWait(interruptMode); + return !timedout; + } + + // support for instrumentation + + /** + * Returns true if this condition was created by the given + * synchronization object. + * + * @return {@code true} if owned + */ + final boolean isOwnedBy(AbstractQueuedSynchronizer sync) { + return sync == AbstractQueuedSynchronizer.this; + } + + /** + * Queries whether any threads are waiting on this condition. + * Implements {@link AbstractQueuedSynchronizer#hasWaiters}. + * + * @return {@code true} if there are any waiting threads + * @throws IllegalMonitorStateException if {@link #isHeldExclusively} + * returns {@code false} + */ + protected final boolean hasWaiters() { + if (!isHeldExclusively()) + throw new IllegalMonitorStateException(); + for (Node w = firstWaiter; w != null; w = w.nextWaiter) { + if (w.waitStatus == Node.CONDITION) + return true; + } + return false; + } + + /** + * Returns an estimate of the number of threads waiting on + * this condition. + * Implements {@link AbstractQueuedSynchronizer#getWaitQueueLength}. + * + * @return the estimated number of waiting threads + * @throws IllegalMonitorStateException if {@link #isHeldExclusively} + * returns {@code false} + */ + protected final int getWaitQueueLength() { + if (!isHeldExclusively()) + throw new IllegalMonitorStateException(); + int n = 0; + for (Node w = firstWaiter; w != null; w = w.nextWaiter) { + if (w.waitStatus == Node.CONDITION) + ++n; + } + return n; + } + + /** + * Returns a collection containing those threads that may be + * waiting on this Condition. + * Implements {@link AbstractQueuedSynchronizer#getWaitingThreads}. + * + * @return the collection of threads + * @throws IllegalMonitorStateException if {@link #isHeldExclusively} + * returns {@code false} + */ + protected final Collection<Thread> getWaitingThreads() { + if (!isHeldExclusively()) + throw new IllegalMonitorStateException(); + ArrayList<Thread> list = new ArrayList<Thread>(); + for (Node w = firstWaiter; w != null; w = w.nextWaiter) { + if (w.waitStatus == Node.CONDITION) { + Thread t = w.thread; + if (t != null) + list.add(t); + } + } + return list; + } + } + + /** + * Setup to support compareAndSet. We need to natively implement + * this here: For the sake of permitting future enhancements, we + * cannot explicitly subclass AtomicInteger, which would be + * efficient and useful otherwise. So, as the lesser of evils, we + * natively implement using hotspot intrinsics API. And while we + * are at it, we do the same for other CASable fields (which could + * otherwise be done with atomic field updaters). + */ + private static final Unsafe unsafe = Unsafe.getUnsafe(); + private static final long stateOffset; + private static final long headOffset; + private static final long tailOffset; + private static final long waitStatusOffset; + + static { + try { + stateOffset = unsafe.objectFieldOffset + (AbstractQueuedSynchronizer.class.getDeclaredField("state")); + headOffset = unsafe.objectFieldOffset + (AbstractQueuedSynchronizer.class.getDeclaredField("head")); + tailOffset = unsafe.objectFieldOffset + (AbstractQueuedSynchronizer.class.getDeclaredField("tail")); + waitStatusOffset = unsafe.objectFieldOffset + (Node.class.getDeclaredField("waitStatus")); + + } catch (Exception ex) { throw new Error(ex); } + } + + /** + * CAS head field. Used only by enq + */ + private final boolean compareAndSetHead(Node update) { + return unsafe.compareAndSwapObject(this, headOffset, null, update); + } + + /** + * CAS tail field. Used only by enq + */ + private final boolean compareAndSetTail(Node expect, Node update) { + return unsafe.compareAndSwapObject(this, tailOffset, expect, update); + } + + /** + * CAS waitStatus field of a node. + */ + private final static boolean compareAndSetWaitStatus(Node node, + int expect, + int update) { + return unsafe.compareAndSwapInt(node, waitStatusOffset, + expect, update); + } +} diff --git a/libjava/classpath/external/jsr166/java/util/concurrent/locks/Condition.java b/libjava/classpath/external/jsr166/java/util/concurrent/locks/Condition.java new file mode 100644 index 000000000..5d24128e1 --- /dev/null +++ b/libjava/classpath/external/jsr166/java/util/concurrent/locks/Condition.java @@ -0,0 +1,435 @@ +/* + * Written by Doug Lea with assistance from members of JCP JSR-166 + * Expert Group and released to the public domain, as explained at + * http://creativecommons.org/licenses/publicdomain + */ + +package java.util.concurrent.locks; +import java.util.concurrent.*; +import java.util.Date; + +/** + * {@code Condition} factors out the {@code Object} monitor + * methods ({@link Object#wait() wait}, {@link Object#notify notify} + * and {@link Object#notifyAll notifyAll}) into distinct objects to + * give the effect of having multiple wait-sets per object, by + * combining them with the use of arbitrary {@link Lock} implementations. + * Where a {@code Lock} replaces the use of {@code synchronized} methods + * and statements, a {@code Condition} replaces the use of the Object + * monitor methods. + * + * <p>Conditions (also known as <em>condition queues</em> or + * <em>condition variables</em>) provide a means for one thread to + * suspend execution (to "wait") until notified by another + * thread that some state condition may now be true. Because access + * to this shared state information occurs in different threads, it + * must be protected, so a lock of some form is associated with the + * condition. The key property that waiting for a condition provides + * is that it <em>atomically</em> releases the associated lock and + * suspends the current thread, just like {@code Object.wait}. + * + * <p>A {@code Condition} instance is intrinsically bound to a lock. + * To obtain a {@code Condition} instance for a particular {@link Lock} + * instance use its {@link Lock#newCondition newCondition()} method. + * + * <p>As an example, suppose we have a bounded buffer which supports + * {@code put} and {@code take} methods. If a + * {@code take} is attempted on an empty buffer, then the thread will block + * until an item becomes available; if a {@code put} is attempted on a + * full buffer, then the thread will block until a space becomes available. + * We would like to keep waiting {@code put} threads and {@code take} + * threads in separate wait-sets so that we can use the optimization of + * only notifying a single thread at a time when items or spaces become + * available in the buffer. This can be achieved using two + * {@link Condition} instances. + * <pre> + * class BoundedBuffer { + * <b>final Lock lock = new ReentrantLock();</b> + * final Condition notFull = <b>lock.newCondition(); </b> + * final Condition notEmpty = <b>lock.newCondition(); </b> + * + * final Object[] items = new Object[100]; + * int putptr, takeptr, count; + * + * public void put(Object x) throws InterruptedException { + * <b>lock.lock(); + * try {</b> + * while (count == items.length) + * <b>notFull.await();</b> + * items[putptr] = x; + * if (++putptr == items.length) putptr = 0; + * ++count; + * <b>notEmpty.signal();</b> + * <b>} finally { + * lock.unlock(); + * }</b> + * } + * + * public Object take() throws InterruptedException { + * <b>lock.lock(); + * try {</b> + * while (count == 0) + * <b>notEmpty.await();</b> + * Object x = items[takeptr]; + * if (++takeptr == items.length) takeptr = 0; + * --count; + * <b>notFull.signal();</b> + * return x; + * <b>} finally { + * lock.unlock(); + * }</b> + * } + * } + * </pre> + * + * (The {@link java.util.concurrent.ArrayBlockingQueue} class provides + * this functionality, so there is no reason to implement this + * sample usage class.) + * + * <p>A {@code Condition} implementation can provide behavior and semantics + * that is + * different from that of the {@code Object} monitor methods, such as + * guaranteed ordering for notifications, or not requiring a lock to be held + * when performing notifications. + * If an implementation provides such specialized semantics then the + * implementation must document those semantics. + * + * <p>Note that {@code Condition} instances are just normal objects and can + * themselves be used as the target in a {@code synchronized} statement, + * and can have their own monitor {@link Object#wait wait} and + * {@link Object#notify notification} methods invoked. + * Acquiring the monitor lock of a {@code Condition} instance, or using its + * monitor methods, has no specified relationship with acquiring the + * {@link Lock} associated with that {@code Condition} or the use of its + * {@linkplain #await waiting} and {@linkplain #signal signalling} methods. + * It is recommended that to avoid confusion you never use {@code Condition} + * instances in this way, except perhaps within their own implementation. + * + * <p>Except where noted, passing a {@code null} value for any parameter + * will result in a {@link NullPointerException} being thrown. + * + * <h3>Implementation Considerations</h3> + * + * <p>When waiting upon a {@code Condition}, a "<em>spurious + * wakeup</em>" is permitted to occur, in + * general, as a concession to the underlying platform semantics. + * This has little practical impact on most application programs as a + * {@code Condition} should always be waited upon in a loop, testing + * the state predicate that is being waited for. An implementation is + * free to remove the possibility of spurious wakeups but it is + * recommended that applications programmers always assume that they can + * occur and so always wait in a loop. + * + * <p>The three forms of condition waiting + * (interruptible, non-interruptible, and timed) may differ in their ease of + * implementation on some platforms and in their performance characteristics. + * In particular, it may be difficult to provide these features and maintain + * specific semantics such as ordering guarantees. + * Further, the ability to interrupt the actual suspension of the thread may + * not always be feasible to implement on all platforms. + * + * <p>Consequently, an implementation is not required to define exactly the + * same guarantees or semantics for all three forms of waiting, nor is it + * required to support interruption of the actual suspension of the thread. + * + * <p>An implementation is required to + * clearly document the semantics and guarantees provided by each of the + * waiting methods, and when an implementation does support interruption of + * thread suspension then it must obey the interruption semantics as defined + * in this interface. + * + * <p>As interruption generally implies cancellation, and checks for + * interruption are often infrequent, an implementation can favor responding + * to an interrupt over normal method return. This is true even if it can be + * shown that the interrupt occurred after another action may have unblocked + * the thread. An implementation should document this behavior. + * + * @since 1.5 + * @author Doug Lea + */ +public interface Condition { + + /** + * Causes the current thread to wait until it is signalled or + * {@linkplain Thread#interrupt interrupted}. + * + * <p>The lock associated with this {@code Condition} is atomically + * released and the current thread becomes disabled for thread scheduling + * purposes and lies dormant until <em>one</em> of four things happens: + * <ul> + * <li>Some other thread invokes the {@link #signal} method for this + * {@code Condition} and the current thread happens to be chosen as the + * thread to be awakened; or + * <li>Some other thread invokes the {@link #signalAll} method for this + * {@code Condition}; or + * <li>Some other thread {@linkplain Thread#interrupt interrupts} the + * current thread, and interruption of thread suspension is supported; or + * <li>A "<em>spurious wakeup</em>" occurs. + * </ul> + * + * <p>In all cases, before this method can return the current thread must + * re-acquire the lock associated with this condition. When the + * thread returns it is <em>guaranteed</em> to hold this lock. + * + * <p>If the current thread: + * <ul> + * <li>has its interrupted status set on entry to this method; or + * <li>is {@linkplain Thread#interrupt interrupted} while waiting + * and interruption of thread suspension is supported, + * </ul> + * then {@link InterruptedException} is thrown and the current thread's + * interrupted status is cleared. It is not specified, in the first + * case, whether or not the test for interruption occurs before the lock + * is released. + * + * <p><b>Implementation Considerations</b> + * + * <p>The current thread is assumed to hold the lock associated with this + * {@code Condition} when this method is called. + * It is up to the implementation to determine if this is + * the case and if not, how to respond. Typically, an exception will be + * thrown (such as {@link IllegalMonitorStateException}) and the + * implementation must document that fact. + * + * <p>An implementation can favor responding to an interrupt over normal + * method return in response to a signal. In that case the implementation + * must ensure that the signal is redirected to another waiting thread, if + * there is one. + * + * @throws InterruptedException if the current thread is interrupted + * (and interruption of thread suspension is supported) + */ + void await() throws InterruptedException; + + /** + * Causes the current thread to wait until it is signalled. + * + * <p>The lock associated with this condition is atomically + * released and the current thread becomes disabled for thread scheduling + * purposes and lies dormant until <em>one</em> of three things happens: + * <ul> + * <li>Some other thread invokes the {@link #signal} method for this + * {@code Condition} and the current thread happens to be chosen as the + * thread to be awakened; or + * <li>Some other thread invokes the {@link #signalAll} method for this + * {@code Condition}; or + * <li>A "<em>spurious wakeup</em>" occurs. + * </ul> + * + * <p>In all cases, before this method can return the current thread must + * re-acquire the lock associated with this condition. When the + * thread returns it is <em>guaranteed</em> to hold this lock. + * + * <p>If the current thread's interrupted status is set when it enters + * this method, or it is {@linkplain Thread#interrupt interrupted} + * while waiting, it will continue to wait until signalled. When it finally + * returns from this method its interrupted status will still + * be set. + * + * <p><b>Implementation Considerations</b> + * + * <p>The current thread is assumed to hold the lock associated with this + * {@code Condition} when this method is called. + * It is up to the implementation to determine if this is + * the case and if not, how to respond. Typically, an exception will be + * thrown (such as {@link IllegalMonitorStateException}) and the + * implementation must document that fact. + */ + void awaitUninterruptibly(); + + /** + * Causes the current thread to wait until it is signalled or interrupted, + * or the specified waiting time elapses. + * + * <p>The lock associated with this condition is atomically + * released and the current thread becomes disabled for thread scheduling + * purposes and lies dormant until <em>one</em> of five things happens: + * <ul> + * <li>Some other thread invokes the {@link #signal} method for this + * {@code Condition} and the current thread happens to be chosen as the + * thread to be awakened; or + * <li>Some other thread invokes the {@link #signalAll} method for this + * {@code Condition}; or + * <li>Some other thread {@linkplain Thread#interrupt interrupts} the + * current thread, and interruption of thread suspension is supported; or + * <li>The specified waiting time elapses; or + * <li>A "<em>spurious wakeup</em>" occurs. + * </ul> + * + * <p>In