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. 19 * 20 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 21 * or visit www.oracle.com if you need additional information or have any 22 * questions. 23 */ 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 36 package java.util.concurrent; 37 38 /** 39 * A {@code Future} represents the result of an asynchronous 40 * computation. Methods are provided to check if the computation is 41 * complete, to wait for its completion, and to retrieve the result of 42 * the computation. The result can only be retrieved using method 43 * {@code get} when the computation has completed, blocking if 44 * necessary until it is ready. Cancellation is performed by the 45 * {@code cancel} method. Additional methods are provided to 46 * determine if the task completed normally or was cancelled. Once a 47 * computation has completed, the computation cannot be cancelled. 48 * If you would like to use a {@code Future} for the sake 49 * of cancellability but not provide a usable result, you can 50 * declare types of the form {@code Future<?>} and 51 * return {@code null} as a result of the underlying task. 52 * 53 * <p> 54 * <b>Sample Usage</b> (Note that the following classes are all 55 * made-up.) 56 * 57 * <pre> {@code 58 * interface ArchiveSearcher { String search(String target); } 59 * class App { 60 * ExecutorService executor = ... 61 * ArchiveSearcher searcher = ... 62 * void showSearch(final String target) 63 * throws InterruptedException { 64 * Future<String> future 65 * = executor.submit(new Callable<String>() { 66 * public String call() { 67 * return searcher.search(target); 68 * }}); 69 * displayOtherThings(); // do other things while searching 70 * try { 71 * displayText(future.get()); // use future 72 * } catch (ExecutionException ex) { cleanup(); return; } 73 * } 74 * }}</pre> 75 * 76 * The {@link FutureTask} class is an implementation of {@code Future} that 77 * implements {@code Runnable}, and so may be executed by an {@code Executor}. 78 * For example, the above construction with {@code submit} could be replaced by: 79 * <pre> {@code 80 * FutureTask<String> future = 81 * new FutureTask<>(new Callable<String>() { 82 * public String call() { 83 * return searcher.search(target); 84 * }}); 85 * executor.execute(future);}</pre> 86 * 87 * <p>Memory consistency effects: Actions taken by the asynchronous computation 88 * <a href="package-summary.html#MemoryVisibility"> <i>happen-before</i></a> 89 * actions following the corresponding {@code Future.get()} in another thread. 90 * 91 * @see FutureTask 92 * @see Executor 93 * @since 1.5 94 * @author Doug Lea 95 * @param <V> The result type returned by this Future's {@code get} method 96 */ 97 public interface Future<V> { 98 99 /** 100 * Attempts to cancel execution of this task. This attempt will 101 * fail if the task has already completed, has already been cancelled, 102 * or could not be cancelled for some other reason. If successful, 103 * and this task has not started when {@code cancel} is called, 104 * this task should never run. If the task has already started, 105 * then the {@code mayInterruptIfRunning} parameter determines 106 * whether the thread executing this task should be interrupted in 107 * an attempt to stop the task. 108 * 109 * <p>After this method returns, subsequent calls to {@link #isDone} will 110 * always return {@code true}. Subsequent calls to {@link #isCancelled} 111 * will always return {@code true} if this method returned {@code true}. 112 * 113 * @param mayInterruptIfRunning {@code true} if the thread executing this 114 * task should be interrupted; otherwise, in-progress tasks are allowed 115 * to complete 116 * @return {@code false} if the task could not be cancelled, 117 * typically because it has already completed normally; 118 * {@code true} otherwise 119 */ cancel(boolean mayInterruptIfRunning)120 boolean cancel(boolean mayInterruptIfRunning); 121 122 /** 123 * Returns {@code true} if this task was cancelled before it completed 124 * normally. 125 * 126 * @return {@code true} if this task was cancelled before it completed 127 */ isCancelled()128 boolean isCancelled(); 129 130 /** 131 * Returns {@code true} if this task completed. 132 * 133 * Completion may be due to normal termination, an exception, or 134 * cancellation -- in all of these cases, this method will return 135 * {@code true}. 136 * 137 * @return {@code true} if this task completed 138 */ isDone()139 boolean isDone(); 140 141 /** 142 * Waits if necessary for the computation to complete, and then 143 * retrieves its result. 144 * 145 * @return the computed result 146 * @throws CancellationException if the computation was cancelled 147 * @throws ExecutionException if the computation threw an 148 * exception 149 * @throws InterruptedException if the current thread was interrupted 150 * while waiting 151 */ get()152 V get() throws InterruptedException, ExecutionException; 153 154 /** 155 * Waits if necessary for at most the given time for the computation 156 * to complete, and then retrieves its result, if available. 157 * 158 * @param timeout the maximum time to wait 159 * @param unit the time unit of the timeout argument 160 * @return the computed result 161 * @throws CancellationException if the computation was cancelled 162 * @throws ExecutionException if the computation threw an 163 * exception 164 * @throws InterruptedException if the current thread was interrupted 165 * while waiting 166 * @throws TimeoutException if the wait timed out 167 */ get(long timeout, TimeUnit unit)168 V get(long timeout, TimeUnit unit) 169 throws InterruptedException, ExecutionException, TimeoutException; 170 } 171