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 */
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 */
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 */
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 */
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 */
159 V get(long timeout, TimeUnit unit)
160 throws InterruptedException, ExecutionException, TimeoutException;
161 }