J. Duke | 319a3b9 | 2007-12-01 00:00:00 +0000 | [diff] [blame^] | 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. Sun designates this |
| 7 | * particular file as subject to the "Classpath" exception as provided |
| 8 | * by Sun 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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara, |
| 21 | * CA 95054 USA or visit www.sun.com if you need additional information or |
| 22 | * have any 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 | } |