| Future.java |
1 /*
2 * %W% %E%
3 *
4 * Copyright (c) 2006, Oracle and/or its affiliates. All rights reserved.
5 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
6 */
7
8 package java.util.concurrent;
9
10 /**
11 * A <tt>Future</tt> represents the result of an asynchronous
12 * computation. Methods are provided to check if the computation is
13 * complete, to wait for its completion, and to retrieve the result of
14 * the computation. The result can only be retrieved using method
15 * <tt>get</tt> when the computation has completed, blocking if
16 * necessary until it is ready. Cancellation is performed by the
17 * <tt>cancel</tt> method. Additional methods are provided to
18 * determine if the task completed normally or was cancelled. Once a
19 * computation has completed, the computation cannot be cancelled.
20 * If you would like to use a <tt>Future</tt> for the sake
21 * of cancellability but not provide a usable result, you can
22 * declare types of the form <tt>Future<?></tt> and
23 * return <tt>null</tt> as a result of the underlying task.
24 *
25 * <p>
26 * <b>Sample Usage</b> (Note that the following classes are all
27 * made-up.) <p>
28 * <pre>
29 * interface ArchiveSearcher { String search(String target); }
30 * class App {
31 * ExecutorService executor = ...
32 * ArchiveSearcher searcher = ...
33 * void showSearch(final String target)
34 * throws InterruptedException {
35 * Future<String> future
36 * = executor.submit(new Callable<String>() {
37 * public String call() {
38 * return searcher.search(target);
39 * }});
40 * displayOtherThings(); // do other things while searching
41 * try {
42 * displayText(future.get()); // use future
43 * } catch (ExecutionException ex) { cleanup(); return; }
44 * }
45 * }
46 * </pre>
47 *
48 * The {@link FutureTask} class is an implementation of <tt>Future</tt> that
49 * implements <tt>Runnable</tt>, and so may be executed by an <tt>Executor</tt>.
50 * For example, the above construction with <tt>submit</tt> could be replaced by:
51 * <pre>
52 * FutureTask<String> future =
53 * new FutureTask<String>(new Callable<String>() {
54 * public String call() {
55 * return searcher.search(target);
56 * }});
57 * executor.execute(future);
58 * </pre>
59 *
60 * <p>Memory consistency effects: Actions taken by the asynchronous computation
61 * <a href="package-summary.html#MemoryVisibility"> <i>happen-before</i></a>
62 * actions following the corresponding {@code Future.get()} in another thread.
63 *
64 * @see FutureTask
65 * @see Executor
66 * @since 1.5
67 * @author Doug Lea
68 * @param <V> The result type returned by this Future's <tt>get</tt> method
69 */
70 public interface Future<V> {
71
72 /**
73 * Attempts to cancel execution of this task. This attempt will
74 * fail if the task has already completed, has already been cancelled,
75 * or could not be cancelled for some other reason. If successful,
76 * and this task has not started when <tt>cancel</tt> is called,
77 * this task should never run. If the task has already started,
78 * then the <tt>mayInterruptIfRunning</tt> parameter determines
79 * whether the thread executing this task should be interrupted in
80 * an attempt to stop the task.
81 *
82 * <p>After this method returns, subsequent calls to {@link #isDone} will
83 * always return <tt>true</tt>. Subsequent calls to {@link #isCancelled}
84 * will always return <tt>true</tt> if this method returned <tt>true</tt>.
85 *
86 * @param mayInterruptIfRunning <tt>true</tt> if the thread executing this
87 * task should be interrupted; otherwise, in-progress tasks are allowed
88 * to complete
89 * @return <tt>false</tt> if the task could not be cancelled,
90 * typically because it has already completed normally;
91 * <tt>true</tt> otherwise
92 */
93 boolean cancel(boolean mayInterruptIfRunning);
94
95 /**
96 * Returns <tt>true</tt> if this task was cancelled before it completed
97 * normally.
98 *
99 * @return <tt>true</tt> if this task was cancelled before it completed
100 */
101 boolean isCancelled();
102
103 /**
104 * Returns <tt>true</tt> if this task completed.
105 *
106 * Completion may be due to normal termination, an exception, or
107 * cancellation -- in all of these cases, this method will return
108 * <tt>true</tt>.
109 *
110 * @return <tt>true</tt> if this task completed
111 */
112 boolean isDone();
113
114 /**
115 * Waits if necessary for the computation to complete, and then
116 * retrieves its result.
117 *
118 * @return the computed result
119 * @throws CancellationException if the computation was cancelled
120 * @throws ExecutionException if the computation threw an
121 * exception
122 * @throws InterruptedException if the current thread was interrupted
123 * while waiting
124 */
125 V get() throws InterruptedException, ExecutionException;
126
127 /**
128 * Waits if necessary for at most the given time for the computation
129 * to complete, and then retrieves its result, if available.
130 *
131 * @param timeout the maximum time to wait
132 * @param unit the time unit of the timeout argument
133 * @return the computed result
134 * @throws CancellationException if the computation was cancelled
135 * @throws ExecutionException if the computation threw an
136 * exception
137 * @throws InterruptedException if the current thread was interrupted
138 * while waiting
139 * @throws TimeoutException if the wait timed out
140 */
141 V get(long timeout, TimeUnit unit)
142 throws InterruptedException, ExecutionException, TimeoutException;
143 }
144