TransferQueue.java revision 29957558cf0db700bfaae360a80c42dc3871d0e5 
1/*
2 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
3 *
4 * This code is free software; you can redistribute it and/or modify it
5 * under the terms of the GNU General Public License version 2 only, as
6 * published by the Free Software Foundation.  Oracle designates this
7 * particular file as subject to the "Classpath" exception as provided
8 * by Oracle in the LICENSE file that accompanied this code.
9 *
10 * This code is distributed in the hope that it will be useful, but WITHOUT
11 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
12 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
13 * version 2 for more details (a copy is included in the LICENSE file that
14 * accompanied this code).
15 *
16 * You should have received a copy of the GNU General Public License version
17 * 2 along with this work; if not, write to the Free Software Foundation,
18 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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20 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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24
25/*
26 * This file is available under and governed by the GNU General Public
27 * License version 2 only, as published by the Free Software Foundation.
28 * However, the following notice accompanied the original version of this
29 * file:
30 *
31 * Written by Doug Lea with assistance from members of JCP JSR-166
32 * Expert Group and released to the public domain, as explained at
33 * http://creativecommons.org/publicdomain/zero/1.0/
34 */
35
36package java.util.concurrent;
37
38// BEGIN android-note
39// removed link to collections framework docs
40// END android-note
41
42/**
43 * A {@link BlockingQueue} in which producers may wait for consumers
44 * to receive elements.  A {@code TransferQueue} may be useful for
45 * example in message passing applications in which producers
46 * sometimes (using method {@link #transfer}) await receipt of
47 * elements by consumers invoking {@code take} or {@code poll}, while
48 * at other times enqueue elements (via method {@code put}) without
49 * waiting for receipt.
50 * {@linkplain #tryTransfer(Object) Non-blocking} and
51 * {@linkplain #tryTransfer(Object,long,TimeUnit) time-out} versions of
52 * {@code tryTransfer} are also available.
53 * A {@code TransferQueue} may also be queried, via {@link
54 * #hasWaitingConsumer}, whether there are any threads waiting for
55 * items, which is a converse analogy to a {@code peek} operation.
56 *
57 * <p>Like other blocking queues, a {@code TransferQueue} may be
58 * capacity bounded.  If so, an attempted transfer operation may
59 * initially block waiting for available space, and/or subsequently
60 * block waiting for reception by a consumer.  Note that in a queue
61 * with zero capacity, such as {@link SynchronousQueue}, {@code put}
62 * and {@code transfer} are effectively synonymous.
63 *
64 * @since 1.7
65 * @author Doug Lea
66 * @param <E> the type of elements held in this queue
67 */
68public interface TransferQueue<E> extends BlockingQueue<E> {
69    /**
70     * Transfers the element to a waiting consumer immediately, if possible.
71     *
72     * <p>More precisely, transfers the specified element immediately
73     * if there exists a consumer already waiting to receive it (in
74     * {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
75     * otherwise returning {@code false} without enqueuing the element.
76     *
77     * @param e the element to transfer
78     * @return {@code true} if the element was transferred, else
79     *         {@code false}
80     * @throws ClassCastException if the class of the specified element
81     *         prevents it from being added to this queue
82     * @throws NullPointerException if the specified element is null
83     * @throws IllegalArgumentException if some property of the specified
84     *         element prevents it from being added to this queue
85     */
86    boolean tryTransfer(E e);
87
88    /**
89     * Transfers the element to a consumer, waiting if necessary to do so.
90     *
91     * <p>More precisely, transfers the specified element immediately
92     * if there exists a consumer already waiting to receive it (in
93     * {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
94     * else waits until the element is received by a consumer.
95     *
96     * @param e the element to transfer
97     * @throws InterruptedException if interrupted while waiting,
98     *         in which case the element is not left enqueued
99     * @throws ClassCastException if the class of the specified element
100     *         prevents it from being added to this queue
101     * @throws NullPointerException if the specified element is null
102     * @throws IllegalArgumentException if some property of the specified
103     *         element prevents it from being added to this queue
104     */
105    void transfer(E e) throws InterruptedException;
106
107    /**
108     * Transfers the element to a consumer if it is possible to do so
109     * before the timeout elapses.
110     *
111     * <p>More precisely, transfers the specified element immediately
112     * if there exists a consumer already waiting to receive it (in
113     * {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
114     * else waits until the element is received by a consumer,
115     * returning {@code false} if the specified wait time elapses
116     * before the element can be transferred.
117     *
118     * @param e the element to transfer
119     * @param timeout how long to wait before giving up, in units of
120     *        {@code unit}
121     * @param unit a {@code TimeUnit} determining how to interpret the
122     *        {@code timeout} parameter
123     * @return {@code true} if successful, or {@code false} if
124     *         the specified waiting time elapses before completion,
125     *         in which case the element is not left enqueued
126     * @throws InterruptedException if interrupted while waiting,
127     *         in which case the element is not left enqueued
128     * @throws ClassCastException if the class of the specified element
129     *         prevents it from being added to this queue
130     * @throws NullPointerException if the specified element is null
131     * @throws IllegalArgumentException if some property of the specified
132     *         element prevents it from being added to this queue
133     */
134    boolean tryTransfer(E e, long timeout, TimeUnit unit)
135        throws InterruptedException;
136
137    /**
138     * Returns {@code true} if there is at least one consumer waiting
139     * to receive an element via {@link #take} or
140     * timed {@link #poll(long,TimeUnit) poll}.
141     * The return value represents a momentary state of affairs.
142     *
143     * @return {@code true} if there is at least one waiting consumer
144     */
145    boolean hasWaitingConsumer();
146
147    /**
148     * Returns an estimate of the number of consumers waiting to
149     * receive elements via {@link #take} or timed
150     * {@link #poll(long,TimeUnit) poll}.  The return value is an
151     * approximation of a momentary state of affairs, that may be
152     * inaccurate if consumers have completed or given up waiting.
153     * The value may be useful for monitoring and heuristics, but
154     * not for synchronization control.  Implementations of this
155     * method are likely to be noticeably slower than those for
156     * {@link #hasWaitingConsumer}.
157     *
158     * @return the number of consumers waiting to receive elements
159     */
160    int getWaitingConsumerCount();
161}
162