Java tutorial
/* * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * * This code is free software; you can redistribute it and/or modify it * under the terms of the GNU General Public License version 2 only, as * published by the Free Software Foundation. Oracle designates this * particular file as subject to the "Classpath" exception as provided * by Oracle in the LICENSE file that accompanied this code. * * This code is distributed in the hope that it will be useful, but WITHOUT * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License * version 2 for more details (a copy is included in the LICENSE file that * accompanied this code). * * You should have received a copy of the GNU General Public License version * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. */ /* * This file is available under and governed by the GNU General Public * License version 2 only, as published by the Free Software Foundation. * However, the following notice accompanied the original version of this * file: * * 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/publicdomain/zero/1.0/ */ package java.util.concurrent; import java.util.Collection; import java.util.Queue; /** * A {@link 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>{@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: * * <table class="plain"> * <caption>Summary of BlockingQueue methods</caption> * <tr> * <td></td> * <th scope="col" style="font-weight:normal; font-style:italic">Throws exception</th> * <th scope="col" style="font-weight:normal; font-style:italic">Special value</th> * <th scope="col" style="font-weight:normal; font-style:italic">Blocks</th> * <th scope="col" style="font-weight:normal; font-style:italic">Times out</th> * </tr> * <tr> * <th scope="row" style="text-align:left">Insert</th> * <td>{@link #add(Object) add(e)}</td> * <td>{@link #offer(Object) offer(e)}</td> * <td>{@link #put(Object) put(e)}</td> * <td>{@link #offer(Object, long, TimeUnit) offer(e, time, unit)}</td> * </tr> * <tr> * <th scope="row" style="text-align:left">Remove</th> * <td>{@link #remove() remove()}</td> * <td>{@link #poll() poll()}</td> * <td>{@link #take() take()}</td> * <td>{@link #poll(long, TimeUnit) poll(time, unit)}</td> * </tr> * <tr> * <th scope="row" style="text-align:left">Examine</th> * <td>{@link #element() element()}</td> * <td>{@link #peek() peek()}</td> * <td style="font-style: italic">not applicable</td> * <td style="font-style: italic">not applicable</td> * </tr> * </table> * * <p>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. * * <p>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}. * * <p>{@code BlockingQueue} implementations are designed to be used * primarily for producer-consumer queues, but additionally support * the {@link 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 * <em>not</em> performed very efficiently, and are intended for only * occasional use, such as when a queued message is cancelled. * * <p>{@code BlockingQueue} 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 {@code addAll}, * {@code containsAll}, {@code retainAll} and {@code removeAll} are * <em>not</em> 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}. * * <p>A {@code BlockingQueue} 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 {@code BlockingQueue} can safely be used with multiple * producers and multiple consumers. * <pre> {@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(); * } * }}</pre> * * <p>Memory consistency effects: As with other concurrent * collections, actions in a thread prior to placing an object into a * {@code BlockingQueue} * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> * actions subsequent to the access or removal of that element from * the {@code BlockingQueue} in another thread. * * <p>This interface is a member of the * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework"> * Java Collections Framework</a>. * * @since 1.5 * @author Doug Lea * @param <E> the type of elements held in this queue */ public interface BlockingQueue<E> extends Queue<E> { /** * Inserts the specified element into this queue if it is possible to do * so immediately without violating capacity restrictions, returning * {@code true} upon success and throwing an * {@code 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 {@code 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 * {@code true} upon success and {@code 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 {@code true} if the element was added to this queue, else * {@code 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 * {@code unit} * @param unit a {@code TimeUnit} determining how to interpret the * {@code timeout} parameter * @return {@code true} if successful, or {@code 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 * {@code unit} * @param unit a {@code TimeUnit} determining how to interpret the * {@code timeout} parameter * @return the head of this queue, or {@code 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 {@code Integer.MAX_VALUE} if there is no intrinsic * limit. * * <p>Note that you <em>cannot</em> 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 * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>) * @throws NullPointerException if the specified element is null * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>) */ 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 * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>) * @throws NullPointerException if the specified element is null * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>) */ 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<? super E> 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<? super E> c, int maxElements); }