all cases, before this method can return the current thread must + * re-acquire the lock associated with this condition. When the + * thread returns it is <em>guaranteed</em> to hold this lock. + * + * <p>If the current thread: + * <ul> + * <li>has its interrupted status set on entry to this method; or + * <li>is {@linkplain Thread#interrupt interrupted} while waiting + * and interruption of thread suspension is supported, + * </ul> + * then {@link InterruptedException} is thrown and the current thread's + * interrupted status is cleared. It is not specified, in the first + * case, whether or not the test for interruption occurs before the lock + * is released. + * + * <p>The method returns an estimate of the number of nanoseconds + * remaining to wait given the supplied {@code nanosTimeout} + * value upon return, or a value less than or equal to zero if it + * timed out. This value can be used to determine whether and how + * long to re-wait in cases where the wait returns but an awaited + * condition still does not hold. Typical uses of this method take + * the following form: + * + * <pre> + * synchronized boolean aMethod(long timeout, TimeUnit unit) { + * long nanosTimeout = unit.toNanos(timeout); + * while (!conditionBeingWaitedFor) { + * if (nanosTimeout > 0) + * nanosTimeout = theCondition.awaitNanos(nanosTimeout); + * else + * return false; + * } + * // ... + * } + * </pre> + * + * <p> Design note: This method requires a nanosecond argument so + * as to avoid truncation errors in reporting remaining times. + * Such precision loss would make it difficult for programmers to + * ensure that total waiting times are not systematically shorter + * than specified when re-waits occur. + * + * <p><b>Implementation Considerations</b> + * + * <p>The current thread is assumed to hold the lock associated with this + * {@code Condition} when this method is called. + * It is up to the implementation to determine if this is + * the case and if not, how to respond. Typically, an exception will be + * thrown (such as {@link IllegalMonitorStateException}) and the + * implementation must document that fact. + * + * <p>An implementation can favor responding to an interrupt over normal + * method return in response to a signal, or over indicating the elapse + * of the specified waiting time. In either case the implementation + * must ensure that the signal is redirected to another waiting thread, if + * there is one. + * + * @param nanosTimeout the maximum time to wait, in nanoseconds + * @return an estimate of the {@code nanosTimeout} value minus + * the time spent waiting upon return from this method. + * A positive value may be used as the argument to a + * subsequent call to this method to finish waiting out + * the desired time. A value less than or equal to zero + * indicates that no time remains. + * @throws InterruptedException if the current thread is interrupted + * (and interruption of thread suspension is supported) + */ + long awaitNanos(long nanosTimeout) throws InterruptedException; + + /** + * Causes the current thread to wait until it is signalled or interrupted, + * or the specified waiting time elapses. This method is behaviorally + * equivalent to:<br> + * <pre> + * awaitNanos(unit.toNanos(time)) > 0 + * </pre> + * @param time the maximum time to wait + * @param unit the time unit of the {@code time} argument + * @return {@code false} if the waiting time detectably elapsed + * before return from the method, else {@code true} + * @throws InterruptedException if the current thread is interrupted + * (and interruption of thread suspension is supported) + */ + boolean await(long time, TimeUnit unit) throws InterruptedException; + + /** + * Causes the current thread to wait until it is signalled or interrupted, + * or the specified deadline elapses. + * + * <p>The lock associated with this condition is atomically + * released and the current thread becomes disabled for thread scheduling + * purposes and lies dormant until <em>one</em> of five things happens: + * <ul> + * <li>Some other thread invokes the {@link #signal} method for this + * {@code Condition} and the current thread happens to be chosen as the + * thread to be awakened; or + * <li>Some other thread invokes the {@link #signalAll} method for this + * {@code Condition}; or + * <li>Some other thread {@linkplain Thread#interrupt interrupts} the + * current thread, and interruption of thread suspension is supported; or + * <li>The specified deadline elapses; or + * <li>A "<em>spurious wakeup</em>" occurs. + * </ul> + * + * <p>In all cases, before this method can return the current thread must + * re-acquire the lock associated with this condition. When the + * thread returns it is <em>guaranteed</em> to hold this lock. + * + * + * <p>If the current thread: + * <ul> + * <li>has its interrupted status set on entry to this method; or + * <li>is {@linkplain Thread#interrupt interrupted} while waiting + * and interruption of thread suspension is supported, + * </ul> + * then {@link InterruptedException} is thrown and the current thread's + * interrupted status is cleared. It is not specified, in the first + * case, whether or not the test for interruption occurs before the lock + * is released. + * + * + * <p>The return value indicates whether the deadline has elapsed, + * which can be used as follows: + * <pre> + * synchronized boolean aMethod(Date deadline) { + * boolean stillWaiting = true; + * while (!conditionBeingWaitedFor) { + * if (stillWaiting) + * stillWaiting = theCondition.awaitUntil(deadline); + * else + * return false; + * } + * // ... + * } + * </pre> + * + * <p><b>Implementation Considerations</b> + * + * <p>The current thread is assumed to hold the lock associated with this + * {@code Condition} when this method is called. + * It is up to the implementation to determine if this is + * the case and if not, how to respond. Typically, an exception will be + * thrown (such as {@link IllegalMonitorStateException}) and the + * implementation must document that fact. + * + * <p>An implementation can favor responding to an interrupt over normal + * method return in response to a signal, or over indicating the passing + * of the specified deadline. In either case the implementation + * must ensure that the signal is redirected to another waiting thread, if + * there is one. + * + * @param deadline the absolute time to wait until + * @return {@code false} if the deadline has elapsed upon return, else + * {@code true} + * @throws InterruptedException if the current thread is interrupted + * (and interruption of thread suspension is supported) + */ + boolean awaitUntil(Date deadline) throws InterruptedException; + + /** + * Wakes up one waiting thread. + * + * <p>If any threads are waiting on this condition then one + * is selected for waking up. That thread must then re-acquire the + * lock before returning from {@code await}. + */ + void signal(); + + /** + * Wakes up all waiting threads. + * + * <p>If any threads are waiting on this condition then they are + * all woken up. Each thread must re-acquire the lock before it can + * return from {@code await}. + */ + void signalAll(); +} diff --git a/libjava/classpath/external/jsr166/java/util/concurrent/locks/Lock.java b/libjava/classpath/external/jsr166/java/util/concurrent/locks/Lock.java new file mode 100644 index 000000000..4b9abd665 --- /dev/null +++ b/libjava/classpath/external/jsr166/java/util/concurrent/locks/Lock.java @@ -0,0 +1,327 @@ +/* + * Written by Doug Lea with assistance from members of JCP JSR-166 + * Expert Group and released to the public domain, as explained at + * http://creativecommons.org/licenses/publicdomain + */ + +package java.util.concurrent.locks; +import java.util.concurrent.TimeUnit; + +/** + * {@code Lock} implementations provide more extensive locking + * operations than can be obtained using {@code synchronized} methods + * and statements. They allow more flexible structuring, may have + * quite different properties, and may support multiple associated + * {@link Condition} objects. + * + * <p>A lock is a tool for controlling access to a shared resource by + * multiple threads. Commonly, a lock provides exclusive access to a + * shared resource: only one thread at a time can acquire the lock and + * all access to the shared resource requires that the lock be + * acquired first. However, some locks may allow concurrent access to + * a shared resource, such as the read lock of a {@link ReadWriteLock}. + * + * <p>The use of {@code synchronized} methods or statements provides + * access to the implicit monitor lock associated with every object, but + * forces all lock acquisition and release to occur in a block-structured way: + * when multiple locks are acquired they must be released in the opposite + * order, and all locks must be released in the same lexical scope in which + * they were acquired. + * + * <p>While the scoping mechanism for {@code synchronized} methods + * and statements makes it much easier to program with monitor locks, + * and helps avoid many common programming errors involving locks, + * there are occasions where you need to work with locks in a more + * flexible way. For example, some algorithms for traversing + * concurrently accessed data structures require the use of + * "hand-over-hand" or "chain locking": you + * acquire the lock of node A, then node B, then release A and acquire + * C, then release B and acquire D and so on. Implementations of the + * {@code Lock} interface enable the use of such techniques by + * allowing a lock to be acquired and released in different scopes, + * and allowing multiple locks to be acquired and released in any + * order. + * + * <p>With this increased flexibility comes additional + * responsibility. The absence of block-structured locking removes the + * automatic release of locks that occurs with {@code synchronized} + * methods and statements. In most cases, the following idiom + * should be used: + * + * <pre><tt> Lock l = ...; + * l.lock(); + * try { + * // access the resource protected by this lock + * } finally { + * l.unlock(); + * } + * </tt></pre> + * + * When locking and unlocking occur in different scopes, care must be + * taken to ensure that all code that is executed while the lock is + * held is protected by try-finally or try-catch to ensure that the + * lock is released when necessary. + * + * <p>{@code Lock} implementations provide additional functionality + * over the use of {@code synchronized} methods and statements by + * providing a non-blocking attempt to acquire a lock ({@link + * #tryLock()}), an attempt to acquire the lock that can be + * interrupted ({@link #lockInterruptibly}, and an attempt to acquire + * the lock that can timeout ({@link #tryLock(long, TimeUnit)}). + * + * <p>A {@code Lock} class can also provide behavior and semantics + * that is quite different from that of the implicit monitor lock, + * such as guaranteed ordering, non-reentrant usage, or deadlock + * detection. If an implementation provides such specialized semantics + * then the implementation must document those semantics. + * + * <p>Note that {@code Lock} instances are just normal objects and can + * themselves be used as the target in a {@code synchronized} statement. + * Acquiring the + * monitor lock of a {@code Lock} instance has no specified relationship + * with invoking any of the {@link #lock} methods of that instance. + * It is recommended that to avoid confusion you never use {@code Lock} + * instances in this way, except within their own implementation. + * + * <p>Except where noted, passing a {@code null} value for any + * parameter will result in a {@link NullPointerException} being + * thrown. + * + * <h3>Memory Synchronization</h3> + * + * <p>All {@code Lock} implementations <em>must</em> enforce the same + * memory synchronization semantics as provided by the built-in monitor + * lock, as described in <a href="http://java.sun.com/docs/books/jls/"> + * The Java Language Specification, Third Edition (17.4 Memory Model)</a>: + * <ul> + * <li>A successful {@code lock} operation has the same memory + * synchronization effects as a successful <em>Lock</em> action. + * <li>A successful {@code unlock} operation has the same + * memory synchronization effects as a successful <em>Unlock</em> action. + * </ul> + * + * Unsuccessful locking and unlocking operations, and reentrant + * locking/unlocking operations, do not require any memory + * synchronization effects. + * + * <h3>Implementation Considerations</h3> + * + * <p> The three forms of lock acquisition (interruptible, + * non-interruptible, and timed) may differ in their performance + * characteristics, ordering guarantees, or other implementation + * qualities. Further, the ability to interrupt the <em>ongoing</em> + * acquisition of a lock may not be available in a given {@code Lock} + * class. Consequently, an implementation is not required to define + * exactly the same guarantees or semantics for all three forms of + * lock acquisition, nor is it required to support interruption of an + * ongoing lock acquisition. An implementation is required to clearly + * document the semantics and guarantees provided by each of the + * locking methods. It must also obey the interruption semantics as + * defined in this interface, to the extent that interruption of lock + * acquisition is supported: which is either totally, or only on + * method entry. + * + * <p>As interruption generally implies cancellation, and checks for + * interruption are often infrequent, an implementation can favor responding + * to an interrupt over normal method return. This is true even if it can be + * shown that the interrupt occurred after another action may have unblocked + * the thread. An implementation should document this behavior. + * + * @see ReentrantLock + * @see Condition + * @see ReadWriteLock + * + * @since 1.5 + * @author Doug Lea + */ +public interface Lock { + + /** + * Acquires the lock. + * + * <p>If the lock is not available then the current thread becomes + * disabled for thread scheduling purposes and lies dormant until the + * lock has been acquired. + * + * <p><b>Implementation Considerations</b> + * + * <p>A {@code Lock} implementation may be able to detect erroneous use + * of the lock, such as an invocation that would cause deadlock, and + * may throw an (unchecked) exception in such circumstances. The + * circumstances and the exception type must be documented by that + * {@code Lock} implementation. + */ + void lock(); + + /** + * Acquires the lock unless the current thread is + * {@linkplain Thread#interrupt interrupted}. + * + * <p>Acquires the lock if it is available and returns immediately. + * + * <p>If the lock is not available then the current thread becomes + * disabled for thread scheduling purposes and lies dormant until + * one of two things happens: + * + * <ul> + * <li>The lock is acquired by the current thread; or + * <li>Some other thread {@linkplain Thread#interrupt interrupts} the + * current thread, and interruption of lock acquisition is supported. + * </ul> + * + * <p>If the current thread: + * <ul> + * <li>has its interrupted status set on entry to this method; or + * <li>is {@linkplain Thread#interrupt interrupted} while acquiring the + * lock, and interruption of lock acquisition is supported, + * </ul> + * then {@link InterruptedException} is thrown and the current thread's + * interrupted status is cleared. + * + * <p><b>Implementation Considerations</b> + * + * <p>The ability to interrupt a lock acquisition in some + * implementations may not be possible, and if possible may be an + * expensive operation. The programmer should be aware that this + * may be the case. An implementation