/*
* @(#)BlockingQueue.java 1.8 04/07/12
*
* Copyright 2004 Sun Microsystems, Inc. All rights reserved.
* SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*/
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.
*
* <p>A <tt>BlockingQueue</tt> does not accept <tt>null</tt> elements.
* Implementations throw <tt>NullPointerException</tt> on attempts
* to <tt>add</tt>, <tt>put</tt> or <tt>offer</tt> a <tt>null</tt>. A
* <tt>null</tt> is used as a sentinel value to indicate failure of
* <tt>poll</tt> operations.
*
* <p>A <tt>BlockingQueue</tt> may be capacity bounded. At any given
* time it may have a <tt>remainingCapacity</tt> beyond which no
* additional elements can be <tt>put</tt> without blocking.
* A <tt>BlockingQueue</tt> without any intrinsic capacity constraints always
* reports a remaining capacity of <tt>Integer.MAX_VALUE</tt>.
*
* <p> <tt>BlockingQueue</tt> 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
* <tt>remove(x)</tt>. However, such operations are in general
* <em>not</em> performed very efficiently, and are intended for only
* occasional use, such as when a queued message is cancelled.
*
* <p> <tt>BlockingQueue</tt> implementations are thread-safe. All
* queuing methods achieve their effects atomically using internal
* locks or other forms of concurrency control. However, the
* <em>bulk</em> Collection operations <tt>addAll</tt>,
* <tt>containsAll</tt>, <tt>retainAll</tt> and <tt>removeAll</tt> are
* <em>not</em> necessarily performed atomically unless specified
* otherwise in an implementation. So it is possible, for example, for
* <tt>addAll(c)</tt> to fail (throwing an exception) after adding
* only some of the elements in <tt>c</tt>.
*
* <p>A <tt>BlockingQueue</tt> does <em>not</em> 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
* <em>end-of-stream</em> or <em>poison</em> objects, that are
* interpreted accordingly when taken by consumers.
*
* <p>
* Usage example, based on a typical producer-consumer scenario.
* Note that a <tt>BlockingQueue</tt> can safely be used with multiple
* producers and multiple consumers.
* <pre>
* 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();
* }
* }
* </pre>
*
* <p>This interface is a member of the
* <a href="{@docRoot}/../guide/collections/index.html">
* Java Collections Framework</a>.
*
* @since 1.5
* @author Doug Lea
* @param <E> the type of elements held in this collection
*/
public interface BlockingQueue<E> extends Queue<E> {
/**
* Inserts the specified element into this queue, if possible. When
* using queues that may impose insertion restrictions (for
* example capacity bounds), method <tt>offer</tt> is generally
* preferable to method {@link Collection#add}, which can fail to
* insert an element only by throwing an exception.
*
* @param o the element to add.
* @return <tt>true</tt> if it was possible to add the element to
* this queue, else <tt>false</tt>
* @throws NullPointerException if the specified element is <tt>null</tt>
*/
boolean offer(E o);
/**
* Inserts the specified element into this queue, waiting if necessary
* up to the specified wait time for space to become available.
* @param o the element to add
* @param timeout how long to wait before giving up, in units of
* <tt>unit</tt>
* @param unit a <tt>TimeUnit</tt> determining how to interpret the
* <tt>timeout</tt> parameter
* @return <tt>true</tt> if successful, or <tt>false</tt> if
* the specified waiting time elapses before space is available.
* @throws InterruptedException if interrupted while waiting.
* @throws NullPointerException if the specified element is <tt>null</tt>.
*/
boolean offer(E o, long timeout, TimeUnit unit)
throws InterruptedException;
/**
* Retrieves and removes the head of this queue, waiting
* if necessary up to the specified wait time if no elements are
* present on this queue.
* @param timeout how long to wait before giving up, in units of
* <tt>unit</tt>
* @param unit a <tt>TimeUnit</tt> determining how to interpret the
* <tt>timeout</tt> parameter
* @return the head of this queue, or <tt>null</tt> if the
* specified waiting time elapses before an element is present.
* @throws InterruptedException if interrupted while waiting.
*/
E poll(long timeout, TimeUnit unit)
throws InterruptedException;
/**
* Retrieves and removes the head of this queue, waiting
* if no elements are present on this queue.
* @return the head of this queue
* @throws InterruptedException if interrupted while waiting.
*/
E take() throws InterruptedException;
/**
* Adds the specified element to this queue, waiting if necessary for
* space to become available.
* @param o the element to add
* @throws InterruptedException if interrupted while waiting.
* @throws NullPointerException if the specified element is <tt>null</tt>.
*/
void put(E o) throws InterruptedException;
/**
* Returns the number of elements that this queue can ideally (in
* the absence of memory or resource constraints) accept without
* blocking, or <tt>Integer.MAX_VALUE</tt> if there is no
* intrinsic limit.
* <p>Note that you <em>cannot</em> always tell if
* an attempt to <tt>add</tt> an element will succeed by
* inspecting <tt>remainingCapacity</tt> because it may be the
* case that another thread is about to <tt>put</tt> or <tt>take</tt> an
* element.
* @return the remaining capacity
*/
int remainingCapacity();
/**
* Adds the specified element to this queue if it is possible to
* do so immediately, returning <tt>true</tt> upon success, else
* throwing an IllegalStateException.
* @param o the element
* @return <tt>true</tt> (as per the general contract of
* <tt>Collection.add</tt>).
*
* @throws NullPointerException if the specified element is <tt>null</tt>
* @throws IllegalStateException if element cannot be added
*/
boolean add(E o);
/**
* Removes all available elements from this queue and adds them
* into the given collection. This operation may be more
* efficient than repeatedly polling this queue. A failure
* encountered while attempting to <tt>add</tt> elements to
* collection <tt>c</tt> 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
* <tt>IllegalArgumentException</tt>. 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 NullPointerException if c is null
* @throws IllegalArgumentException if c is this queue
*
*/
int drainTo(Collection<? super E> c);
/**
* Removes at most the given number of available elements from
* this queue and adds them into the given collection. A failure
* encountered while attempting to <tt>add</tt> elements to
* collection <tt>c</tt> 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
* <tt>IllegalArgumentException</tt>. 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 NullPointerException if c is null
* @throws IllegalArgumentException if c is this queue
*/
int drainTo(Collection<? super E> c, int maxElements);
}
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