util/concurrent/Future.java

/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent;

/**
 * A {@code Future} represents the result of an asynchronous
 * computation.  Methods are provided to check if the computation is
 * complete, to wait for its completion, and to retrieve the result of
 * the computation.  The result can only be retrieved using method
 * {@code get} when the computation has completed, blocking if
 * necessary until it is ready.  Cancellation is performed by the
 * {@code cancel} method.  Additional methods are provided to
 * determine if the task completed normally or was cancelled. Once a
 * computation has completed, the computation cannot be cancelled.
 * If you would like to use a {@code Future} for the sake
 * of cancellability but not provide a usable result, you can
 * declare types of the form {@code Future<?>} and
 * return {@code null} as a result of the underlying task.
 *
 * <p>
 * <b>Sample Usage</b> (Note that the following classes are all
 * made-up.)
 *
 * <pre> {@code
 * interface ArchiveSearcher { String search(String target); }
 * class App {
 *   ExecutorService executor = ...
 *   ArchiveSearcher searcher = ...
 *   void showSearch(final String target)
 *       throws InterruptedException {
 *     Future<String> future
 *       = executor.submit(new Callable<String>() {
 *         public String call() {
 *             return searcher.search(target);
 *         }});
 *     displayOtherThings(); // do other things while searching
 *     try {
 *       displayText(future.get()); // use future
 *     } catch (ExecutionException ex) { cleanup(); return; }
 *   }
 * }}</pre>
 *
 * The {@link FutureTask} class is an implementation of {@code Future} that
 * implements {@code Runnable}, and so may be executed by an {@code Executor}.
 * For example, the above construction with {@code submit} could be replaced by:
 * <pre> {@code
 * FutureTask<String> future =
 *   new FutureTask<>(new Callable<String>() {
 *     public String call() {
 *       return searcher.search(target);
 *   }});
 * executor.execute(future);}</pre>
 *
 * <p>Memory consistency effects: Actions taken by the asynchronous computation
 * <a href="package-summary.html#MemoryVisibility"> <i>happen-before</i></a>
 * actions following the corresponding {@code Future.get()} in another thread.
 *
 * @see FutureTask
 * @see Executor
 * @since 1.5
 * @author Doug Lea
 * @param <V> The result type returned by this Future's {@code get} method
 */
public interface Future<V> {

    /**
     * Attempts to cancel execution of this task.  This attempt will
     * fail if the task has already completed, has already been cancelled,
     * or could not be cancelled for some other reason. If successful,
     * and this task has not started when {@code cancel} is called,
     * this task should never run.  If the task has already started,
     * then the {@code mayInterruptIfRunning} parameter determines
     * whether the thread executing this task should be interrupted in
     * an attempt to stop the task.
     *
     * <p>After this method returns, subsequent calls to {@link #isDone} will
     * always return {@code true}.  Subsequent calls to {@link #isCancelled}
     * will always return {@code true} if this method returned {@code true}.
     *
     * @param mayInterruptIfRunning {@code true} if the thread executing this
     * task should be interrupted; otherwise, in-progress tasks are allowed
     * to complete
     * @return {@code false} if the task could not be cancelled,
     * typically because it has already completed normally;
     * {@code true} otherwise
     */
    boolean cancel(boolean mayInterruptIfRunning);

    /**
     * Returns {@code true} if this task was cancelled before it completed
     * normally.
     *
     * @return {@code true} if this task was cancelled before it completed
     */
    boolean isCancelled();

    /**
     * Returns {@code true} if this task completed.
     *
     * Completion may be due to normal termination, an exception, or
     * cancellation -- in all of these cases, this method will return
     * {@code true}.
     *
     * @return {@code true} if this task completed
     */
    boolean isDone();

    /**
     * Waits if necessary for the computation to complete, and then
     * retrieves its result.
     *
     * @return the computed result
     * @throws CancellationException if the computation was cancelled
     * @throws ExecutionException if the computation threw an
     * exception
     * @throws InterruptedException if the current thread was interrupted
     * while waiting
     */
    V get() throws InterruptedException, ExecutionException;