should document when this is + * the case. + * + * <p>An implementation can favor responding to an interrupt over + * normal method return. + * + * <p>A {@code Lock} implementation may be able to detect + * erroneous use of the lock, such as an invocation that would + * cause deadlock, and may throw an (unchecked) exception in such + * circumstances. The circumstances and the exception type must + * be documented by that {@code Lock} implementation. + * + * @throws InterruptedException if the current thread is + * interrupted while acquiring the lock (and interruption + * of lock acquisition is supported). + */ + void lockInterruptibly() throws InterruptedException; + + /** + * Acquires the lock only if it is free at the time of invocation. + * + * <p>Acquires the lock if it is available and returns immediately + * with the value {@code true}. + * If the lock is not available then this method will return + * immediately with the value {@code false}. + * + * <p>A typical usage idiom for this method would be: + * <pre> + * Lock lock = ...; + * if (lock.tryLock()) { + * try { + * // manipulate protected state + * } finally { + * lock.unlock(); + * } + * } else { + * // perform alternative actions + * } + * </pre> + * This usage ensures that the lock is unlocked if it was acquired, and + * doesn't try to unlock if the lock was not acquired. + * + * @return {@code true} if the lock was acquired and + * {@code false} otherwise + */ + boolean tryLock(); + + /** + * Acquires the lock if it is free within the given waiting time and the + * current thread has not been {@linkplain Thread#interrupt interrupted}. + * + * <p>If the lock is available this method returns immediately + * with the value {@code true}. + * If the lock is not available then + * the current thread becomes disabled for thread scheduling + * purposes and lies dormant until one of three things happens: + * <ul> + * <li>The lock is acquired by the current thread; or + * <li>Some other thread {@linkplain Thread#interrupt interrupts} the + * current thread, and interruption of lock acquisition is supported; or + * <li>The specified waiting time elapses + * </ul> + * + * <p>If the lock is acquired then the value {@code true} is returned. + * + * <p>If the current thread: + * <ul> + * <li>has its interrupted status set on entry to this method; or + * <li>is {@linkplain Thread#interrupt interrupted} while acquiring + * the lock, and interruption of lock acquisition is supported, + * </ul> + * then {@link InterruptedException} is thrown and the current thread's + * interrupted status is cleared. + * + * <p>If the specified waiting time elapses then the value {@code false} + * is returned. + * If the time is + * less than or equal to zero, the method will not wait at all. + * + * <p><b>Implementation Considerations</b> + * + * <p>The ability to interrupt a lock acquisition in some implementations + * may not be possible, and if possible may + * be an expensive operation. + * The programmer should be aware that this may be the case. An + * implementation should document when this is the case. + * + * <p>An implementation can favor responding to an interrupt over normal + * method return, or reporting a timeout. + * + * <p>A {@code Lock} implementation may be able to detect + * erroneous use of the lock, such as an invocation that would cause + * deadlock, and may throw an (unchecked) exception in such circumstances. + * The circumstances and the exception type must be documented by that + * {@code Lock} implementation. + * + * @param time the maximum time to wait for the lock + * @param unit the time unit of the {@code time} argument + * @return {@code true} if the lock was acquired and {@code false} + * if the waiting time elapsed before the lock was acquired + * + * @throws InterruptedException if the current thread is interrupted + * while acquiring the lock (and interruption of lock + * acquisition is supported) + */ + boolean tryLock(long time, TimeUnit unit) throws InterruptedException; + + /** + * Releases the lock. + * + * <p><b>Implementation Considerations</b> + * + * <p>A {@code Lock} implementation will usually impose + * restrictions on which thread can release a lock (typically only the + * holder of the lock can release it) and may throw + * an (unchecked) exception if the restriction is violated. + * Any restrictions and the exception + * type must be documented by that {@code Lock} implementation. + */ + void unlock(); + + /** + * Returns a new {@link Condition} instance that is bound to this + * {@code Lock} instance. + * + * <p>Before waiting on the condition the lock must be held by the + * current thread. + * A call to {@link Condition#await()} will atomically release the lock + * before waiting and re-acquire the lock before the wait returns. + * + * <p><b>Implementation Considerations</b> + * + * <p>The exact operation of the {@link Condition} instance depends on + * the {@code Lock} implementation and must be documented by that + * implementation. + * + * @return A new {@link Condition} instance for this {@code Lock} instance + * @throws UnsupportedOperationException if this {@code Lock} + * implementation does not support conditions + */ + Condition newCondition(); +} diff --git a/libjava/classpath/external/jsr166/java/util/concurrent/locks/LockSupport.java b/libjava/classpath/external/jsr166/java/util/concurrent/locks/LockSupport.java new file mode 100644 index 000000000..28728ae2b --- /dev/null +++ b/libjava/classpath/external/jsr166/java/util/concurrent/locks/LockSupport.java @@ -0,0 +1,352 @@ +/* + * Written by Doug Lea with assistance from members of JCP JSR-166 + * Expert Group and released to the public domain, as explained at + * http://creativecommons.org/licenses/publicdomain + */ + +package java.util.concurrent.locks; +import java.util.concurrent.*; +import sun.misc.Unsafe; + + +/** + * Basic thread blocking primitives for creating locks and other + * synchronization classes. + * + * <p>This class associates, with each thread that uses it, a permit + * (in the sense of the {@link java.util.concurrent.Semaphore + * Semaphore} class). A call to {@code park} will return immediately + * if the permit is available, consuming it in the process; otherwise + * it <em>may</em> block. A call to {@code unpark} makes the permit + * available, if it was not already available. (Unlike with Semaphores + * though, permits do not accumulate. There is at most one.) + * + * <p>Methods {@code park} and {@code unpark} provide efficient + * means of blocking and unblocking threads that do not encounter the + * problems that cause the deprecated methods {@code Thread.suspend} + * and {@code Thread.resume} to be unusable for such purposes: Races + * between one thread invoking {@code park} and another thread trying + * to {@code unpark} it will preserve liveness, due to the + * permit. Additionally, {@code park} will return if the caller's + * thread was interrupted, and timeout versions are supported. The + * {@code park} method may also return at any other time, for "no + * reason", so in general must be invoked within a loop that rechecks + * conditions upon return. In this sense {@code park} serves as an + * optimization of a "busy wait" that does not waste as much time + * spinning, but must be paired with an {@code unpark} to be + * effective. + * + * <p>The three forms of {@code park} each also support a + * {@code blocker} object parameter. This object is recorded while + * the thread is blocked to permit monitoring and diagnostic tools to + * identify the reasons that threads are blocked. (Such tools may + * access blockers using method {@link #getBlocker}.) The use of these + * forms rather than the original forms without this parameter is + * strongly encouraged. The normal argument to supply as a + * {@code blocker} within a lock implementation is {@code this}. + * + * <p>These methods are designed to be used as tools for creating + * higher-level synchronization utilities, and are not in themselves + * useful for most concurrency control applications. The {@code park} + * method is designed for use only in constructions of the form: + * <pre>while (!canProceed()) { ... LockSupport.park(this); }</pre> + * where neither {@code canProceed} nor any other actions prior to the + * call to {@code park} entail locking or blocking. Because only one + * permit is associated with each thread, any intermediary uses of + * {@code park} could interfere with its intended effects. + * + * <p><b>Sample Usage.</b> Here is a sketch of a first-in-first-out + * non-reentrant lock class: + * <pre>{@code + * class FIFOMutex { + * private final AtomicBoolean locked = new AtomicBoolean(false); + * private final Queue<Thread> waiters + * = new ConcurrentLinkedQueue<Thread>(); + * + * public void lock() { + * boolean wasInterrupted = false; + * Thread current = Thread.currentThread(); + * waiters.add(current); + * + * // Block while not first in queue or cannot acquire lock + * while (waiters.peek() != current || + * !locked.compareAndSet(false, true)) { + * LockSupport.park(this); + * if (Thread.interrupted()) // ignore interrupts while waiting + * wasInterrupted = true; + * } + * + * waiters.remove(); + * if (wasInterrupted) // reassert interrupt status on exit + * current.interrupt(); + * } + * + * public void unlock() { + * locked.set(false); + * LockSupport.unpark(waiters.peek()); + * } + * }}</pre> + */ + +public class LockSupport { + private LockSupport() {} // Cannot be instantiated. + + // Hotspot implementation via intrinsics API + private static final Unsafe unsafe = Unsafe.getUnsafe(); + private static final long parkBlockerOffset; + + static { + try { + parkBlockerOffset = unsafe.objectFieldOffset + (java.lang.Thread.class.getDeclaredField("parkBlocker")); + } catch (Exception ex) { throw new Error(ex); } + } + + private static void setBlocker(Thread t, Object arg) { + // Even though volatile, hotspot doesn't need a write barrier here. + unsafe.putObject(t, parkBlockerOffset, arg); + } + + /** + * Makes available the permit for the given thread, if it + * was not already available. If the thread was blocked on + * {@code park} then it will unblock. Otherwise, its next call + * to {@code park} is guaranteed not to block. This operation + * is not guaranteed to have any effect at all if the given + * thread has not been started. + * + * @param thread the thread to unpark, or {@code null}, in which case + * this operation has no effect + */ + public static void unpark(Thread thread) { + if (thread != null) + unsafe.unpark(thread); + } + + /** + * Disables the current thread for thread scheduling purposes unless the + * permit is available. + * + * <p>If the permit is available then it is consumed and the call returns + * immediately; otherwise + * the current thread becomes disabled for thread scheduling + * purposes and lies dormant until one of three things happens: + * + * <ul> + * <li>Some other thread invokes {@link #unpark unpark} with the + * current thread as the target; or + * + * <li>Some other thread {@linkplain Thread#interrupt interrupts} + * the current thread; or + * + * <li>The call spuriously (that is, for no reason) returns. + * </ul> + * + * <p>This method does <em>not</em> report which of these caused the + * method to return. Callers should re-check the conditions which caused + * the thread to park in the first place. Callers may also determine, + * for example, the interrupt status of the thread upon return. + * + * @param blocker the synchronization object responsible for this + * thread parking + * @since 1.6 + */ + public static void park(Object blocker) { + Thread t = Thread.currentThread(); + setBlocker(t, blocker); + unsafe.park(false, 0L); + setBlocker(t, null); + } + + /** + * Disables the current thread for thread scheduling purposes, for up to + * the specified waiting time, unless the permit is available. + * + * <p>If the permit is available then it is consumed and the call + * returns immediately; otherwise the current thread becomes disabled + * for thread scheduling purposes and lies dormant until one of four + * things happens: + * + * <ul> + * <li>Some other thread invokes {@link #unpark unpark} with the + * current thread as the target; or + * + * <li>Some other thread {@linkplain Thread#interrupt interrupts} the current + * thread; or + * + * <li>The specified waiting time elapses; or + * + * <li>The call spuriously (that is, for no reason) returns. + * </ul> + * + * <p>This method does <em>not</em> report which of these caused the + * method to return. Callers should re-check the conditions which caused + * the thread to park in the first place. Callers may also determine, + * for example, the interrupt status of the thread, or the elapsed time + * upon return. + * + * @param blocker the synchronization object responsible for this + * thread parking + * @param nanos the maximum number of nanoseconds to wait + * @since 1.6 + */ + public static void parkNanos(Object blocker, long nanos) { + if (nanos > 0) { + Thread t = Thread.currentThread(); + setBlocker(t, blocker); + unsafe.park(false, nanos); + setBlocker(t, null); + } + } + + /** + * Disables the current thread for thread scheduling purposes, until + * the specified deadline, unless the permit is available. + * + * <p>If the permit is available then it is consumed and the call + * returns immediately; otherwise the current thread becomes disabled + * for thread scheduling purposes and lies dormant until one of four + * things happens: + * + * <ul> + * <li>Some other thread invokes {@link #unpark unpark} with the + * current thread as the target; or + * + * <li>Some other thread {@linkplain Thread#interrupt interrupts} the + * current thread; or + * + * <li>The specified deadline passes; or + * + * <li>The call spuriously (that is, for no reason) returns. + * </ul> + * + * <p>This method does <em>not</em> report which of these caused the + * method to return. Callers should re-check the conditions which caused + * the thread to park in the first place. Callers may also determine, + * for example, the interrupt status of the thread, or the current time + * upon return. + * + * @param blocker the synchronization object responsible for this + * thread parking + * @param deadline the absolute time, in milliseconds from the Epoch, + * to wait until + * @since 1.6 + */ + public static void parkUntil(Object blocker, long deadline) { + Thread t = Thread.currentThread(); + setBlocker(t, blocker); + unsafe.park(true, deadline); + setBlocker(t, null); + } + + /** + * Returns the blocker object supplied to the most recent + * invocation of a park method that has not yet unblocked, or null + * if not blocked. The value returned is just a momentary + * snapshot -- the thread may have since unblocked or blocked on a + * different blocker object. + * + * @return the blocker + * @since 1.6 + */ + public static Object getBlocker(Thread t) { + return unsafe.getObjectVolatile(t, parkBlockerOffset); + } + + /** + * Disables the current thread for thread scheduling purposes unless the + * permit is available. + * + * <p>If the permit is available then it is consumed and the call + * returns immediately; otherwise the current thread becomes disabled + * for thread scheduling purposes and lies dormant until one of three + * things happens: + * + * <ul> + * + * <li>Some other thread invokes {@link #unpark unpark} with the + * current thread as the target; or + * + * <li>Some other thread {@linkplain Thread#interrupt interrupts} + * the current thread; or + * + * <li>The call spuriously (that is, for no reason) returns. + * </ul> + * + * <p>This method does <em>not</em> report which of these caused the + * method to return. Callers should