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 /** 10 * A {@code Future} represents the result of an asynchronous 11 * computation. Methods are provided to check if the computation is 12 * complete, to wait for its completion, and to retrieve the result of 13 * the computation. The result can only be retrieved using method 14 * {@code get} when the computation has completed, blocking if 15 * necessary until it is ready. Cancellation is performed by the 16 * {@code cancel} method. Additional methods are provided to 17 * determine if the task completed normally or was cancelled. Once a 18 * computation has completed, the computation cannot be cancelled. 19 * If you would like to use a {@code Future} for the sake 20 * of cancellability but not provide a usable result, you can 21 * declare types of the form {@code Future<?>} and 22 * return {@code null} as a result of the underlying task. 23 * 24 * <p> 25 * <b>Sample Usage</b> (Note that the following classes are all 26 * made-up.) <p> 27 * <pre> {@code 28 * interface ArchiveSearcher { String search(String target); } 29 * class App { 30 * ExecutorService executor = ... 31 * ArchiveSearcher searcher = ... 32 * void showSearch(final String target) 33 * throws InterruptedException { 34 * Future<String> future 35 * = executor.submit(new Callable<String>() { 36 * public String call() { 37 * return searcher.search(target); 38 * }}); 39 * displayOtherThings(); // do other things while searching 40 * try { 41 * displayText(future.get()); // use future 42 * } catch (ExecutionException ex) { cleanup(); return; } 43 * } 44 * }}</pre> 45 * 46 * The {@link FutureTask} class is an implementation of {@code Future} that 47 * implements {@code Runnable}, and so may be executed by an {@code Executor}. 48 * For example, the above construction with {@code submit} could be replaced by: 49 * <pre> {@code 50 * FutureTask<String> future = 51 * new FutureTask<String>(new Callable<String>() { 52 * public String call() { 53 * return searcher.search(target); 54 * }}); 55 * executor.execute(future);}</pre> 56 * 57 * <p>Memory consistency effects: Actions taken by the asynchronous computation 58 * <a href="package-summary.html#MemoryVisibility"> <i>happen-before</i></a> 59 * actions following the corresponding {@code Future.get()} in another thread. 60 * 61 * @see FutureTask 62 * @see Executor 63 * @since 1.5 64 * @author Doug Lea 65 * @param <V> The result type returned by this Future's {@code get} method 66 */ 67 public interface Future<V> { 68 69 /** 70 * Attempts to cancel execution of this task. This attempt will 71 * fail if the task has already completed, has already been cancelled, 72 * or could not be cancelled for some other reason. If successful, 73 * and this task has not started when {@code cancel} is called, 74 * this task should never run. If the task has already started, 75 * then the {@code mayInterruptIfRunning} parameter determines 76 * whether the thread executing this task should be interrupted in 77 * an attempt to stop the task. 78 * 79 * <p>After this method returns, subsequent calls to {@link #isDone} will 80 * always return {@code true}. Subsequent calls to {@link #isCancelled} 81 * will always return {@code true} if this method returned {@code true}. 82 * 83 * @param mayInterruptIfRunning {@code true} if the thread executing this 84 * task should be interrupted; otherwise, in-progress tasks are allowed 85 * to complete 86 * @return {@code false} if the task could not be cancelled, 87 * typically because it has already completed normally; 88 * {@code true} otherwise 89 */ 90 boolean cancel(boolean mayInterruptIfRunning); 91 92 /** 93 * Returns {@code true} if this task was cancelled before it completed 94 * normally. 95 * 96 * @return {@code true} if this task was cancelled before it completed 97 */ 98 boolean isCancelled(); 99 100 /** 101 * Returns {@code true} if this task completed. 102 * 103 * Completion may be due to normal termination, an exception, or 104 * cancellation -- in all of these cases, this method will return 105 * {@code true}. 106 * 107 * @return {@code true} if this task completed 108 */ 109 boolean isDone(); 110 111 /** 112 * Waits if necessary for the computation to complete, and then 113 * retrieves its result. 114 * 115 * @return the computed result 116 * @throws CancellationException if the computation was cancelled 117 * @throws ExecutionException if the computation threw an 118 * exception 119 * @throws InterruptedException if the current thread was interrupted 120 * while waiting 121 */ 122 V get() throws InterruptedException, ExecutionException; 123 124 /** 125 * Waits if necessary for at most the given time for the computation 126 * to complete, and then retrieves its result, if available. 127 * 128 * @param timeout the maximum time to wait 129 * @param unit the time unit of the timeout argument 130 * @return the computed result 131 * @throws CancellationException if the computation was cancelled 132 * @throws ExecutionException if the computation threw an 133 * exception 134 * @throws InterruptedException if the current thread was interrupted 135 * while waiting 136 * @throws TimeoutException if the wait timed out 137 */ 138 V get(long timeout, TimeUnit unit) 139 throws InterruptedException, ExecutionException, TimeoutException; 140 } 141
