/* * 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; import java.util.Collection; import java.util.Queue; /** * A {@link java.util.Queue} that additionally supports operations * that wait for the queue to become non-empty when retrieving an * element, and wait for space to become available in the queue when * storing an element. * *

BlockingQueue methods come in four forms, with different ways * of handling operations that cannot be satisfied immediately, but may be * satisfied at some point in the future: * one throws an exception, the second returns a special value (either * null or false, depending on the operation), the third * blocks the current thread indefinitely until the operation can succeed, * and the fourth blocks for only a given maximum time limit before giving * up. These methods are summarized in the following table: * *

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
Throws exceptionSpecial valueBlocksTimes out
Insert{@link #add add(e)}{@link #offer offer(e)}{@link #put put(e)}{@link #offer(Object, long, TimeUnit) offer(e, time, unit)}
Remove{@link #remove remove()}{@link #poll poll()}{@link #take take()}{@link #poll(long, TimeUnit) poll(time, unit)}
Examine{@link #element element()}{@link #peek peek()}not applicablenot applicable
* *

A BlockingQueue does not accept null elements. * Implementations throw NullPointerException on attempts * to add, put or offer a null. A * null is used as a sentinel value to indicate failure of * poll operations. * *

A BlockingQueue may be capacity bounded. At any given * time it may have a remainingCapacity beyond which no * additional elements can be put without blocking. * A BlockingQueue without any intrinsic capacity constraints always * reports a remaining capacity of Integer.MAX_VALUE. * *

BlockingQueue implementations are designed to be used * primarily for producer-consumer queues, but additionally support * the {@link java.util.Collection} interface. So, for example, it is * possible to remove an arbitrary element from a queue using * remove(x). However, such operations are in general * not performed very efficiently, and are intended for only * occasional use, such as when a queued message is cancelled. * *

BlockingQueue implementations are thread-safe. All * queuing methods achieve their effects atomically using internal * locks or other forms of concurrency control. However, the * bulk Collection operations addAll, * containsAll, retainAll and removeAll are * not necessarily performed atomically unless specified * otherwise in an implementation. So it is possible, for example, for * addAll(c) to fail (throwing an exception) after adding * only some of the elements in c. * *

A BlockingQueue does not intrinsically support * any kind of "close" or "shutdown" operation to * indicate that no more items will be added. The needs and usage of * such features tend to be implementation-dependent. For example, a * common tactic is for producers to insert special * end-of-stream or poison objects, that are * interpreted accordingly when taken by consumers. * *

* Usage example, based on a typical producer-consumer scenario. * Note that a BlockingQueue can safely be used with multiple * producers and multiple consumers. *

 * class Producer implements Runnable {
 *   private final BlockingQueue queue;
 *   Producer(BlockingQueue q) { queue = q; }
 *   public void run() {
 *     try {
 *       while (true) { queue.put(produce()); }
 *     } catch (InterruptedException ex) { ... handle ...}
 *   }
 *   Object produce() { ... }
 * }
 *
 * class Consumer implements Runnable {
 *   private final BlockingQueue queue;
 *   Consumer(BlockingQueue q) { queue = q; }
 *   public void run() {
 *     try {
 *       while (true) { consume(queue.take()); }
 *     } catch (InterruptedException ex) { ... handle ...}
 *   }
 *   void consume(Object x) { ... }
 * }
 *
 * class Setup {
 *   void main() {
 *     BlockingQueue q = new SomeQueueImplementation();
 *     Producer p = new Producer(q);
 *     Consumer c1 = new Consumer(q);
 *     Consumer c2 = new Consumer(q);
 *     new Thread(p).start();
 *     new Thread(c1).start();
 *     new Thread(c2).start();
 *   }
 * }
 * 
* *

Memory consistency effects: As with other concurrent * collections, actions in a thread prior to placing an object into a * {@code BlockingQueue} * happen-before * actions subsequent to the access or removal of that element from * the {@code BlockingQueue} in another thread. * *