re-check the conditions which caused + * the thread to park in the first place. Callers may also determine, + * for example, the interrupt status of the thread upon return. + */ + public static void park() { + unsafe.park(false, 0L); + } + + /** + * Disables the current thread for thread scheduling purposes, for up to + * the specified waiting time, unless the permit is available. + * + * <p>If the permit is available then it is consumed and the call + * returns immediately; otherwise the current thread becomes disabled + * for thread scheduling purposes and lies dormant until one of four + * things happens: + * + * <ul> + * <li>Some other thread invokes {@link #unpark unpark} with the + * current thread as the target; or + * + * <li>Some other thread {@linkplain Thread#interrupt interrupts} + * the current thread; or + * + * <li>The specified waiting time elapses; or + * + * <li>The call spuriously (that is, for no reason) returns. + * </ul> + * + * <p>This method does <em>not</em> report which of these caused the + * method to return. Callers should re-check the conditions which caused + * the thread to park in the first place. Callers may also determine, + * for example, the interrupt status of the thread, or the elapsed time + * upon return. + * + * @param nanos the maximum number of nanoseconds to wait + */ + public static void parkNanos(long nanos) { + if (nanos > 0) + unsafe.park(false, nanos); + } + + /** + * Disables the current thread for thread scheduling purposes, until + * the specified deadline, unless the permit is available. + * + * <p>If the permit is available then it is consumed and the call + * returns immediately; otherwise the current thread becomes disabled + * for thread scheduling purposes and lies dormant until one of four + * things happens: + * + * <ul> + * <li>Some other thread invokes {@link #unpark unpark} with the + * current thread as the target; or + * + * <li>Some other thread {@linkplain Thread#interrupt interrupts} + * the current thread; or + * + * <li>The specified deadline passes; or + * + * <li>The call spuriously (that is, for no reason) returns. + * </ul> + * + * <p>This method does <em>not</em> report which of these caused the + * method to return. Callers should re-check the conditions which caused + * the thread to park in the first place. Callers may also determine, + * for example, the interrupt status of the thread, or the current time + * upon return. + * + * @param deadline the absolute time, in milliseconds from the Epoch, + * to wait until + */ + public static void parkUntil(long deadline) { + unsafe.park(true, deadline); + } +} diff --git a/libjava/classpath/external/jsr166/java/util/concurrent/locks/ReadWriteLock.java b/libjava/classpath/external/jsr166/java/util/concurrent/locks/ReadWriteLock.java new file mode 100644 index 000000000..484f68d15 --- /dev/null +++ b/libjava/classpath/external/jsr166/java/util/concurrent/locks/ReadWriteLock.java @@ -0,0 +1,104 @@ +/* + * Written by Doug Lea with assistance from members of JCP JSR-166 + * Expert Group and released to the public domain, as explained at + * http://creativecommons.org/licenses/publicdomain + */ + +package java.util.concurrent.locks; + +/** + * A <tt>ReadWriteLock</tt> maintains a pair of associated {@link + * Lock locks}, one for read-only operations and one for writing. + * The {@link #readLock read lock} may be held simultaneously by + * multiple reader threads, so long as there are no writers. The + * {@link #writeLock write lock} is exclusive. + * + * <p>All <tt>ReadWriteLock</tt> implementations must guarantee that + * the memory synchronization effects of <tt>writeLock</tt> operations + * (as specified in the {@link Lock} interface) also hold with respect + * to the associated <tt>readLock</tt>. That is, a thread successfully + * acquiring the read lock will see all updates made upon previous + * release of the write lock. + * + * <p>A read-write lock allows for a greater level of concurrency in + * accessing shared data than that permitted by a mutual exclusion lock. + * It exploits the fact that while only a single thread at a time (a + * <em>writer</em> thread) can modify the shared data, in many cases any + * number of threads can concurrently read the data (hence <em>reader</em> + * threads). + * In theory, the increase in concurrency permitted by the use of a read-write + * lock will lead to performance improvements over the use of a mutual + * exclusion lock. In practice this increase in concurrency will only be fully + * realized on a multi-processor, and then only if the access patterns for + * the shared data are suitable. + * + * <p>Whether or not a read-write lock will improve performance over the use + * of a mutual exclusion lock depends on the frequency that the data is + * read compared to being modified, the duration of the read and write + * operations, and the contention for the data - that is, the number of + * threads that will try to read or write the data at the same time. + * For example, a collection that is initially populated with data and + * thereafter infrequently modified, while being frequently searched + * (such as a directory of some kind) is an ideal candidate for the use of + * a read-write lock. However, if updates become frequent then the data + * spends most of its time being exclusively locked and there is little, if any + * increase in concurrency. Further, if the read operations are too short + * the overhead of the read-write lock implementation (which is inherently + * more complex than a mutual exclusion lock) can dominate the execution + * cost, particularly as many read-write lock implementations still serialize + * all threads through a small section of code. Ultimately, only profiling + * and measurement will establish whether the use of a read-write lock is + * suitable for your application. + * + * + * <p>Although the basic operation of a read-write lock is straight-forward, + * there are many policy decisions that an implementation must make, which + * may affect the effectiveness of the read-write lock in a given application. + * Examples of these policies include: + * <ul> + * <li>Determining whether to grant the read lock or the write lock, when + * both readers and writers are waiting, at the time that a writer releases + * the write lock. Writer preference is common, as writes are expected to be + * short and infrequent. Reader preference is less common as it can lead to + * lengthy delays for a write if the readers are frequent and long-lived as + * expected. Fair, or "in-order" implementations are also possible. + * + * <li>Determining whether readers that request the read lock while a + * reader is active and a writer is waiting, are granted the read lock. + * Preference to the reader can delay the writer indefinitely, while + * preference to the writer can reduce the potential for concurrency. + * + * <li>Determining whether the locks are reentrant: can a thread with the + * write lock reacquire it? Can it acquire a read lock while holding the + * write lock? Is the read lock itself reentrant? + * + * <li>Can the write lock be downgraded to a read lock without allowing + * an intervening writer? Can a read lock be upgraded to a write lock, + * in preference to other waiting readers or writers? + * + * </ul> + * You should consider all of these things when evaluating the suitability + * of a given implementation for your application. + * + * @see ReentrantReadWriteLock + * @see Lock + * @see ReentrantLock + * + * @since 1.5 + * @author Doug Lea + */ +public interface ReadWriteLock { + /** + * Returns the lock used for reading. + * + * @return the lock used for reading. + */ + Lock readLock(); + + /** + * Returns the lock used for writing. + * + * @return the lock used for writing. + */ + Lock writeLock(); +} diff --git a/libjava/classpath/external/jsr166/java/util/concurrent/locks/ReentrantLock.java b/libjava/classpath/external/jsr166/java/util/concurrent/locks/ReentrantLock.java new file mode 100644 index 000000000..4a2fc175c --- /dev/null +++ b/libjava/classpath/external/jsr166/java/util/concurrent/locks/ReentrantLock.java @@ -0,0 +1,740 @@ +/* + * Written by Doug Lea with assistance from members of JCP JSR-166 + * Expert Group and released to the public domain, as explained at + * http://creativecommons.org/licenses/publicdomain + */ + +package java.util.concurrent.locks; +import java.util.*; +import java.util.concurrent.*; +import java.util.concurrent.atomic.*; + +/** + * A reentrant mutual exclusion {@link Lock} with the same basic + * behavior and semantics as the implicit monitor lock accessed using + * {@code synchronized} methods and statements, but with extended + * capabilities. + * + * <p>A {@code ReentrantLock} is <em>owned</em> by the thread last + * successfully locking, but not yet unlocking it. A thread invoking + * {@code lock} will return, successfully acquiring the lock, when + * the lock is not owned by another thread. The method will return + * immediately if the current thread already owns the lock. This can + * be checked using methods {@link #isHeldByCurrentThread}, and {@link + * #getHoldCount}. + * + * <p>The constructor for this class accepts an optional + * <em>fairness</em> parameter. When set {@code true}, under + * contention, locks favor granting access to the longest-waiting + * thread. Otherwise this lock does not guarantee any particular + * access order. Programs using fair locks accessed by many threads + * may display lower overall throughput (i.e., are slower; often much + * slower) than those using the default setting, but have smaller + * variances in times to obtain locks and guarantee lack of + * starvation. Note however, that fairness of locks does not guarantee + * fairness of thread scheduling. Thus, one of many threads using a + * fair lock may obtain it multiple times in succession while other + * active threads are not progressing and not currently holding the + * lock. + * Also note that the untimed {@link #tryLock() tryLock} method does not + * honor the fairness setting. It will succeed if the lock + * is available even if other threads are waiting. + * + * <p>It is recommended practice to <em>always</em> immediately + * follow a call to {@code lock} with a {@code try} block, most + * typically in a before/after construction such as: + * + * <pre> + * class X { + * private final ReentrantLock lock = new ReentrantLock(); + * // ... + * + * public void m() { + * lock.lock(); // block until condition holds + * try { + * // ... method body + * } finally { + * lock.unlock() + * } + * } + * } + * </pre> + * + * <p>In addition to implementing the {@link Lock} interface, this + * class defines methods {@code isLocked} and + * {@code getLockQueueLength}, as well as some associated + * {@code protected} access methods that may be useful for + * instrumentation and monitoring. + * + * <p>Serialization of this class behaves in the same way as built-in + * locks: a deserialized lock is in the unlocked state, regardless of + * its state when serialized. + * + * <p>This lock supports a maximum of 2147483647 recursive locks by + * the same thread. Attempts to exceed this limit result in + * {@link Error} throws from locking methods. + * + * @since 1.5 + * @author Doug Lea + */ +public class ReentrantLock implements Lock, java.io.Serializable { + private static final long serialVersionUID = 7373984872572414699L; + /** Synchronizer providing all implementation mechanics */ + private final Sync sync; + + /** + * Base of synchronization control for this lock. Subclassed + * into fair and nonfair versions below. Uses AQS state to + * represent the number of holds on the lock. + */ + static abstract class Sync extends AbstractQueuedSynchronizer { + private static final long serialVersionUID = -5179523762034025860L; + + /** + * Performs {@link Lock#lock}. The main reason for subclassing + * is to allow fast path for nonfair version. + */ + abstract void lock(); + + /** + * Performs non-fair tryLock. tryAcquire is + * implemented in subclasses, but both need nonfair + * try for trylock method. + */ + final boolean nonfairTryAcquire(int acquires) { + final Thread current = Thread.currentThread(); + int c = getState(); + if (c == 0) { + if (compareAndSetState(0, acquires)) { + setExclusiveOwnerThread(current); + return true; + } + } + else if (current == getExclusiveOwnerThread()) { + int nextc = c + acquires; + if (nextc < 0) // overflow + throw new Error("Maximum lock count exceeded"); + setState(nextc); + return true; + } + return false; + } + + protected final boolean tryRelease(int releases) { + int c = getState() - releases; + if (Thread.currentThread() != getExclusiveOwnerThread()) + throw new IllegalMonitorStateException(); + boolean free = false; + if (c == 0) { + free = true; + setExclusiveOwnerThread(null); + } + setState(c); + return free; + } + + protected final boolean isHeldExclusively() { + // While we must in general read state before owner, + // we don't need to do so to check if current thread is owner + return getExclusiveOwnerThread() == Thread.currentThread(); + } + + final ConditionObject newCondition() { + return new ConditionObject(); + } + + // Methods relayed from outer class + + final Thread getOwner() { + return getState() == 0 ? null : getExclusiveOwnerThread(); + } + + final int getHoldCount() { + return isHeldExclusively() ? getState() : 0; + } + + final boolean isLocked() { + return getState() != 0; + } + + /** + * Reconstitutes this lock instance from a stream. + * @param s the stream + */ + private void readObject(java.io.ObjectInputStream s) + throws java.io.IOException, ClassNotFoundException { + s.defaultReadObject(); + setState(0); // reset to unlocked state + } + } + + /** + * Sync object for non-fair locks + */ + final static class NonfairSync extends Sync { + private static final long serialVersionUID = 7316153563782823691L; + + /** + * Performs lock. Try immediate barge, backing up to normal + * acquire on failure. + */ + final void lock() { + if (compareAndSetState(0, 1)) + setExclusiveOwnerThread(Thread.currentThread()); + else + acquire(1); + } + + protected final boolean tryAcquire(int acquires) { + return nonfairTryAcquire(acquires); + } + } + + /** + * Sync object for fair locks + */ + final static class FairSync extends Sync { + private static final long serialVersionUID = -3000897897090466540L; + + final void lock() { + acquire(1); + } + + /** + * Fair version of tryAcquire. Don't grant access unless + * recursive call or no waiters or is first. + */ + protected final boolean tryAcquire(int acquires) { + final Thread current = Thread.currentThread(); + int c = getState(); + if (c == 0) { + if (isFirst(current) && + compareAndSetState(0, acquires)) { + setExclusiveOwnerThread(current); + return true; + } + } + else if (current == getExclusiveOwnerThread()) { + int nextc = c + acquires; + if (nextc < 0) + throw new Error("Maximum lock count exceeded"); + setState(nextc); + return true; + } + return false; + } + } + + /** + * Creates an instance of {@code ReentrantLock}. + * This is equivalent to using {@code ReentrantLock(false)}. + */ + public ReentrantLock() { + sync = new NonfairSync(); + } + + /** + * Creates an instance of {@code ReentrantLock} with the + * given fairness policy. + * + * @param fair {@code true} if this lock should use a fair ordering policy + */ + public ReentrantLock(boolean fair) { + sync = (fair)? new FairSync() : new NonfairSync(); + } + + /** + * Acquires the lock. + * + * <p>Acquires the lock if it is not held by another thread and returns + * immediately, setting the lock hold count to one. + * + * <p>If the current thread already holds the lock then the hold + * count is incremented by