    /**
     * Waits if necessary for at most the given time for the computation
     * to complete, and then retrieves its result, if available.
     *
     * @param timeout the maximum time to wait
     * @param unit the time unit of the timeout argument
     * @return the computed result
     * @throws CancellationException if the computation was cancelled
     * @throws ExecutionException if the computation threw an
     * exception
     * @throws InterruptedException if the current thread was interrupted
     * while waiting
     * @throws TimeoutException if the wait timed out
     */
    V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException;
}
Revision:	1.40
Committed:	Tue Feb 17 18:55:39 2015 UTC (9 years, 3 months ago) by jsr166
Branch:	MAIN
Changes since 1.39:	+2 -2 lines
Log Message:	standardize code sample idiom: * <pre> {@code
#	User	Rev	Content
1	tim	1.1	/*
2	dl	1.12	* Written by Doug Lea with assistance from members of JCP JSR-166
3	dl	1.22	* Expert Group and released to the public domain, as explained at
4	jsr166	1.35	* http://creativecommons.org/publicdomain/zero/1.0/
5	tim	1.1	*/
6
7			package java.util.concurrent;
8
9			/**
10	jsr166	1.37	* A {@code Future} represents the result of an asynchronous
11	dl	1.12	* computation. Methods are provided to check if the computation is
12			* complete, to wait for its completion, and to retrieve the result of
13			* the computation. The result can only be retrieved using method
14	jsr166	1.37	* {@code get} when the computation has completed, blocking if
15	dl	1.17	* necessary until it is ready. Cancellation is performed by the
16	jsr166	1.37	* {@code cancel} method. Additional methods are provided to
17	dl	1.17	* determine if the task completed normally or was cancelled. Once a
18			* computation has completed, the computation cannot be cancelled.
19	jsr166	1.37	* If you would like to use a {@code Future} for the sake
20	dl	1.21	* of cancellability but not provide a usable result, you can
21	jsr166	1.34	* declare types of the form {@code Future<?>} and
22	jsr166	1.37	* return {@code null} as a result of the underlying task.
23	tim	1.1	*
24			* <p>
25	dl	1.5	* <b>Sample Usage</b> (Note that the following classes are all
26	jsr166	1.38	* made-up.)
27			*
28	jsr166	1.40	* <pre> {@code
29	dl	1.10	* interface ArchiveSearcher { String search(String target); }
30	tim	1.1	* class App {
31	tim	1.18	* ExecutorService executor = ...
32	dl	1.5	* ArchiveSearcher searcher = ...
33	jsr166	1.26	* void showSearch(final String target)
34			* throws InterruptedException {
35	jsr166	1.34	* Future<String> future
36			* = executor.submit(new Callable<String>() {
37	jsr166	1.27	* public String call() {
38			* return searcher.search(target);
39			* }});
40	dl	1.5	* displayOtherThings(); // do other things while searching
41	tim	1.1	* try {
42	dl	1.5	* displayText(future.get()); // use future
43	tim	1.8	* } catch (ExecutionException ex) { cleanup(); return; }
44	tim	1.1	* }
45	jsr166	1.34	* }}</pre>
46	tim	1.1	*
47	jsr166	1.37	* The {@link FutureTask} class is an implementation of {@code Future} that
48			* implements {@code Runnable}, and so may be executed by an {@code Executor}.
49			* For example, the above construction with {@code submit} could be replaced by:
50	jsr166	1.40	* <pre> {@code
51	jsr166	1.36	* FutureTask<String> future =
52	jsr166	1.39	* new FutureTask<>(new Callable<String>() {
53	jsr166	1.36	* public String call() {
54			* return searcher.search(target);
55			* }});
56			* executor.execute(future);}</pre>
57	brian	1.29	*
58	jsr166	1.31	* <p>Memory consistency effects: Actions taken by the asynchronous computation