This interface is a member of the * * Java Collections Framework. * * @since 1.5 * @author Doug Lea * @param the type of elements held in this collection */ public interface BlockingQueue extends Queue { /** * Inserts the specified element into this queue if it is possible to do * so immediately without violating capacity restrictions, returning * true upon success and throwing an * IllegalStateException if no space is currently available. * When using a capacity-restricted queue, it is generally preferable to * use {@link #offer(Object) offer}. * * @param e the element to add * @return true (as specified by {@link Collection#add}) * @throws IllegalStateException if the element cannot be added at this * time due to capacity restrictions * @throws ClassCastException if the class of the specified element * prevents it from being added to this queue * @throws NullPointerException if the specified element is null * @throws IllegalArgumentException if some property of the specified * element prevents it from being added to this queue */ boolean add(E e); /** * Inserts the specified element into this queue if it is possible to do * so immediately without violating capacity restrictions, returning * true upon success and false if no space is currently * available. When using a capacity-restricted queue, this method is * generally preferable to {@link #add}, which can fail to insert an * element only by throwing an exception. * * @param e the element to add * @return true if the element was added to this queue, else * false * @throws ClassCastException if the class of the specified element * prevents it from being added to this queue * @throws NullPointerException if the specified element is null * @throws IllegalArgumentException if some property of the specified * element prevents it from being added to this queue */ boolean offer(E e); /** * Inserts the specified element into this queue, waiting if necessary * for space to become available. * * @param e the element to add * @throws InterruptedException if interrupted while waiting * @throws ClassCastException if the class of the specified element * prevents it from being added to this queue * @throws NullPointerException if the specified element is null * @throws IllegalArgumentException if some property of the specified * element prevents it from being added to this queue */ void put(E e) throws InterruptedException; /** * Inserts the specified element into this queue, waiting up to the * specified wait time if necessary for space to become available. * * @param e the element to add * @param timeout how long to wait before giving up, in units of * unit * @param unit a TimeUnit determining how to interpret the * timeout parameter * @return true if successful, or false if * the specified waiting time elapses before space is available * @throws InterruptedException if interrupted while waiting * @throws ClassCastException if the class of the specified element * prevents it from being added to this queue * @throws NullPointerException if the specified element is null * @throws IllegalArgumentException if some property of the specified * element prevents it from being added to this queue */ boolean offer(E e, long timeout, TimeUnit unit) throws InterruptedException; /** * Retrieves and removes the head of this queue, waiting if necessary * until an element becomes available. * * @return the head of this queue * @throws InterruptedException if interrupted while waiting */ E take() throws InterruptedException; /** * Retrieves and removes the head of this queue, waiting up to the * specified wait time if necessary for an element to become available. * * @param timeout how long to wait before giving up, in units of * unit * @param unit a TimeUnit determining how to interpret the * timeout parameter * @return the head of this queue, or null if the * specified waiting time elapses before an element is available * @throws InterruptedException if interrupted while waiting */ E poll(long timeout, TimeUnit unit) throws InterruptedException; /** * Returns the number of additional elements that this queue can ideally * (in the absence of memory or resource constraints) accept without * blocking, or Integer.MAX_VALUE if there is no intrinsic * limit. * *

Note that you cannot always tell if an attempt to insert * an element will succeed by inspecting remainingCapacity * because it may be the case that another thread is about to * insert or remove an element. * * @return the remaining capacity */ int remainingCapacity(); /** * Removes a single instance of the specified element from this queue, * if it is present. More formally, removes an element e such * that o.equals(e), if this queue contains one or more such * elements. * Returns true if this queue contained the specified element * (or equivalently, if this queue changed as a result of the call). * * @param o element to be removed from this queue, if present * @return true if this queue changed as a result of the call * @throws ClassCastException if the class of the specified element * is incompatible with this queue (optional) * @throws NullPointerException if the specified element is null (optional) */ boolean remove(Object o); /** * Returns true if this queue contains the specified element. * More formally, returns true if and only if this queue contains * at least one element e such that o.equals(e). * * @param o object to be checked for containment in this queue * @return true if this queue contains the specified element * @throws ClassCastException if the class of the specified element * is incompatible with this queue (optional) * @throws NullPointerException if the specified element is null (optional) */ public boolean contains(Object o); /** * Removes all available elements from this queue and adds them * to the given collection. This operation may be more * efficient than repeatedly polling this queue. A failure * encountered while attempting to add elements to * collection c may result in elements being in neither, * either or both collections when the associated exception is * thrown. Attempts to drain a queue to itself result in * IllegalArgumentException. Further, the behavior of * this operation is undefined if the specified collection is * modified while the operation is in progress. * * @param c the collection to transfer elements into * @return the number of elements transferred * @throws UnsupportedOperationException if addition of elements * is not supported by the specified collection * @throws ClassCastException if the class of an element of this queue * prevents it from being added to the specified collection * @throws NullPointerException if the specified collection is null * @throws IllegalArgumentException if the specified collection is this * queue, or some property of an element of this queue prevents * it from being added to the specified collection */ int drainTo(Collection c); /** * Removes at most the given number of available elements from * this queue and adds them to the given collection. A failure * encountered while attempting to add elements to * collection c may result in elements being in neither, * either or both collections when the associated exception is * thrown. Attempts to drain a queue to itself result in * IllegalArgumentException. Further, the behavior of * this operation is undefined if the specified collection is * modified while the operation is in progress. * * @param c the collection to transfer elements into * @param maxElements the maximum number of elements to transfer * @return the number of elements transferred * @throws UnsupportedOperationException if addition of elements * is not supported by the specified collection * @throws ClassCastException if the class of an element of this queue * prevents it from being added to the specified collection * @throws NullPointerException if the specified collection is null * @throws IllegalArgumentException if the specified collection is this * queue, or some property of an element of this queue prevents * it from being added to the specified collection */ int drainTo(Collection c, int maxElements); }