one and the method returns immediately. + * + * <p>If the lock is held by another thread then the + * current thread becomes disabled for thread scheduling + * purposes and lies dormant until the lock has been acquired, + * at which time the lock hold count is set to one. + */ + public void lock() { + sync.lock(); + } + + /** + * Acquires the lock unless the current thread is + * {@linkplain Thread#interrupt interrupted}. + * + * <p>Acquires the lock if it is not held by another thread and returns + * immediately, setting the lock hold count to one. + * + * <p>If the current thread already holds this lock then the hold count + * is incremented by one and the method returns immediately. + * + * <p>If the lock is held by another thread then the + * current thread becomes disabled for thread scheduling + * purposes and lies dormant until one of two things happens: + * + * <ul> + * + * <li>The lock is acquired by the current thread; or + * + * <li>Some other thread {@linkplain Thread#interrupt interrupts} the + * current thread. + * + * </ul> + * + * <p>If the lock is acquired by the current thread then the lock hold + * count is set to one. + * + * <p>If the current thread: + * + * <ul> + * + * <li>has its interrupted status set on entry to this method; or + * + * <li>is {@linkplain Thread#interrupt interrupted} while acquiring + * the lock, + * + * </ul> + * + * then {@link InterruptedException} is thrown and the current thread's + * interrupted status is cleared. + * + * <p>In this implementation, as this method is an explicit + * interruption point, preference is given to responding to the + * interrupt over normal or reentrant acquisition of the lock. + * + * @throws InterruptedException if the current thread is interrupted + */ + public void lockInterruptibly() throws InterruptedException { + sync.acquireInterruptibly(1); + } + + /** + * Acquires the lock only if it is not held by another thread at the time + * of invocation. + * + * <p>Acquires the lock if it is not held by another thread and + * returns immediately with the value {@code true}, setting the + * lock hold count to one. Even when this lock has been set to use a + * fair ordering policy, a call to {@code tryLock()} <em>will</em> + * immediately acquire the lock if it is available, whether or not + * other threads are currently waiting for the lock. + * This "barging" behavior can be useful in certain + * circumstances, even though it breaks fairness. If you want to honor + * the fairness setting for this lock, then use + * {@link #tryLock(long, TimeUnit) tryLock(0, TimeUnit.SECONDS) } + * which is almost equivalent (it also detects interruption). + * + * <p> If the current thread already holds this lock then the hold + * count is incremented by one and the method returns {@code true}. + * + * <p>If the lock is held by another thread then this method will return + * immediately with the value {@code false}. + * + * @return {@code true} if the lock was free and was acquired by the + * current thread, or the lock was already held by the current + * thread; and {@code false} otherwise + */ + public boolean tryLock() { + return sync.nonfairTryAcquire(1); + } + + /** + * Acquires the lock if it is not held by another thread within the given + * waiting time and the current thread has not been + * {@linkplain Thread#interrupt interrupted}. + * + * <p>Acquires the lock if it is not held by another thread and returns + * immediately with the value {@code true}, setting the lock hold count + * to one. If this lock has been set to use a fair ordering policy then + * an available lock <em>will not</em> be acquired if any other threads + * are waiting for the lock. This is in contrast to the {@link #tryLock()} + * method. If you want a timed {@code tryLock} that does permit barging on + * a fair lock then combine the timed and un-timed forms together: + * + * <pre>if (lock.tryLock() || lock.tryLock(timeout, unit) ) { ... } + * </pre> + * + * <p>If the current thread + * already holds this lock then the hold count is incremented by one and + * the method returns {@code true}. + * + * <p>If the lock is held by another thread then the + * current thread becomes disabled for thread scheduling + * purposes and lies dormant until one of three things happens: + * + * <ul> + * + * <li>The lock is acquired by the current thread; or + * + * <li>Some other thread {@linkplain Thread#interrupt interrupts} + * the current thread; or + * + * <li>The specified waiting time elapses + * + * </ul> + * + * <p>If the lock is acquired then the value {@code true} is returned and + * the lock hold count is set to one. + * + * <p>If the current thread: + * + * <ul> + * + * <li>has its interrupted status set on entry to this method; or + * + * <li>is {@linkplain Thread#interrupt interrupted} while + * acquiring the lock, + * + * </ul> + * then {@link InterruptedException} is thrown and the current thread's + * interrupted status is cleared. + * + * <p>If the specified waiting time elapses then the value {@code false} + * is returned. If the time is less than or equal to zero, the method + * will not wait at all. + * + * <p>In this implementation, as this method is an explicit + * interruption point, preference is given to responding to the + * interrupt over normal or reentrant acquisition of the lock, and + * over reporting the elapse of the waiting time. + * + * @param timeout the time to wait for the lock + * @param unit the time unit of the timeout argument + * @return {@code true} if the lock was free and was acquired by the + * current thread, or the lock was already held by the current + * thread; and {@code false} if the waiting time elapsed before + * the lock could be acquired + * @throws InterruptedException if the current thread is interrupted + * @throws NullPointerException if the time unit is null + * + */ + public boolean tryLock(long timeout, TimeUnit unit) throws InterruptedException { + return sync.tryAcquireNanos(1, unit.toNanos(timeout)); + } + + /** + * Attempts to release this lock. + * + * <p>If the current thread is the holder of this lock then the hold + * count is decremented. If the hold count is now zero then the lock + * is released. If the current thread is not the holder of this + * lock then {@link IllegalMonitorStateException} is thrown. + * + * @throws IllegalMonitorStateException if the current thread does not + * hold this lock + */ + public void unlock() { + sync.release(1); + } + + /** + * Returns a {@link Condition} instance for use with this + * {@link Lock} instance. + * + * <p>The returned {@link Condition} instance supports the same + * usages as do the {@link Object} monitor methods ({@link + * Object#wait() wait}, {@link Object#notify notify}, and {@link + * Object#notifyAll notifyAll}) when used with the built-in + * monitor lock. + * + * <ul> + * + * <li>If this lock is not held when any of the {@link Condition} + * {@linkplain Condition#await() waiting} or {@linkplain + * Condition#signal signalling} methods are called, then an {@link + * IllegalMonitorStateException} is thrown. + * + * <li>When the condition {@linkplain Condition#await() waiting} + * methods are called the lock is released and, before they + * return, the lock is reacquired and the lock hold count restored + * to what it was when the method was called. + * + * <li>If a thread is {@linkplain Thread#interrupt interrupted} + * while waiting then the wait will terminate, an {@link + * InterruptedException} will be thrown, and the thread's + * interrupted status will be cleared. + * + * <li> Waiting threads are signalled in FIFO order. + * + * <li>The ordering of lock reacquisition for threads returning + * from waiting methods is the same as for threads initially + * acquiring the lock, which is in the default case not specified, + * but for <em>fair</em> locks favors those threads that have been + * waiting the longest. + * + * </ul> + * + * @return the Condition object + */ + public Condition newCondition() { + return sync.newCondition(); + } + + /** + * Queries the number of holds on this lock by the current thread. + * + * <p>A thread has a hold on a lock for each lock action that is not + * matched by an unlock action. + * + * <p>The hold count information is typically only used for testing and + * debugging purposes. For example, if a certain section of code should + * not be entered with the lock already held then we can assert that + * fact: + * + * <pre> + * class X { + * ReentrantLock lock = new ReentrantLock(); + * // ... + * public void m() { + * assert lock.getHoldCount() == 0; + * lock.lock(); + * try { + * // ... method body + * } finally { + * lock.unlock(); + * } + * } + * } + * </pre> + * + * @return the number of holds on this lock by the current thread, + * or zero if this lock is not held by the current thread + */ + public int getHoldCount() { + return sync.getHoldCount(); + } + + /** + * Queries if this lock is held by the current thread. + * + * <p>Analogous to the {@link Thread#holdsLock} method for built-in + * monitor locks, this method is typically used for debugging and + * testing. For example, a method that should only be called while + * a lock is held can assert that this is the case: + * + * <pre> + * class X { + * ReentrantLock lock = new ReentrantLock(); + * // ... + * + * public void m() { + * assert lock.isHeldByCurrentThread(); + * // ... method body + * } + * } + * </pre> + * + * <p>It can also be used to ensure that a reentrant lock is used + * in a non-reentrant manner, for example: + * + * <pre> + * class X { + * ReentrantLock lock = new ReentrantLock(); + * // ... + * + * public void m() { + * assert !lock.isHeldByCurrentThread(); + * lock.lock(); + * try { + * // ... method body + * } finally { + * lock.unlock(); + * } + * } + * } + * </pre> + * + * @return {@code true} if current thread holds this lock and + * {@code false} otherwise + */ + public boolean isHeldByCurrentThread() { + return sync.isHeldExclusively(); + } + + /** + * Queries if this lock is held by any thread. This method is + * designed for use in monitoring of the system state, + * not for synchronization control. + * + * @return {@code true} if any thread holds this lock and + * {@code false} otherwise + */ + public boolean isLocked() { + return sync.isLocked(); + } + + /** + * Returns {@code true} if this lock has fairness set true. + * + * @return {@code true} if this lock has fairness set true + */ + public final boolean isFair() { + return sync instanceof FairSync; + } + + /** + * Returns the thread that currently owns this lock, or + * {@code null} if not owned. When this method is called by a + * thread that is not the owner, the return value reflects a + * best-effort approximation of current lock status. For example, + * the owner may be momentarily {@code null} even if there are + * threads trying to acquire the lock but have not yet done so. + * This method is designed to facilitate construction of + * subclasses that provide more extensive lock monitoring + * facilities. + * + * @return the owner, or {@code null} if not owned + */ + protected Thread getOwner() { + return sync.getOwner(); + } + + /** + * Queries whether any threads are waiting to acquire this lock. Note that + * because cancellations may occur at any time, a {@code true} + * return does not guarantee that any other thread will ever + * acquire this lock. This method is designed primarily for use in + * monitoring of the system state. + * + * @return {@code true} if there may be other threads waiting to + * acquire the lock + */ + public final boolean hasQueuedThreads() { + return sync.hasQueuedThreads(); + } + + + /** + * Queries whether the given thread is waiting to acquire this + * lock. Note that because cancellations may occur at any time, a + * {@code true} return does not guarantee that this thread + * will ever acquire this lock. This method is designed primarily for use + * in monitoring of the system state. + * + * @param thread the thread + * @return {@code true} if the given thread is queued waiting for this lock + * @throws NullPointerException if the thread is null + */ + public final boolean hasQueuedThread(Thread thread) { + return sync.isQueued(thread); + } + + + /** + * Returns an estimate of the number of threads waiting to + * acquire this lock. The value is only an estimate because the number of + * threads may change dynamically while this method traverses + * internal data structures. This method is designed for use in + * monitoring of the system state, not for synchronization + * control. + * + * @return the estimated number of threads waiting for this lock + */ + public final int getQueueLength() { + return sync.getQueueLength(); + } + + /** + * Returns a collection containing threads that may be waiting to + * acquire this lock. Because the actual set of threads may change + * dynamically while constructing this result, the returned + * collection is only a best-effort estimate. The elements of the + * returned collection are in no particular order. This method is + * designed to facilitate construction of subclasses that provide + * more extensive monitoring facilities. + * + * @return the collection of threads + */ + protected Collection<Thread> getQueuedThreads() { + return sync.getQueuedThreads(); + } + + /** + * Queries whether any threads are waiting on the given condition + * associated with this lock. Note that because timeouts and + * interrupts may occur at any time, a {@code true} return does + * not guarantee that a future {@code signal} will awaken any + * threads. This method is designed primarily for use in + * monitoring of the system state. + * + * @param condition the condition + * @return {@code true} if there are any waiting threads + * @throws IllegalMonitorStateException if this lock is not held + * @throws IllegalArgumentException if the given condition is + * not associated with this lock + * @throws NullPointerException if the condition is null + */ + public boolean hasWaiters(Condition condition) { + if (condition == null) + throw new NullPointerException(); + if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject)) + throw new IllegalArgumentException("not owner"); + return sync.hasWaiters((AbstractQueuedSynchronizer.ConditionObject)condition); + } + + /** + * Returns an estimate of the number of threads waiting on the + * given condition associated with this lock. Note that because + * timeouts and interrupts may occur at any time, the estimate + * serves only as an upper bound on the actual number of waiters. + * This method is designed for use in monitoring of the system + * state, not for synchronization control. + * + * @param condition the condition + * @return the estimated number of waiting threads + * @throws IllegalMonitorStateException if this lock is not held + * @throws IllegalArgumentException if the given condition is + * not associated with this lock + * @throws NullPointerException if the condition is null + */ + public int getWaitQueueLength(Condition condition) { + if (condition == null) + throw new NullPointerException(); + if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject)) + throw new IllegalArgumentException("not owner"); + return sync.getWaitQueueLength((AbstractQueuedSynchronizer.ConditionObject)condition); + } + + /** + * Returns a collection containing those threads that may be + * waiting on the given condition associated with this lock. + * Because the actual set of threads may change dynamically while + * constructing this result, the returned collection is only a + * best-effort estimate. The elements of the returned collection + * are in no particular order. This method is designed to + * facilitate construction of subclasses that provide more + * extensive condition monitoring facilities. + * + * @param condition the condition + * @return the collection of threads + * @throws IllegalMonitorStateException if this lock is not held + * @throws IllegalArgumentException if the given condition is + * not associated with this lock + * @throws NullPointerException if the condition is null + */ + protected Collection<Thread> getWaitingThreads(Condition condition) { + if (condition == null) + throw new NullPointerException(); + if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject)) + throw new IllegalArgumentException("not owner"); + return sync.getWaitingThreads((AbstractQueuedSynchronizer.ConditionObject)condition); + } + + /** + * Returns a string identifying this lock, as well as its lock state. + * The state, in brackets, includes either the String {@code "Unlocked"} + * or the String {@code "Locked by"} followed by the + * {@linkplain Thread#getName name} of the owning thread. + * + * @return a string identifying this lock, as well as its lock state + */ + public String toString() { + Thread o = sync.getOwner(); + return super.toString() + ((o == null) ? + "[Unlocked]" : + "[Locked by thread " + o.getName() + "]"); + } +} diff --git a/libjava/classpath/external/jsr166/java/util/concurrent/locks/ReentrantReadWriteLock.java b/libjava/classpath/external/jsr166/java/util/concurrent/locks/ReentrantReadWriteLock.java new file mode 100644 index 000000000..a6eadff5b --- /dev/null +++ b/libjava/classpath/external/jsr166/java/util/concurrent/locks/ReentrantReadWriteLock.java @@ -0,0 +1,1346 @@ +/* + * Written by Doug Lea with assistance from members of JCP JSR-166 + * Expert Group and released to the public domain, as explained at + * http://creativecommons.org/licenses/publicdomain + */ + +package java.util.concurrent.locks; +import java.util.concurrent.