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><b>Sample Usage</b> (Note that the following classes are all 54 * made-up.) 55 * 56 * <pre> {@code 57 * interface ArchiveSearcher { String search(String target); } 58 * class App { 59 * ExecutorService executor = ... 60 * ArchiveSearcher searcher = ... 61 * void showSearch(String target) throws InterruptedException { 62 * Callable<String> task = () -> searcher.search(target); 63 * Future<String> future = executor.submit(task); 64 * displayOtherThings(); // do other things while searching 65 * try { 66 * displayText(future.get()); // use future 67 * } catch (ExecutionException ex) { cleanup(); return; } 68 * } 69 * }}</pre> 70 * 71 * The {@link FutureTask} class is an implementation of {@code Future} that 72 * implements {@code Runnable}, and so may be executed by an {@code Executor}. 73 * For example, the above construction with {@code submit} could be replaced by: 74 * <pre> {@code 75 * FutureTask<String> future = new FutureTask<>(task); 76 * executor.execute(future);}</pre> 77 * 78 * <p>Memory consistency effects: Actions taken by the asynchronous computation 79 * <a href="package-summary.html#MemoryVisibility"> <i>happen-before</i></a> 80 * actions following the corresponding {@code Future.get()} in another thread. 81 * 82 * @see FutureTask 83 * @see Executor 84 * @since 1.5 85 * @author Doug Lea 86 * @param <V> The result type returned by this Future's {@code get} method 87 */ 88 public interface Future<V> { 89 90 /** 91 * Attempts to cancel execution of this task. This attempt will 92 * fail if the task has already completed, has already been cancelled, 93 * or could not be cancelled for some other reason. If successful, 94 * and this task has not started when {@code cancel} is called, 95 * this task should never run. If the task has already started, 96 * then the {@code mayInterruptIfRunning} parameter determines 97 * whether the thread executing this task should be interrupted in 98 * an attempt to stop the task. 99 * 100 * <p>After this method returns, subsequent calls to {@link #isDone} will 101 * always return {@code true}. Subsequent calls to {@link #isCancelled} 102 * will always return {@code true} if this method returned {@code true}. 103 * 104 * @param mayInterruptIfRunning {@code true} if the thread executing this 105 * task should be interrupted; otherwise, in-progress tasks are allowed 106 * to complete 107 * @return {@code false} if the task could not be cancelled, 108 * typically because it has already completed normally; 109 * {@code true} otherwise 110 */ cancel(boolean mayInterruptIfRunning)111 boolean cancel(boolean mayInterruptIfRunning); 112 113 /** 114 * Returns {@code true} if this task was cancelled before it completed 115 * normally. 116 * 117 * @return {@code true} if this task was cancelled before it completed 118 */ isCancelled()119 boolean isCancelled(); 120 121 /** 122 * Returns {@code true} if this task completed. 123 * 124 * Completion may be due to normal termination, an exception, or 125 * cancellation -- in all of these cases, this method will return 126 * {@code true}. 127 * 128 * @return {@code true} if this task completed 129 */ isDone()130 boolean isDone(); 131 132 /** 133 * Waits if necessary for the computation to complete, and then 134 * retrieves its result. 135 * 136 * @return the computed result 137 * @throws CancellationException if the computation was cancelled 138 * @throws ExecutionException if the computation threw an 139 * exception 140 * @throws InterruptedException if the current thread was interrupted 141 * while waiting 142 */ get()143 V get() throws InterruptedException, ExecutionException; 144 145 /** 146 * Waits if necessary for at most the given time for the computation 147 * to complete, and then retrieves its result, if available. 148 * 149 * @param timeout the maximum time to wait 150 * @param unit the time unit of the timeout argument 151 * @return the computed result 152 * @throws CancellationException if the computation was cancelled 153 * @throws ExecutionException if the computation threw an 154 * exception 155 * @throws InterruptedException if the current thread was interrupted 156 * while waiting 157 * @throws TimeoutException if the wait timed out 158 */ get(long timeout, TimeUnit unit)159 V get(long timeout, TimeUnit unit) 160 throws InterruptedException, ExecutionException, TimeoutException; 161 } 162