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.) <p>
 *  <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<String>(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:	1.2
Committed:	Wed Jan 16 01:39:37 2013 UTC (11 years, 4 months ago) by jsr166
Branch:	MAIN
CVS Tags:	HEAD
Changes since 1.1:	+21 -21 lines
Log Message:	<tt> -> {@code
#	User	Rev	Content
1	dl	1.1	/*
2			* Written by Doug Lea with assistance from members of JCP JSR-166
3			* Expert Group and released to the public domain, as explained at
4			* http://creativecommons.org/publicdomain/zero/1.0/
5			*/
6
7			package java.util.concurrent;
8
9			/**
10	jsr166	1.2	* A {@code Future} represents the result of an asynchronous
11	dl	1.1	* 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	1.2	* {@code get} when the computation has completed, blocking if
15	dl	1.1	* necessary until it is ready. Cancellation is performed by the
16	jsr166	1.2	* {@code cancel} method. Additional methods are provided to
17	dl	1.1	* determine if the task completed normally or was cancelled. Once a
18			* computation has completed, the computation cannot be cancelled.
19	jsr166	1.2	* If you would like to use a {@code Future} for the sake
20	dl	1.1	* of cancellability but not provide a usable result, you can
21			* declare types of the form {@code Future<?>} and
22	jsr166	1.2	* return {@code null} as a result of the underlying task.
23	dl	1.1	*
24			* <p>
25			* <b>Sample Usage</b> (Note that the following classes are all
26			* made-up.) <p>
27			* <pre> {@code
28			* interface ArchiveSearcher { String search(String target); }
29			* class App {
30			* ExecutorService executor = ...
31			* ArchiveSearcher searcher = ...
32			* void showSearch(final String target)
33			* throws InterruptedException {
34			* Future<String> future
35			* = executor.submit(new Callable<String>() {
36			* public String call() {
37			* return searcher.search(target);
38			* }});
39			* displayOtherThings(); // do other things while searching
40			* try {
41			* displayText(future.get()); // use future
42			* } catch (ExecutionException ex) { cleanup(); return; }
43			* }
44			* }}</pre>
45			*
46	jsr166	1.2	* The {@link FutureTask} class is an implementation of {@code Future} that
47			* implements {@code Runnable}, and so may be executed by an {@code Executor}.
48			* For example, the above construction with {@code submit} could be replaced by:
49	dl	1.1	* <pre> {@code
50			* FutureTask<String> future =
51			* new FutureTask<String>(new Callable<String>() {
52			* public String call() {
53			* return searcher.search(target);
54			* }});
55			* executor.execute(future);}</pre>
56			*
57			* <p>Memory consistency effects: Actions taken by the asynchronous computation
58			* <a href="package-summary.html#MemoryVisibility"> <i>happen-before</i></a>
59			* actions following the corresponding {@code Future.get()} in another thread.
60			*
61			* @see FutureTask
62			* @see Executor
63			* @since 1.5
64			* @author Doug Lea
65	jsr166	1.2	* @param <V> The result type returned by this Future's {@code get} method
66	dl	1.1	*/
67			public interface Future<V> {
68
69			/**
70			* Attempts to cancel execution of this task. This attempt will
71			* fail if the task has already completed, has already been cancelled,
72			* or could not be cancelled for some other reason. If successful,
73	jsr166	1.2	* and this task has not started when {@code cancel} is called,
74	dl	1.1	* this task should never run. If the task has already started,
75	jsr166	1.2	* then the {@code mayInterruptIfRunning} parameter determines
76	dl	1.1	* whether the thread executing this task should be interrupted in
77			* an attempt to stop the task.
78			*
79			* <p>After this method returns, subsequent calls to {@link #isDone} will
80	jsr166	1.2	* always return {@code true}. Subsequent calls to {@link #isCancelled}
81			* will always return {@code true} if this method returned {@code true}.
82	dl	1.1	*
83	jsr166	1.2	* @param mayInterruptIfRunning {@code true} if the thread executing this
84	dl	1.1	* task should be interrupted; otherwise, in-progress tasks are allowed
85			* to complete
86	jsr166	1.2	* @return {@code false} if the task could not be cancelled,
87	dl	1.1	* typically because it has already completed normally;
88	jsr166	1.2	* {@code true} otherwise
89	dl	1.1	*/
90			boolean cancel(boolean mayInterruptIfRunning);
91
92			/**
93	jsr166	1.2	* Returns {@code true} if this task was cancelled before it completed
94	dl	1.1	* normally.
95			*
96	jsr166	1.2	* @return {@code true} if this task was cancelled before it completed
97	dl	1.1	*/
98			boolean isCancelled();
99
100			/**
101	jsr166	1.2	* Returns {@code true} if this task completed.
102	dl	1.1	*
103			* Completion may be due to normal termination, an exception, or
104			* cancellation -- in all of these cases, this method will return
105	jsr166	1.2	* {@code true}.
106	dl	1.1	*
107	jsr166	1.2	* @return {@code true} if this task completed
108	dl	1.1	*/
109			boolean isDone();
110
111			/**
112			* Waits if necessary for the computation to complete, and then
113			* retrieves its result.
114			*
115			* @return the computed result
116			* @throws CancellationException if the computation was cancelled
117			* @throws ExecutionException if the computation threw an
118			* exception
119			* @throws InterruptedException if the current thread was interrupted
120			* while waiting
121			*/
122			V get() throws InterruptedException, ExecutionException;
123
124			/**
125			* Waits if necessary for at most the given time for the computation
126			* to complete, and then retrieves its result, if available.
127			*
128			* @param timeout the maximum time to wait
129			* @param unit the time unit of the timeout argument
130			* @return the computed result
131			* @throws CancellationException if the computation was cancelled
132			* @throws ExecutionException if the computation threw an
133			* exception
134			* @throws InterruptedException if the current thread was interrupted
135			* while waiting
136			* @throws TimeoutException if the wait timed out
137			*/
138			V get(long timeout, TimeUnit unit)
139			throws InterruptedException, ExecutionException, TimeoutException;
140			}