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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 &quot;barging&quot; 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 &quot;barging&quot;
+         * 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 + "]";
+    }
+
+}