*; +import java.util.concurrent.atomic.*; +import java.util.*; + +/** + * An implementation of {@link ReadWriteLock} supporting similar + * semantics to {@link ReentrantLock}. + * <p>This class has the following properties: + * + * <ul> + * <li><b>Acquisition order</b> + * + * <p> This class does not impose a reader or writer preference + * ordering for lock access. However, it does support an optional + * <em>fairness</em> policy. + * + * <dl> + * <dt><b><i>Non-fair mode (default)</i></b> + * <dd>When constructed as non-fair (the default), the order of entry + * to the read and write lock is unspecified, subject to reentrancy + * constraints. A nonfair lock that is continously contended may + * indefinitely postpone one or more reader or writer threads, but + * will normally have higher throughput than a fair lock. + * <p> + * + * <dt><b><i>Fair mode</i></b> + * <dd> When constructed as fair, threads contend for entry using an + * approximately arrival-order policy. When the currently held lock + * is released either the longest-waiting single writer thread will + * be assigned the write lock, or if there is a group of reader threads + * waiting longer than all waiting writer threads, that group will be + * assigned the read lock. + * + * <p>A thread that tries to acquire a fair read lock (non-reentrantly) + * will block if either the write lock is held, or there is a waiting + * writer thread. The thread will not acquire the read lock until + * after the oldest currently waiting writer thread has acquired and + * released the write lock. Of course, if a waiting writer abandons + * its wait, leaving one or more reader threads as the longest waiters + * in the queue with the write lock free, then those readers will be + * assigned the read lock. + * + * <p>A thread that tries to acquire a fair write lock (non-reentrantly) + * will block unless both the read lock and write lock are free (which + * implies there are no waiting threads). (Note that the non-blocking + * {@link ReadLock#tryLock()} and {@link WriteLock#tryLock()} methods + * do not honor this fair setting and will acquire the lock if it is + * possible, regardless of waiting threads.) + * <p> + * </dl> + * + * <li><b>Reentrancy</b> + * + * <p>This lock allows both readers and writers to reacquire read or + * write locks in the style of a {@link ReentrantLock}. Non-reentrant + * readers are not allowed until all write locks held by the writing + * thread have been released. + * + * <p>Additionally, a writer can acquire the read lock, but not + * vice-versa. Among other applications, reentrancy can be useful + * when write locks are held during calls or callbacks to methods that + * perform reads under read locks. If a reader tries to acquire the + * write lock it will never succeed. + * + * <li><b>Lock downgrading</b> + * <p>Reentrancy also allows downgrading from the write lock to a read lock, + * by acquiring the write lock, then the read lock and then releasing the + * write lock. However, upgrading from a read lock to the write lock is + * <b>not</b> possible. + * + * <li><b>Interruption of lock acquisition</b> + * <p>The read lock and write lock both support interruption during lock + * acquisition. + * + * <li><b>{@link Condition} support</b> + * <p>The write lock provides a {@link Condition} implementation that + * behaves in the same way, with respect to the write lock, as the + * {@link Condition} implementation provided by + * {@link ReentrantLock#newCondition} does for {@link ReentrantLock}. + * This {@link Condition} can, of course, only be used with the write lock. + * + * <p>The read lock does not support a {@link Condition} and + * {@code readLock().newCondition()} throws + * {@code UnsupportedOperationException}. + * + * <li><b>Instrumentation</b> + * <p>This class supports methods to determine whether locks + * are held or contended. These methods are designed for monitoring + * system state, not for synchronization control. + * </ul> + * + * <p>Serialization of this class behaves in the same way as built-in + * locks: a deserialized lock is in the unlocked state, regardless of + * its state when serialized. + * + * <p><b>Sample usages</b>. Here is a code sketch showing how to exploit + * reentrancy to perform lock downgrading after updating a cache (exception + * handling is elided for simplicity): + * <pre> + * class CachedData { + * Object data; + * volatile boolean cacheValid; + * ReentrantReadWriteLock rwl = new ReentrantReadWriteLock(); + * + * void processCachedData() { + * rwl.readLock().lock(); + * if (!cacheValid) { + * // Must release read lock before acquiring write lock + * rwl.readLock().unlock(); + * rwl.writeLock().lock(); + * // Recheck state because another thread might have acquired + * // write lock and changed state before we did. + * if (!cacheValid) { + * data = ... + * cacheValid = true; + * } + * // Downgrade by acquiring read lock before releasing write lock + * rwl.readLock().lock(); + * rwl.writeLock().unlock(); // Unlock write, still hold read + * } + * + * use(data); + * rwl.readLock().unlock(); + * } + * } + * </pre> + * + * ReentrantReadWriteLocks can be used to improve concurrency in some + * uses of some kinds of Collections. This is typically worthwhile + * only when the collections are expected to be large, accessed by + * more reader threads than writer threads, and entail operations with + * overhead that outweighs synchronization overhead. For example, here + * is a class using a TreeMap that is expected to be large and + * concurrently accessed. + * + * <pre>{@code + * class RWDictionary { + * private final Map<String, Data> m = new TreeMap<String, Data>(); + * private final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock(); + * private final Lock r = rwl.readLock(); + * private final Lock w = rwl.writeLock(); + * + * public Data get(String key) { + * r.lock(); + * try { return m.get(key); } + * finally { r.unlock(); } + * } + * public String[] allKeys() { + * r.lock(); + * try { return m.keySet().toArray(); } + * finally { r.unlock(); } + * } + * public Data put(String key, Data value) { + * w.lock(); + * try { return m.put(key, value); } + * finally { w.unlock(); } + * } + * public void clear() { + * w.lock(); + * try { m.clear(); } + * finally { w.unlock(); } + * } + * }}</pre> + * + * <h3>Implementation Notes</h3> + * + * <p>This lock supports a maximum of 65535 recursive write locks + * and 65535 read locks. Attempts to exceed these limits result in + * {@link Error} throws from locking methods. + * + * @since 1.5 + * @author Doug Lea + * + */ +public class ReentrantReadWriteLock implements ReadWriteLock, java.io.Serializable { + private static final long serialVersionUID = -6992448646407690164L; + /** Inner class providing readlock */ + private final ReentrantReadWriteLock.ReadLock readerLock; + /** Inner class providing writelock */ + private final ReentrantReadWriteLock.WriteLock writerLock; + /** Performs all synchronization mechanics */ + private final Sync sync; + + /** + * Creates a new {@code ReentrantReadWriteLock} with + * default (nonfair) ordering properties. + */ + public ReentrantReadWriteLock() { + this(false); + } + + /** + * Creates a new {@code ReentrantReadWriteLock} with + * the given fairness policy. + * + * @param fair {@code true} if this lock should use a fair ordering policy + */ + public ReentrantReadWriteLock(boolean fair) { + sync = (fair)? new FairSync() : new NonfairSync(); + readerLock = new ReadLock(this); + writerLock = new WriteLock(this); + } + + public ReentrantReadWriteLock.WriteLock writeLock() { return writerLock; } + public ReentrantReadWriteLock.ReadLock readLock() { return readerLock; } + + /** + * Synchronization implementation for ReentrantReadWriteLock. + * Subclassed into fair and nonfair versions. + */ + static abstract class Sync extends AbstractQueuedSynchronizer { + private static final long serialVersionUID = 6317671515068378041L; + + /* + * Read vs write count extraction constants and functions. + * Lock state is logically divided into two shorts: The lower + * one representing the exclusive (writer) lock hold count, + * and the upper the shared (reader) hold count. + */ + + static final int SHARED_SHIFT = 16; + static final int SHARED_UNIT = (1 << SHARED_SHIFT); + static final int MAX_COUNT = (1 << SHARED_SHIFT) - 1; + static final int EXCLUSIVE_MASK = (1 << SHARED_SHIFT) - 1; + + /** Returns the number of shared holds represented in count */ + static int sharedCount(int c) { return c >>> SHARED_SHIFT; } + /** Returns the number of exclusive holds represented in count */ + static int exclusiveCount(int c) { return c & EXCLUSIVE_MASK; } + + /** + * A counter for per-thread read hold counts. + * Maintained as a ThreadLocal; cached in cachedHoldCounter + */ + static final class HoldCounter { + int count; + // Use id, not reference, to avoid garbage retention + final long tid = Thread.currentThread().getId(); + /** Decrement if positive; return previous value */ + int tryDecrement() { + int c = count; + if (c > 0) + count = c - 1; + return c; + } + } + + /** + * ThreadLocal subclass. Easiest to explicitly define for sake + * of deserialization mechanics. + */ + static final class ThreadLocalHoldCounter + extends ThreadLocal<HoldCounter> { + public HoldCounter initialValue() { + return new HoldCounter(); + } + } + + /** + * The number of read locks held by current thread. + * Initialized only in constructor and readObject. + */ + transient ThreadLocalHoldCounter readHolds; + + /** + * The hold count of the last thread to successfully acquire + * readLock. This saves ThreadLocal lookup in the common case + * where the next thread to release is the last one to + * acquire. This is non-volatile since it is just used + * as a heuristic, and would be great for threads to cache. + */ + transient HoldCounter cachedHoldCounter; + + Sync() { + readHolds = new ThreadLocalHoldCounter(); + setState(getState()); // ensures visibility of readHolds + } + + /* + * Acquires and releases use the same code for fair and + * nonfair locks, but differ in whether/how they allow barging + * when queues are non-empty. + */ + + /** + * Return true if a reader thread that is otherwise + * eligible for lock should block because of policy + * for overtaking other waiting threads. + */ + abstract boolean readerShouldBlock(Thread current); + + /** + * Return true if a writer thread that is otherwise + * eligible for lock should block because of policy + * for overtaking other waiting threads. + */ + abstract boolean writerShouldBlock(Thread current); + + /* + * Note that tryRelease and tryAcquire can be called by + * Conditions. So it is possible that their arguments contain + * both read and write holds that are all released during a + * condition wait and re-established in tryAcquire. + */ + + protected final boolean tryRelease(int releases) { + int nextc = getState() - releases; + if (Thread.currentThread() != getExclusiveOwnerThread()) + throw new IllegalMonitorStateException(); + if (exclusiveCount(nextc) == 0) { + setExclusiveOwnerThread(null); + setState(nextc); + return true; + } else { + setState(nextc); + return false; + } + } + + protected final boolean tryAcquire(int acquires) { + /* + * Walkthrough: + * 1. if read count nonzero or write count nonzero + * and owner is a different thread, fail. + * 2. If count would saturate, fail. (This can only + * happen if count is already nonzero.) + * 3. Otherwise, this thread is eligible for lock if + * it is either a reentrant acquire or + * queue policy allows it. If so, update state + * and set owner. + */ + Thread current = Thread.currentThread(); + int c = getState(); + int w = exclusiveCount(c); + if (c != 0) { + // (Note: if c != 0 and w == 0 then shared count != 0) + if (w == 0 || current != getExclusiveOwnerThread()) + return false; + if (w + exclusiveCount(acquires) > MAX_COUNT) + throw new Error("Maximum lock count exceeded"); + } + if ((w == 0 && writerShouldBlock(current)) || + !compareAndSetState(c, c + acquires)) + return false; + setExclusiveOwnerThread(current); + return true; + } + + protected final boolean tryReleaseShared(int unused) { + HoldCounter rh = cachedHoldCounter; + Thread current = Thread.currentThread(); + if (rh == null || rh.tid != current.getId()) + rh = readHolds.get(); + if (rh.tryDecrement() <= 0) + throw new IllegalMonitorStateException(); + for (;;) { + int c = getState(); + int nextc = c - SHARED_UNIT; + if (compareAndSetState(c, nextc)) + return nextc == 0; + } + } + + protected final int tryAcquireShared(int unused) { + /* + * Walkthrough: + * 1. If write lock held by another thread, fail + * 2. If count saturated, throw error + * 3. Otherwise, this thread is eligible for + * lock wrt state, so ask if it should block + * because of queue policy. If not, try + * to grant by CASing state and updating count. + * Note that step does not check for reentrant + * acquires, which is postponed to full version + * to avoid having to check hold count in + * the more typical non-reentrant case. + * 4. If step 3 fails either because thread + * apparently not eligible or CAS fails, + * chain to version with full retry loop. + */ + Thread current = Thread.currentThread(); + int c = getState(); + if (exclusiveCount(c) != 0 && + getExclusiveOwnerThread() != current) + return -1; + if (sharedCount(c) == MAX_COUNT) + throw new Error("Maximum lock count exceeded"); + if (!readerShouldBlock(current) && + compareAndSetState(c, c + SHARED_UNIT)) { + HoldCounter rh = cachedHoldCounter; + if (rh == null || rh.tid != current.getId()) + cachedHoldCounter = rh = readHolds.get(); + rh.count++; + return 1; + } + return fullTryAcquireShared(current); + } + + /** + * Full version of acquire for reads, that handles CAS misses + * and reentrant reads not dealt with in tryAcquireShared. + */ + final int fullTryAcquireShared(Thread current) { + /* + * This code is