59			* <a href="package-summary.html#MemoryVisibility"> <i>happen-before</i></a>
60			* actions following the corresponding {@code Future.get()} in another thread.
61	dl	1.30	*
62	tim	1.1	* @see FutureTask
63			* @see Executor
64	dl	1.12	* @since 1.5
65	dl	1.4	* @author Doug Lea
66	jsr166	1.37	* @param <V> The result type returned by this Future's {@code get} method
67	tim	1.1	*/
68	dl	1.17	public interface Future<V> {
69
70			/**
71			* Attempts to cancel execution of this task. This attempt will
72	jsr166	1.33	* fail if the task has already completed, has already been cancelled,
73	dl	1.17	* or could not be cancelled for some other reason. If successful,
74	jsr166	1.37	* and this task has not started when {@code cancel} is called,
75	dl	1.17	* this task should never run. If the task has already started,
76	jsr166	1.37	* then the {@code mayInterruptIfRunning} parameter determines
77	dl	1.17	* whether the thread executing this task should be interrupted in
78			* an attempt to stop the task.
79			*
80	jsr166	1.27	* <p>After this method returns, subsequent calls to {@link #isDone} will
81	jsr166	1.37	* always return {@code true}. Subsequent calls to {@link #isCancelled}
82			* will always return {@code true} if this method returned {@code true}.
83	jsr166	1.27	*
84	jsr166	1.37	* @param mayInterruptIfRunning {@code true} if the thread executing this
85	dl	1.17	* task should be interrupted; otherwise, in-progress tasks are allowed
86			* to complete
87	jsr166	1.37	* @return {@code false} if the task could not be cancelled,
88	jozart	1.19	* typically because it has already completed normally;
89	jsr166	1.37	* {@code true} otherwise
90	dl	1.17	*/
91			boolean cancel(boolean mayInterruptIfRunning);
92
93			/**
94	jsr166	1.37	* Returns {@code true} if this task was cancelled before it completed
95	dl	1.17	* normally.
96			*
97	jsr166	1.37	* @return {@code true} if this task was cancelled before it completed
98	dl	1.17	*/
99			boolean isCancelled();
100
101			/**
102	jsr166	1.37	* Returns {@code true} if this task completed.
103	dl	1.17	*
104			* Completion may be due to normal termination, an exception, or
105			* cancellation -- in all of these cases, this method will return
106	jsr166	1.37	* {@code true}.
107	jsr166	1.24	*
108	jsr166	1.37	* @return {@code true} if this task completed
109	dl	1.17	*/
110			boolean isDone();
111	tim	1.1
112			/**
113	dl	1.23	* Waits if necessary for the computation to complete, and then
114	tim	1.1	* retrieves its result.
115			*
116	dl	1.23	* @return the computed result
117			* @throws CancellationException if the computation was cancelled
118			* @throws ExecutionException if the computation threw an
119	dl	1.4	* exception
120	dl	1.23	* @throws InterruptedException if the current thread was interrupted
121	dl	1.4	* while waiting
122	tim	1.1	*/
123			V get() throws InterruptedException, ExecutionException;
124
125			/**
126			* Waits if necessary for at most the given time for the computation
127	dl	1.23	* to complete, and then retrieves its result, if available.
128	tim	1.1	*
129			* @param timeout the maximum time to wait
130	dl	1.14	* @param unit the time unit of the timeout argument
131	dl	1.23	* @return the computed result
132			* @throws CancellationException if the computation was cancelled
133			* @throws ExecutionException if the computation threw an
134	dl	1.4	* exception
135	dl	1.23	* @throws InterruptedException if the current thread was interrupted
136	dl	1.4	* while waiting
137	tim	1.1	* @throws TimeoutException if the wait timed out
138			*/
139	dl	1.14	V get(long timeout, TimeUnit unit)
140	tim	1.1	throws InterruptedException, ExecutionException, TimeoutException;
141			}