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; /** * A {@link BlockingQueue} in which producers may wait for consumers * to receive elements. A {@code TransferQueue} may be useful for * example in message passing applications in which producers * sometimes (using method {@link #transfer}) await receipt of * elements by consumers invoking {@code take} or {@code poll}, while * at other times enqueue elements (via method {@code put}) without * waiting for receipt. * {@linkplain #tryTransfer(Object) Non-blocking} and * {@linkplain #tryTransfer(Object,long,TimeUnit) time-out} versions of * {@code tryTransfer} are also available. * A {@code TransferQueue} may also be queried, via {@link * #hasWaitingConsumer}, whether there are any threads waiting for * items, which is a converse analogy to a {@code peek} operation. * * <p>Like other blocking queues, a {@code TransferQueue} may be * capacity bounded. If so, an attempted transfer operation may * initially block waiting for available space, and/or subsequently * block waiting for reception by a consumer. Note that in a queue * with zero capacity, such as {@link SynchronousQueue}, {@code put} * and {@code transfer} are effectively synonymous. * * <p>This interface is a member of the * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework"> * Java Collections Framework</a>. * * @since 1.7 * @author Doug Lea * @param <E> the type of elements held in this queue */ public interface TransferQueue<E> extends BlockingQueue<E> { /** * Transfers the element to a waiting consumer immediately, if possible. * * <p>More precisely, transfers the specified element immediately * if there exists a consumer already waiting to receive it (in * {@link #take} or timed {@link #poll(long,TimeUnit) poll}), * otherwise returning {@code false} without enqueuing the element. * * @param e the element to transfer * @return {@code true} if the element was transferred, 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 tryTransfer(E e); /** * Transfers the element to a consumer, waiting if necessary to do so. * * <p>More precisely, transfers the specified element immediately * if there exists a consumer already waiting to receive it (in * {@link #take} or timed {@link #poll(long,TimeUnit) poll}), * else waits until the element is received by a consumer. * * @param e the element to transfer * @throws InterruptedException if interrupted while waiting, * in which case the element is not left enqueued * @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 transfer(E e) throws InterruptedException; /** * Transfers the element to a consumer if it is possible to do so * before the timeout elapses. * * <p>More precisely, transfers the specified element immediately * if there exists a consumer already waiting to receive it (in * {@link #take} or timed {@link #poll(long,TimeUnit) poll}), * else waits until the element is received by a consumer, * returning {@code false} if the specified wait time elapses * before the element can be transferred. * * @param e the element to transfer * @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 completion, * in which case the element is not left enqueued * @throws InterruptedException if interrupted while waiting, * in which case the element is not left enqueued * @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 tryTransfer(E e, long timeout, TimeUnit unit) throws InterruptedException; /** * Returns {@code true} if there is at least one consumer waiting * to receive an element via {@link #take} or * timed {@link #poll(long,TimeUnit) poll}. * The return value represents a momentary state of affairs. * * @return {@code true} if there is at least one waiting consumer */ boolean hasWaitingConsumer(); /** * Returns an estimate of the number of consumers waiting to * receive elements via {@link #take} or timed * {@link #poll(long,TimeUnit) poll}. The return value is an * approximation of a momentary state of affairs, that may be * inaccurate if consumers have completed or given up waiting. * The value may be useful for monitoring and heuristics, but * not for synchronization control. Implementations of this * method are likely to be noticeably slower than those for * {@link #hasWaitingConsumer}. * * @return the number of consumers waiting to receive elements */ int getWaitingConsumerCount(); }