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/licenses/publicdomain 34 */ 35 36 package java.util.concurrent; 37 38 /** 39 * A <tt>Future</tt> 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 * <tt>get</tt> when the computation has completed, blocking if 44 * necessary until it is ready. Cancellation is performed by the 45 * <tt>cancel</tt> 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 <tt>Future</tt> for the sake 49 * of cancellability but not provide a usable result, you can 50 * declare types of the form <tt>Future<?></tt> and 51 * return <tt>null</tt> 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.) <p> 56 * <pre> 57 * interface ArchiveSearcher { String search(String target); } 58 * class App { 59 * ExecutorService executor = ... 60 * ArchiveSearcher searcher = ... 61 * void showSearch(final String target) 62 * throws InterruptedException { 63 * Future<String> future 64 * = executor.submit(new Callable<String>() { 65 * public String call() { 66 * return searcher.search(target); 67 * }}); 68 * displayOtherThings(); // do other things while searching 69 * try { 70 * displayText(future.get()); // use future 71 * } catch (ExecutionException ex) { cleanup(); return; } 72 * } 73 * } 74 * </pre> 75 * 76 * The {@link FutureTask} class is an implementation of <tt>Future</tt> that 77 * implements <tt>Runnable</tt>, and so may be executed by an <tt>Executor</tt>. 78 * For example, the above construction with <tt>submit</tt> could be replaced by: 79 * <pre> 80 * FutureTask<String> future = 81 * new FutureTask<String>(new Callable<String>() { 82 * public String call() { 83 * return searcher.search(target); 84 * }}); 85 * executor.execute(future); 86 * </pre> 87 * 88 * <p>Memory consistency effects: Actions taken by the asynchronous computation 89 * <a href="package-summary.html#MemoryVisibility"> <i>happen-before</i></a> 90 * actions following the corresponding {@code Future.get()} in another thread. 91 * 92 * @see FutureTask 93 * @see Executor 94 * @since 1.5 95 * @author Doug Lea 96 * @param <V> The result type returned by this Future's <tt>get</tt> method 97 */ 98 public interface Future<V> { 99 100 /** 101 * Attempts to cancel execution of this task. This attempt will 102 * fail if the task has already completed, has already been cancelled, 103 * or could not be cancelled for some other reason. If successful, 104 * and this task has not started when <tt>cancel</tt> is called, 105 * this task should never run. If the task has already started, 106 * then the <tt>mayInterruptIfRunning</tt> parameter determines 107 * whether the thread executing this task should be interrupted in 108 * an attempt to stop the task. 109 * 110 * <p>After this method returns, subsequent calls to {@link #isDone} will 111 * always return <tt>true</tt>. Subsequent calls to {@link #isCancelled} 112 * will always return <tt>true</tt> if this method returned <tt>true</tt>. 113 * 114 * @param mayInterruptIfRunning <tt>true</tt> if the thread executing this 115 * task should be interrupted; otherwise, in-progress tasks are allowed 116 * to complete 117 * @return <tt>false</tt> if the task could not be cancelled, 118 * typically because it has already completed normally; 119 * <tt>true</tt> otherwise 120 */ 121 boolean cancel(boolean mayInterruptIfRunning); 122 123 /** 124 * Returns <tt>true</tt> if this task was cancelled before it completed 125 * normally. 126 * 127 * @return <tt>true</tt> if this task was cancelled before it completed 128 */ 129 boolean isCancelled(); 130 131 /** 132 * Returns <tt>true</tt> if this task completed. 133 * 134 * Completion may be due to normal termination, an exception, or 135 * cancellation -- in all of these cases, this method will return 136 * <tt>true</tt>. 137 * 138 * @return <tt>true</tt> if this task completed 139 */ 140 boolean isDone(); 141 142 /** 143 * Waits if necessary for the computation to complete, and then 144 * retrieves its result. 145 * 146 * @return the computed result 147 * @throws CancellationException if the computation was cancelled 148 * @throws ExecutionException if the computation threw an 149 * exception 150 * @throws InterruptedException if the current thread was interrupted 151 * while waiting 152 */ 153 V get() throws InterruptedException, ExecutionException; 154 155 /** 156 * Waits if necessary for at most the given time for the computation 157 * to complete, and then retrieves its result, if available. 158 * 159 * @param timeout the maximum time to wait 160 * @param unit the time unit of the timeout argument 161 * @return the computed result 162 * @throws CancellationException if the computation was cancelled 163 * @throws ExecutionException if the computation threw an 164 * exception 165 * @throws InterruptedException if the current thread was interrupted 166 * while waiting 167 * @throws TimeoutException if the wait timed out 168 */ 169 V get(long timeout, TimeUnit unit) 170 throws InterruptedException, ExecutionException, TimeoutException; 171 }