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 method has no
92 * effect if the task is already completed or cancelled, or could
93 * not be cancelled for some other reason. Otherwise, if this
94 * task has not started when {@code cancel} is called, this task
95 * should never run. If the task has already started, then the
96 * {@code mayInterruptIfRunning} parameter determines whether the
97 * thread executing this task (when known by the implementation)
98 * is interrupted in an attempt to stop the task.
99 *
100 * <p>The return value from this method does not necessarily
101 * indicate whether the task is now cancelled; use {@link
102 * #isCancelled}.
103 *
104 * @param mayInterruptIfRunning {@code true} if the thread
105 * executing this task should be interrupted (if the thread is
106 * known to the implementation); otherwise, in-progress tasks are
107 * allowed to complete
108 * @return {@code false} if the task could not be cancelled,
109 * typically because it has already completed; {@code true}
110 * otherwise. If two or more threads cause a task to be cancelled,
111 * then at least one of them returns {@code true}. Implementations
112 * may provide stronger guarantees.
113 */
114 boolean cancel(boolean mayInterruptIfRunning);
115
116 /**
117 * Returns {@code true} if this task was cancelled before it completed
118 * normally.
119 *
120 * @return {@code true} if this task was cancelled before it completed
121 */
122 boolean isCancelled();
123
124 /**
125 * Returns {@code true} if this task completed.
126 *
127 * Completion may be due to normal termination, an exception, or
128 * cancellation -- in all of these cases, this method will return
129 * {@code true}.
130 *
131 * @return {@code true} if this task completed
132 */
133 boolean isDone();
134
135 /**
136 * Waits if necessary for the computation to complete, and then
137 * retrieves its result.
138 *
139 * @return the computed result
140 * @throws CancellationException if the computation was cancelled
141 * @throws ExecutionException if the computation threw an
142 * exception
143 * @throws InterruptedException if the current thread was interrupted
144 * while waiting
145 */
146 V get() throws InterruptedException, ExecutionException;
147
148 /**
149 * Waits if necessary for at most the given time for the computation
150 * to complete, and then retrieves its result, if available.
151 *
152 * @param timeout the maximum time to wait
153 * @param unit the time unit of the timeout argument
154 * @return the computed result
155 * @throws CancellationException if the computation was cancelled
156 * @throws ExecutionException if the computation threw an
157 * exception
158 * @throws InterruptedException if the current thread was interrupted
159 * while waiting
160 * @throws TimeoutException if the wait timed out
161 */
162 V get(long timeout, TimeUnit unit)
163 throws InterruptedException, ExecutionException, TimeoutException;
164 }