{@code 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 * {@code null} or {@code 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 exception | *Special value | *Blocks | *Times 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 applicable | *not applicable | *
A {@code BlockingQueue} does not accept {@code null} elements. * Implementations throw {@code NullPointerException} on attempts * to {@code add}, {@code put} or {@code offer} a {@code null}. A * {@code null} is used as a sentinel value to indicate failure of * {@code poll} operations. * *
A {@code BlockingQueue} may be capacity bounded. At any given * time it may have a {@code remainingCapacity} beyond which no * additional elements can be {@code put} without blocking. * A {@code BlockingQueue} without any intrinsic capacity constraints always * reports a remaining capacity of {@code Integer.MAX_VALUE}. * *
{@code 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 * {@code 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. * *
{@code 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 {@code addAll}, * {@code containsAll}, {@code retainAll} and {@code removeAll} are * not necessarily performed atomically unless specified * otherwise in an implementation. So it is possible, for example, for * {@code addAll(c)} to fail (throwing an exception) after adding * only some of the elements in {@code c}. * *
A {@code 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 {@code BlockingQueue} can safely be used with multiple * producers and multiple consumers. *
{@code
* 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.
*
* @since 1.5
* @author Doug Lea
* @param Note that you cannot always tell if an attempt to insert
* an element will succeed by inspecting {@code 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 {@code e} such
* that {@code o.equals(e)}, if this queue contains one or more such
* elements.
* Returns {@code 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 {@code 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 {@code true} if this queue contains the specified element.
* More formally, returns {@code true} if and only if this queue contains
* at least one element {@code e} such that {@code o.equals(e)}.
*
* @param o object to be checked for containment in this queue
* @return {@code 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)
*/
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 {@code 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
* {@code 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 {@code 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
* {@code 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);
}