in part redundant with that in + * tryAcquireShared but is simpler overall by not + * complicating tryAcquireShared with interactions between + * retries and lazily reading hold counts. + */ + HoldCounter rh = cachedHoldCounter; + if (rh == null || rh.tid != current.getId()) + rh = readHolds.get(); + for (;;) { + int c = getState(); + int w = exclusiveCount(c); + if ((w != 0 && getExclusiveOwnerThread() != current) || + ((rh.count | w) == 0 && readerShouldBlock(current))) + return -1; + if (sharedCount(c) == MAX_COUNT) + throw new Error("Maximum lock count exceeded"); + if (compareAndSetState(c, c + SHARED_UNIT)) { + cachedHoldCounter = rh; // cache for release + rh.count++; + return 1; + } + } + } + + /** + * Performs tryLock for write, enabling barging in both modes. + * This is identical in effect to tryAcquire except for lack + * of calls to writerShouldBlock + */ + final boolean tryWriteLock() { + Thread current = Thread.currentThread(); + int c = getState(); + if (c != 0) { + int w = exclusiveCount(c); + if (w == 0 ||current != getExclusiveOwnerThread()) + return false; + if (w == MAX_COUNT) + throw new Error("Maximum lock count exceeded"); + } + if (!compareAndSetState(c, c + 1)) + return false; + setExclusiveOwnerThread(current); + return true; + } + + /** + * Performs tryLock for read, enabling barging in both modes. + * This is identical in effect to tryAcquireShared except for + * lack of calls to readerShouldBlock + */ + final boolean tryReadLock() { + Thread current = Thread.currentThread(); + for (;;) { + int c = getState(); + if (exclusiveCount(c) != 0 && + getExclusiveOwnerThread() != current) + return false; + if (sharedCount(c) == MAX_COUNT) + throw new Error("Maximum lock count exceeded"); + if (compareAndSetState(c, c + SHARED_UNIT)) { + HoldCounter rh = cachedHoldCounter; + if (rh == null || rh.tid != current.getId()) + cachedHoldCounter = rh = readHolds.get(); + rh.count++; + return true; + } + } + } + + protected final boolean isHeldExclusively() { + // While we must in general read state before owner, + // we don't need to do so to check if current thread is owner + return getExclusiveOwnerThread() == Thread.currentThread(); + } + + // Methods relayed to outer class + + final ConditionObject newCondition() { + return new ConditionObject(); + } + + final Thread getOwner() { + // Must read state before owner to ensure memory consistency + return ((exclusiveCount(getState()) == 0)? + null : + getExclusiveOwnerThread()); + } + + final int getReadLockCount() { + return sharedCount(getState()); + } + + final boolean isWriteLocked() { + return exclusiveCount(getState()) != 0; + } + + final int getWriteHoldCount() { + return isHeldExclusively() ? exclusiveCount(getState()) : 0; + } + + final int getReadHoldCount() { + return getReadLockCount() == 0? 0 : readHolds.get().count; + } + + /** + * Reconstitute this lock instance from a stream + * @param s the stream + */ + private void readObject(java.io.ObjectInputStream s) + throws java.io.IOException, ClassNotFoundException { + s.defaultReadObject(); + readHolds = new ThreadLocalHoldCounter(); + setState(0); // reset to unlocked state + } + + final int getCount() { return getState(); } + } + + /** + * Nonfair version of Sync + */ + final static class NonfairSync extends Sync { + private static final long serialVersionUID = -8159625535654395037L; + final boolean writerShouldBlock(Thread current) { + return false; // writers can always barge + } + final boolean readerShouldBlock(Thread current) { + /* As a heuristic to avoid indefinite writer starvation, + * block if the thread that momentarily appears to be head + * of queue, if one exists, is a waiting writer. This is + * only a probablistic effect since a new reader will not + * block if there is a waiting writer behind other enabled + * readers that have not yet drained from the queue. + */ + return apparentlyFirstQueuedIsExclusive(); + } + } + + /** + * Fair version of Sync + */ + final static class FairSync extends Sync { + private static final long serialVersionUID = -2274990926593161451L; + final boolean writerShouldBlock(Thread current) { + // only proceed if queue is empty or current thread at head + return !isFirst(current); + } + final boolean readerShouldBlock(Thread current) { + // only proceed if queue is empty or current thread at head + return !isFirst(current); + } + } + + /** + * The lock returned by method {@link ReentrantReadWriteLock#readLock}. + */ + public static class ReadLock implements Lock, java.io.Serializable { + private static final long serialVersionUID = -5992448646407690164L; + private final Sync sync; + + /** + * Constructor for use by subclasses + * + * @param lock the outer lock object + * @throws NullPointerException if the lock is null + */ + protected ReadLock(ReentrantReadWriteLock lock) { + sync = lock.sync; + } + + /** + * Acquires the read lock. + * + * <p>Acquires the read lock if the write lock is not held by + * another thread and returns immediately. + * + * <p>If the write lock is held by another thread then + * the current thread becomes disabled for thread scheduling + * purposes and lies dormant until the read lock has been acquired. + */ + public void lock() { + sync.acquireShared(1); + } + + /** + * Acquires the read lock unless the current thread is + * {@linkplain Thread#interrupt interrupted}. + * + * <p>Acquires the read lock if the write lock is not held + * by another thread and returns immediately. + * + * <p>If the write lock is held by another thread then the + * current thread becomes disabled for thread scheduling + * purposes and lies dormant until one of two things happens: + * + * <ul> + * + * <li>The read lock is acquired by the current thread; or + * + * <li>Some other thread {@linkplain Thread#interrupt interrupts} + * the current thread. + * + * </ul> + * + * <p>If the current thread: + * + * <ul> + * + * <li>has its interrupted status set on entry to this method; or + * + * <li>is {@linkplain Thread#interrupt interrupted} while + * acquiring the read lock, + * + * </ul> + * + * then {@link InterruptedException} is thrown and the current + * thread's interrupted status is cleared. + * + * <p>In this implementation, as this method is an explicit + * interruption point, preference is given to responding to + * the interrupt over normal or reentrant acquisition of the + * lock. + * + * @throws InterruptedException if the current thread is interrupted + */ + public void lockInterruptibly() throws InterruptedException { + sync.acquireSharedInterruptibly(1); + } + + /** + * Acquires the read lock only if the write lock is not held by + * another thread at the time of invocation. + * + * <p>Acquires the read lock if the write lock is not held by + * another thread and returns immediately with the value + * {@code true}. Even when this lock has been set to use a + * fair ordering policy, a call to {@code tryLock()} + * <em>will</em> immediately acquire the read lock if it is + * available, whether or not other threads are currently + * waiting for the read lock. This "barging" behavior + * can be useful in certain circumstances, even though it + * breaks fairness. If you want to honor the fairness setting + * for this lock, then use {@link #tryLock(long, TimeUnit) + * tryLock(0, TimeUnit.SECONDS) } which is almost equivalent + * (it also detects interruption). + * + * <p>If the write lock is held by another thread then + * this method will return immediately with the value + * {@code false}. + * + * @return {@code true} if the read lock was acquired + */ + public boolean tryLock() { + return sync.tryReadLock(); + } + + /** + * Acquires the read lock if the write lock is not held by + * another thread within the given waiting time and the + * current thread has not been {@linkplain Thread#interrupt + * interrupted}. + * + * <p>Acquires the read lock if the write lock is not held by + * another thread and returns immediately with the value + * {@code true}. If this lock has been set to use a fair + * ordering policy then an available lock <em>will not</em> be + * acquired if any other threads are waiting for the + * lock. This is in contrast to the {@link #tryLock()} + * method. If you want a timed {@code tryLock} that does + * permit barging on a fair lock then combine the timed and + * un-timed forms together: + * + * <pre>if (lock.tryLock() || lock.tryLock(timeout, unit) ) { ... } + * </pre> + * + * <p>If the write lock is held by another thread then the + * current thread becomes disabled for thread scheduling + * purposes and lies dormant until one of three things happens: + * + * <ul> + * + * <li>The read lock is acquired by the current thread; or + * + * <li>Some other thread {@linkplain Thread#interrupt interrupts} + * the current thread; or + * + * <li>The specified waiting time elapses. + * + * </ul> + * + * <p>If the read lock is acquired then the value {@code true} is + * returned. + * + * <p>If the current thread: + * + * <ul> + * + * <li>has its interrupted status set on entry to this method; or + * + * <li>is {@linkplain Thread#interrupt interrupted} while + * acquiring the read lock, + * + * </ul> then {@link InterruptedException} is thrown and the + * current thread's interrupted status is cleared. + * + * <p>If the specified waiting time elapses then the value + * {@code false} is returned. If the time is less than or + * equal to zero, the method will not wait at all. + * + * <p>In this implementation, as this method is an explicit + * interruption point, preference is given to responding to + * the interrupt over normal or reentrant acquisition of the + * lock, and over reporting the elapse of the waiting time. + * + * @param timeout the time to wait for the read lock + * @param unit the time unit of the timeout argument + * @return {@code true} if the read lock was acquired + * @throws InterruptedException if the current thread is interrupted + * @throws NullPointerException if the time unit is null + * + */ + public boolean tryLock(long timeout, TimeUnit unit) throws InterruptedException { + return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout)); + } + + /** + * Attempts to release this lock. + * + * <p> If the number of readers is now zero then the lock + * is made available for write lock attempts. + */ + public void unlock() { + sync.releaseShared(1); + } + + /** + * Throws {@code UnsupportedOperationException} because + * {@code ReadLocks} do not support conditions. + * + * @throws UnsupportedOperationException always + */ + public Condition newCondition() { + throw new UnsupportedOperationException(); + } + + /** + * Returns a string identifying this lock, as well as its lock state. + * The state, in brackets, includes the String {@code "Read locks ="} + * followed by the number of held read locks. + * + * @return a string identifying this lock, as well as its lock state + */ + public String toString() { + int r = sync.getReadLockCount(); + return super.toString() + + "[Read locks = " + r + "]"; + } + } + + /** + * The lock returned by method {@link ReentrantReadWriteLock#writeLock}. + */ + public static class WriteLock implements Lock, java.io.Serializable { + private static final long serialVersionUID = -4992448646407690164L; + private final Sync sync; + + /** + * Constructor for use by subclasses + * + * @param lock the outer lock object + * @throws NullPointerException if the lock is null + */ + protected WriteLock(ReentrantReadWriteLock lock) { + sync = lock.sync; + } + + /** + * Acquires the write lock. + * + * <p>Acquires the write lock if neither the read nor write lock + * are held by another thread + * and returns immediately, setting the write lock hold count to + * one. + * + * <p>If the current thread already holds the write lock then the + * hold count is incremented by one and the method returns + * immediately. + * + * <p>If the lock is held by another thread then the current + * thread becomes disabled for thread scheduling purposes and + * lies dormant until the write lock has been acquired, at which + * time the write lock hold count is set to one. + */ + public void lock() { + sync.acquire(1); + } + + /** + * Acquires the write lock unless the current thread is + * {@linkplain Thread#interrupt interrupted}. + * + * <p>Acquires the write lock if neither the read nor write lock + * are held by another thread + * and returns immediately, setting the write lock hold count to + * one. + * + * <p>If the current thread already holds this lock then the + * hold count is incremented by one and the method returns + * immediately. + * + * <p>If the lock is held by another thread then the current + * thread becomes disabled for thread scheduling purposes and + * lies dormant until one of two things happens: + * + * <ul> + * + * <li>The write lock is acquired by the current thread; or + * + * <li>Some other thread {@linkplain Thread#interrupt interrupts} + * the current thread. + * + * </ul> + * + * <p>If the write lock is acquired by the current thread then the + * lock hold count is set to one. + * + * <p>If the current thread: + * + * <ul> + * + * <li>has its interrupted status set on entry to this method; + * or + * + * <li>is {@linkplain Thread#interrupt interrupted} while + * acquiring the write lock, + * + * </ul> + * + * then {@link InterruptedException} is thrown and the current + * thread's interrupted status is cleared. + * + * <p>In this implementation, as this method is an explicit + * interruption point, preference is given to responding to + * the interrupt over normal or reentrant acquisition of the + * lock. + * + * @throws InterruptedException if the current thread is interrupted + */ + public void lockInterruptibly() throws InterruptedException { + sync.acquireInterruptibly(1); + } + + /** + * Acquires the write lock only if it is not held by another thread + * at the time of invocation. + * + * <p>Acquires the write lock if neither the read nor write lock + * are held by another thread + * and returns immediately with the value {@code true}, + * setting the write lock hold count to one. Even when this lock has + * been set to use a fair ordering policy, a call to + * {@code tryLock()} <em>will</em> immediately acquire the + * lock if it is available, whether or not other threads are + * currently waiting for the write lock. This "barging" + * behavior can be useful in certain circumstances, even + * though it breaks fairness. If you want to honor the + * fairness setting for this lock, then use {@link + * #tryLock(long, TimeUnit) tryLock(0, TimeUnit.SECONDS) } + * which is almost equivalent (it also detects interruption). + * + * <p> If the current thread already holds this lock then the + * hold count is incremented by one and the method returns + * {@code true}. + * + * <p>If the lock is held by another thread then this method + * will return immediately with the value {@code false}. + * + * @return {@code true} if the lock was free and was acquired + * by the current thread, or the write lock was already held + * by the current thread; and {@code false} otherwise. + */ + public boolean tryLock( ) { + return sync.tryWriteLock(); + } + + /** + * Acquires the write lock if it is not held by another thread + * within the given waiting time and the current thread has + * not been {@linkplain Thread#interrupt interrupted}. + * + * <p>Acquires the write lock if neither the read nor write lock + * are held by another thread + * and returns immediately with the value {@code true}, + * setting the write lock hold count to one. If this lock has been + * set to use a fair ordering policy then an available lock + * <em>will not</em> be acquired if any other threads are + * waiting for the write lock. This is in contrast to the {@link + * #tryLock()} method. If