1 /* 2 * Written by Doug Lea with assistance from members of JCP JSR-166 3 * Expert Group and released to the public domain, as explained at 4 * http://creativecommons.org/publicdomain/zero/1.0/ 5 */ 6 7 package java.util.concurrent; 8 9 // BEGIN android-note 10 // removed link to collections framework docs 11 // END android-note 12 13 /** 14 * A {@link BlockingQueue} in which producers may wait for consumers 15 * to receive elements. A {@code TransferQueue} may be useful for 16 * example in message passing applications in which producers 17 * sometimes (using method {@link #transfer}) await receipt of 18 * elements by consumers invoking {@code take} or {@code poll}, while 19 * at other times enqueue elements (via method {@code put}) without 20 * waiting for receipt. 21 * {@linkplain #tryTransfer(Object) Non-blocking} and 22 * {@linkplain #tryTransfer(Object,long,TimeUnit) time-out} versions of 23 * {@code tryTransfer} are also available. 24 * A {@code TransferQueue} may also be queried, via {@link 25 * #hasWaitingConsumer}, whether there are any threads waiting for 26 * items, which is a converse analogy to a {@code peek} operation. 27 * 28 * <p>Like other blocking queues, a {@code TransferQueue} may be 29 * capacity bounded. If so, an attempted transfer operation may 30 * initially block waiting for available space, and/or subsequently 31 * block waiting for reception by a consumer. Note that in a queue 32 * with zero capacity, such as {@link SynchronousQueue}, {@code put} 33 * and {@code transfer} are effectively synonymous. 34 * 35 * @since 1.7 36 * @author Doug Lea 37 * @param <E> the type of elements held in this collection 38 */ 39 public interface TransferQueue<E> extends BlockingQueue<E> { 40 /** 41 * Transfers the element to a waiting consumer immediately, if possible. 42 * 43 * <p>More precisely, transfers the specified element immediately 44 * if there exists a consumer already waiting to receive it (in 45 * {@link #take} or timed {@link #poll(long,TimeUnit) poll}), 46 * otherwise returning {@code false} without enqueuing the element. 47 * 48 * @param e the element to transfer 49 * @return {@code true} if the element was transferred, else 50 * {@code false} 51 * @throws ClassCastException if the class of the specified element 52 * prevents it from being added to this queue 53 * @throws NullPointerException if the specified element is null 54 * @throws IllegalArgumentException if some property of the specified 55 * element prevents it from being added to this queue 56 */ 57 boolean tryTransfer(E e); 58 59 /** 60 * Transfers the element to a consumer, waiting if necessary to do so. 61 * 62 * <p>More precisely, transfers the specified element immediately 63 * if there exists a consumer already waiting to receive it (in 64 * {@link #take} or timed {@link #poll(long,TimeUnit) poll}), 65 * else waits until the element is received by a consumer. 66 * 67 * @param e the element to transfer 68 * @throws InterruptedException if interrupted while waiting, 69 * in which case the element is not left enqueued 70 * @throws ClassCastException if the class of the specified element 71 * prevents it from being added to this queue 72 * @throws NullPointerException if the specified element is null 73 * @throws IllegalArgumentException if some property of the specified 74 * element prevents it from being added to this queue 75 */ 76 void transfer(E e) throws InterruptedException; 77 78 /** 79 * Transfers the element to a consumer if it is possible to do so 80 * before the timeout elapses. 81 * 82 * <p>More precisely, transfers the specified element immediately 83 * if there exists a consumer already waiting to receive it (in 84 * {@link #take} or timed {@link #poll(long,TimeUnit) poll}), 85 * else waits until the element is received by a consumer, 86 * returning {@code false} if the specified wait time elapses 87 * before the element can be transferred. 88 * 89 * @param e the element to transfer 90 * @param timeout how long to wait before giving up, in units of 91 * {@code unit} 92 * @param unit a {@code TimeUnit} determining how to interpret the 93 * {@code timeout} parameter 94 * @return {@code true} if successful, or {@code false} if 95 * the specified waiting time elapses before completion, 96 * in which case the element is not left enqueued 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 boolean tryTransfer(E e, long timeout, TimeUnit unit) 106 throws InterruptedException; 107 108 /** 109 * Returns {@code true} if there is at least one consumer waiting 110 * to receive an element via {@link #take} or 111 * timed {@link #poll(long,TimeUnit) poll}. 112 * The return value represents a momentary state of affairs. 113 * 114 * @return {@code true} if there is at least one waiting consumer 115 */ 116 boolean hasWaitingConsumer(); 117 118 /** 119 * Returns an estimate of the number of consumers waiting to 120 * receive elements via {@link #take} or timed 121 * {@link #poll(long,TimeUnit) poll}. The return value is an 122 * approximation of a momentary state of affairs, that may be 123 * inaccurate if consumers have completed or given up waiting. 124 * The value may be useful for monitoring and heuristics, but 125 * not for synchronization control. Implementations of this 126 * method are likely to be noticeably slower than those for 127 * {@link #hasWaitingConsumer}. 128 * 129 * @return the number of consumers waiting to receive elements 130 */ 131 int getWaitingConsumerCount(); 132 } 133