you want a timed {@code tryLock} + * that does permit barging on a fair lock then combine the + * timed and un-timed forms together: + * + * <pre>if (lock.tryLock() || lock.tryLock(timeout, unit) ) { ... } + * </pre> + * + * <p>If the current thread already holds this lock then the + * hold count is incremented by one and the method returns + * {@code true}. + * + * <p>If the lock is held by another thread then the current + * thread becomes disabled for thread scheduling purposes and + * lies dormant until one of three things happens: + * + * <ul> + * + * <li>The write lock is acquired by the current thread; or + * + * <li>Some other thread {@linkplain Thread#interrupt interrupts} + * the current thread; or + * + * <li>The specified waiting time elapses + * + * </ul> + * + * <p>If the write lock is acquired then the value {@code true} is + * returned and the write lock hold count is set to one. + * + * <p>If the current thread: + * + * <ul> + * + * <li>has its interrupted status set on entry to this method; + * or + * + * <li>is {@linkplain Thread#interrupt interrupted} while + * acquiring the write lock, + * + * </ul> + * + * then {@link InterruptedException} is thrown and the current + * thread's interrupted status is cleared. + * + * <p>If the specified waiting time elapses then the value + * {@code false} is returned. If the time is less than or + * equal to zero, the method will not wait at all. + * + * <p>In this implementation, as this method is an explicit + * interruption point, preference is given to responding to + * the interrupt over normal or reentrant acquisition of the + * lock, and over reporting the elapse of the waiting time. + * + * @param timeout the time to wait for the write lock + * @param unit the time unit of the timeout argument + * + * @return {@code true} if the lock was free and was acquired + * by the current thread, or the write lock was already held by the + * current thread; and {@code false} if the waiting time + * elapsed before the lock could be acquired. + * + * @throws InterruptedException if the current thread is interrupted + * @throws NullPointerException if the time unit is null + * + */ + public boolean tryLock(long timeout, TimeUnit unit) throws InterruptedException { + return sync.tryAcquireNanos(1, unit.toNanos(timeout)); + } + + /** + * Attempts to release this lock. + * + * <p>If the current thread is the holder of this lock then + * the hold count is decremented. If the hold count is now + * zero then the lock is released. If the current thread is + * not the holder of this lock then {@link + * IllegalMonitorStateException} is thrown. + * + * @throws IllegalMonitorStateException if the current thread does not + * hold this lock. + */ + public void unlock() { + sync.release(1); + } + + /** + * Returns a {@link Condition} instance for use with this + * {@link Lock} instance. + * <p>The returned {@link Condition} instance supports the same + * usages as do the {@link Object} monitor methods ({@link + * Object#wait() wait}, {@link Object#notify notify}, and {@link + * Object#notifyAll notifyAll}) when used with the built-in + * monitor lock. + * + * <ul> + * + * <li>If this write lock is not held when any {@link + * Condition} method is called then an {@link + * IllegalMonitorStateException} is thrown. (Read locks are + * held independently of write locks, so are not checked or + * affected. However it is essentially always an error to + * invoke a condition waiting method when the current thread + * has also acquired read locks, since other threads that + * could unblock it will not be able to acquire the write + * lock.) + * + * <li>When the condition {@linkplain Condition#await() waiting} + * methods are called the write lock is released and, before + * they return, the write lock is reacquired and the lock hold + * count restored to what it was when the method was called. + * + * <li>If a thread is {@linkplain Thread#interrupt interrupted} while + * waiting then the wait will terminate, an {@link + * InterruptedException} will be thrown, and the thread's + * interrupted status will be cleared. + * + * <li> Waiting threads are signalled in FIFO order. + * + * <li>The ordering of lock reacquisition for threads returning + * from waiting methods is the same as for threads initially + * acquiring the lock, which is in the default case not specified, + * but for <em>fair</em> locks favors those threads that have been + * waiting the longest. + * + * </ul> + * + * @return the Condition object + */ + public Condition newCondition() { + return sync.newCondition(); + } + + /** + * Returns a string identifying this lock, as well as its lock + * state. The state, in brackets includes either the String + * {@code "Unlocked"} or the String {@code "Locked by"} + * followed by the {@linkplain Thread#getName name} of the owning thread. + * + * @return a string identifying this lock, as well as its lock state + */ + public String toString() { + Thread o = sync.getOwner(); + return super.toString() + ((o == null) ? + "[Unlocked]" : + "[Locked by thread " + o.getName() + "]"); + } + + /** + * Queries if this write lock is held by the current thread. + * Identical in effect to {@link + * ReentrantReadWriteLock#isWriteLockedByCurrentThread}. + * + * @return {@code true} if the current thread holds this lock and + * {@code false} otherwise + * @since 1.6 + */ + public boolean isHeldByCurrentThread() { + return sync.isHeldExclusively(); + } + + /** + * Queries the number of holds on this write lock by the current + * thread. A thread has a hold on a lock for each lock action + * that is not matched by an unlock action. Identical in effect + * to {@link ReentrantReadWriteLock#getWriteHoldCount}. + * + * @return the number of holds on this lock by the current thread, + * or zero if this lock is not held by the current thread + * @since 1.6 + */ + public int getHoldCount() { + return sync.getWriteHoldCount(); + } + } + + // Instrumentation and status + + /** + * Returns {@code true} if this lock has fairness set true. + * + * @return {@code true} if this lock has fairness set true + */ + public final boolean isFair() { + return sync instanceof FairSync; + } + + /** + * Returns the thread that currently owns the write lock, or + * {@code null} if not owned. When this method is called by a + * thread that is not the owner, the return value reflects a + * best-effort approximation of current lock status. For example, + * the owner may be momentarily {@code null} even if there are + * threads trying to acquire the lock but have not yet done so. + * This method is designed to facilitate construction of + * subclasses that provide more extensive lock monitoring + * facilities. + * + * @return the owner, or {@code null} if not owned + */ + protected Thread getOwner() { + return sync.getOwner(); + } + + /** + * Queries the number of read locks held for this lock. This + * method is designed for use in monitoring system state, not for + * synchronization control. + * @return the number of read locks held. + */ + public int getReadLockCount() { + return sync.getReadLockCount(); + } + + /** + * Queries if the write lock is held by any thread. This method is + * designed for use in monitoring system state, not for + * synchronization control. + * + * @return {@code true} if any thread holds the write lock and + * {@code false} otherwise + */ + public boolean isWriteLocked() { + return sync.isWriteLocked(); + } + + /** + * Queries if the write lock is held by the current thread. + * + * @return {@code true} if the current thread holds the write lock and + * {@code false} otherwise + */ + public boolean isWriteLockedByCurrentThread() { + return sync.isHeldExclusively(); + } + + /** + * Queries the number of reentrant write holds on this lock by the + * current thread. A writer thread has a hold on a lock for + * each lock action that is not matched by an unlock action. + * + * @return the number of holds on the write lock by the current thread, + * or zero if the write lock is not held by the current thread + */ + public int getWriteHoldCount() { + return sync.getWriteHoldCount(); + } + + /** + * Queries the number of reentrant read holds on this lock by the + * current thread. A reader thread has a hold on a lock for + * each lock action that is not matched by an unlock action. + * + * @return the number of holds on the read lock by the current thread, + * or zero if the read lock is not held by the current thread + * @since 1.6 + */ + public int getReadHoldCount() { + return sync.getReadHoldCount(); + } + + /** + * Returns a collection containing threads that may be waiting to + * acquire the write lock. Because the actual set of threads may + * change dynamically while constructing this result, the returned + * collection is only a best-effort estimate. The elements of the + * returned collection are in no particular order. This method is + * designed to facilitate construction of subclasses that provide + * more extensive lock monitoring facilities. + * + * @return the collection of threads + */ + protected Collection<Thread> getQueuedWriterThreads() { + return sync.getExclusiveQueuedThreads(); + } + + /** + * Returns a collection containing threads that may be waiting to + * acquire the read lock. Because the actual set of threads may + * change dynamically while constructing this result, the returned + * collection is only a best-effort estimate. The elements of the + * returned collection are in no particular order. This method is + * designed to facilitate construction of subclasses that provide + * more extensive lock monitoring facilities. + * + * @return the collection of threads + */ + protected Collection<Thread> getQueuedReaderThreads() { + return sync.getSharedQueuedThreads(); + } + + /** + * Queries whether any threads are waiting to acquire the read or + * write lock. Note that because cancellations may occur at any + * time, a {@code true} return does not guarantee that any other + * thread will ever acquire a lock. This method is designed + * primarily for use in monitoring of the system state. + * + * @return {@code true} if there may be other threads waiting to + * acquire the lock + */ + public final boolean hasQueuedThreads() { + return sync.hasQueuedThreads(); + } + + /** + * Queries whether the given thread is waiting to acquire either + * the read or write lock. Note that because cancellations may + * occur at any time, a {@code true} return does not guarantee + * that this thread will ever acquire a lock. This method is + * designed primarily for use in monitoring of the system state. + * + * @param thread the thread + * @return {@code true} if the given thread is queued waiting for this lock + * @throws NullPointerException if the thread is null + */ + public final boolean hasQueuedThread(Thread thread) { + return sync.isQueued(thread); + } + + /** + * Returns an estimate of the number of threads waiting to acquire + * either the read or write lock. The value is only an estimate + * because the number of threads may change dynamically while this + * method traverses internal data structures. This method is + * designed for use in monitoring of the system state, not for + * synchronization control. + * + * @return the estimated number of threads waiting for this lock + */ + public final int getQueueLength() { + return sync.getQueueLength(); + } + + /** + * Returns a collection containing threads that may be waiting to + * acquire either the read or write lock. Because the actual set + * of threads may change dynamically while constructing this + * result, the returned collection is only a best-effort estimate. + * The elements of the returned collection are in no particular + * order. This method is designed to facilitate construction of + * subclasses that provide more extensive monitoring facilities. + * + * @return the collection of threads + */ + protected Collection<Thread> getQueuedThreads() { + return sync.getQueuedThreads(); + } + + /** + * Queries whether any threads are waiting on the given condition + * associated with the write lock. Note that because timeouts and + * interrupts may occur at any time, a {@code true} return does + * not guarantee that a future {@code signal} will awaken any + * threads. This method is designed primarily for use in + * monitoring of the system state. + * + * @param condition the condition + * @return {@code true} if there are any waiting threads + * @throws IllegalMonitorStateException if this lock is not held + * @throws IllegalArgumentException if the given condition is + * not associated with this lock + * @throws NullPointerException if the condition is null + */ + public boolean hasWaiters(Condition condition) { + if (condition == null) + throw new NullPointerException(); + if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject)) + throw new IllegalArgumentException("not owner"); + return sync.hasWaiters((AbstractQueuedSynchronizer.ConditionObject)condition); + } + + /** + * Returns an estimate of the number of threads waiting on the + * given condition associated with the write lock. Note that because + * timeouts and interrupts may occur at any time, the estimate + * serves only as an upper bound on the actual number of waiters. + * This method is designed for use in monitoring of the system + * state, not for synchronization control. + * + * @param condition the condition + * @return the estimated number of waiting threads + * @throws IllegalMonitorStateException if this lock is not held + * @throws IllegalArgumentException if the given condition is + * not associated with this lock + * @throws NullPointerException if the condition is null + */ + public int getWaitQueueLength(Condition condition) { + if (condition == null) + throw new NullPointerException(); + if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject)) + throw new IllegalArgumentException("not owner"); + return sync.getWaitQueueLength((AbstractQueuedSynchronizer.ConditionObject)condition); + } + + /** + * Returns a collection containing those threads that may be + * waiting on the given condition associated with the write lock. + * Because the actual set of threads may change dynamically while + * constructing this result, the returned collection is only a + * best-effort estimate. The elements of the returned collection + * are in no particular order. This method is designed to + * facilitate construction of subclasses that provide more + * extensive condition monitoring facilities. + * + * @param condition the condition + * @return the collection of threads + * @throws IllegalMonitorStateException if this lock is not held + * @throws IllegalArgumentException if the given condition is + * not associated with this lock + * @throws NullPointerException if the condition is null + */ + protected Collection<Thread> getWaitingThreads(Condition condition) { + if (condition == null) + throw new NullPointerException(); + if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject)) + throw new IllegalArgumentException("not owner"); + return sync.getWaitingThreads((AbstractQueuedSynchronizer.ConditionObject)condition); + } + + /** + * Returns a string identifying this lock, as well as its lock state. + * The state, in brackets, includes the String {@code "Write locks ="} + * followed by the number of reentrantly held write locks, and the + * String {@code "Read locks ="} followed by the number of held + * read locks. + * + * @return a string identifying this lock, as well as its lock state + */ + public String toString() { + int c = sync.getCount(); + int w = Sync.exclusiveCount(c); + int r = Sync.sharedCount(c); + + return super.toString() + + "[Write locks = " + w + ", Read locks = " + r + "]"; + } + +} |