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/licenses/publicdomain
 */

package java.util.concurrent;

/**
 * A <tt>Future</tt> 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
 * <tt>get</tt> when the computation has completed, blocking if
 * necessary until it is ready.  Cancellation is performed by the
 * <tt>cancel</tt> 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 <tt>Future</tt> for the sake
 * of cancellability but not provide a usable result, you can
 * declare types of the form <tt>Future&lt;?&gt;</tt> and
 * return <tt>null</tt> as a result of the underlying task.
 *
 * <p>
 * <b>Sample Usage</b> (Note that the following classes are all
 * made-up.) <p>
 * <pre>
 * interface ArchiveSearcher { String search(String target); }
 * class App {
 *   ExecutorService executor = ...
 *   ArchiveSearcher searcher = ...
 *   void showSearch(final String target)
 *       throws InterruptedException {
 *     Future&lt;String&gt; future
 *       = executor.submit(new Callable&lt;String&gt;() {
 *         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 <tt>Future</tt> that
 * implements <tt>Runnable</tt>, and so may be executed by an <tt>Executor</tt>.
 * For example, the above construction with <tt>submit</tt> could be replaced by:
 * <pre>
 *     FutureTask&lt;String&gt; future =
 *       new FutureTask&lt;String&gt;(new Callable&lt;String&gt;() {
 *         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 <tt>get</tt> 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 <tt>cancel</tt> is called,
     * this task should never run.  If the task has already started,
     * then the <tt>mayInterruptIfRunning</tt> 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 <tt>true</tt>.  Subsequent calls to {@link #isCancelled}
     * will always return <tt>true</tt> if this method returned <tt>true</tt>.
     *
     * @param mayInterruptIfRunning <tt>true</tt> if the thread executing this
     * task should be interrupted; otherwise, in-progress tasks are allowed
     * to complete
     * @return <tt>false</tt> if the task could not be cancelled,
     * typically because it has already completed normally;
     * <tt>true</tt> otherwise
     */
    boolean cancel(boolean mayInterruptIfRunning);

    /**
     * Returns <tt>true</tt> if this task was cancelled before it completed
     * normally.
     *
     * @return <tt>true</tt> if this task was cancelled before it completed
     */
    boolean isCancelled();

    /**
     * Returns <tt>true</tt> if this task completed.
     *
     * Completion may be due to normal termination, an exception, or
     * cancellation -- in all of these cases, this method will return
     * <tt>true</tt>.
     *
     * @return <tt>true</tt> 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:	1.33
Committed:	Fri Mar 24 05:51:49 2006 UTC (18 years, 2 months ago) by jsr166
Branch:	MAIN
Changes since 1.32:	+1 -1 lines
Log Message:	6373953: Future.cancel doc "has" small grammar problem
#	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			* http://creativecommons.org/licenses/publicdomain
5	tim	1.1	*/
6
7			package java.util.concurrent;
8
9			/**
10	dl	1.12	* A <tt>Future</tt> represents the result of an asynchronous
11			* 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			* <tt>get</tt> when the computation has completed, blocking if
15	dl	1.17	* necessary until it is ready. Cancellation is performed by the
16			* <tt>cancel</tt> method. Additional methods are provided to
17			* determine if the task completed normally or was cancelled. Once a
18			* computation has completed, the computation cannot be cancelled.
19	dl	1.21	* If you would like to use a <tt>Future</tt> for the sake
20			* of cancellability but not provide a usable result, you can
21			* declare types of the form <tt>Future<?></tt> and
22			* return <tt>null</tt> 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			* made-up.) <p>
27	tim	1.1	* <pre>
28	dl	1.10	* interface ArchiveSearcher { String search(String target); }
29	tim	1.1	* class App {
30	tim	1.18	* ExecutorService executor = ...
31	dl	1.5	* ArchiveSearcher searcher = ...
32	jsr166	1.26	* void showSearch(final String target)
33			* throws InterruptedException {
34			* Future<String> future
35			* = executor.submit(new Callable<String>() {
36	jsr166	1.27	* public String call() {
37			* return searcher.search(target);
38			* }});
39	dl	1.5	* displayOtherThings(); // do other things while searching
40	tim	1.1	* try {
41	dl	1.5	* displayText(future.get()); // use future
42	tim	1.8	* } catch (ExecutionException ex) { cleanup(); return; }
43	tim	1.1	* }
44			* }
45			* </pre>
46			*
47	jsr166	1.24	* The {@link FutureTask} class is an implementation of <tt>Future</tt> that
48			* implements <tt>Runnable</tt>, and so may be executed by an <tt>Executor</tt>.
49	tim	1.18	* For example, the above construction with <tt>submit</tt> could be replaced by:
50	dl	1.10	* <pre>
51	tim	1.18	* FutureTask<String> future =
52			* new FutureTask<String>(new Callable<String>() {
53			* public String call() {
54			* return searcher.search(target);
55			* }});
56			* executor.execute(future);
57	dl	1.10	* </pre>
58	brian	1.29	*
59	jsr166	1.31	* <p>Memory consistency effects: Actions taken by the asynchronous computation
60			* <a href="package-summary.html#MemoryVisibility"> <i>happen-before</i></a>
61			* actions following the corresponding {@code Future.get()} in another thread.
62	dl	1.30	*
63	tim	1.1	* @see FutureTask
64			* @see Executor
65	dl	1.12	* @since 1.5
66	dl	1.4	* @author Doug Lea
67	dl	1.16	* @param <V> The result type returned by this Future's <tt>get</tt> method
68	tim	1.1	*/
69	dl	1.17	public interface Future<V> {
70
71			/**
72			* Attempts to cancel execution of this task. This attempt will
73	jsr166	1.33	* fail if the task has already completed, has already been cancelled,
74	dl	1.17	* or could not be cancelled for some other reason. If successful,
75			* and this task has not started when <tt>cancel</tt> is called,
76			* this task should never run. If the task has already started,
77	dl	1.23	* then the <tt>mayInterruptIfRunning</tt> parameter determines
78	dl	1.17	* whether the thread executing this task should be interrupted in
79			* an attempt to stop the task.
80			*
81	jsr166	1.27	* <p>After this method returns, subsequent calls to {@link #isDone} will
82			* always return <tt>true</tt>. Subsequent calls to {@link #isCancelled}
83			* will always return <tt>true</tt> if this method returned <tt>true</tt>.
84			*
85	dl	1.17	* @param mayInterruptIfRunning <tt>true</tt> if the thread executing this
86			* task should be interrupted; otherwise, in-progress tasks are allowed
87			* to complete
88			* @return <tt>false</tt> if the task could not be cancelled,
89	jozart	1.19	* typically because it has already completed normally;
90	dl	1.17	* <tt>true</tt> otherwise
91			*/
92			boolean cancel(boolean mayInterruptIfRunning);
93
94			/**
95			* Returns <tt>true</tt> if this task was cancelled before it completed
96			* normally.
97			*
98	jsr166	1.28	* @return <tt>true</tt> if this task was cancelled before it completed
99	dl	1.17	*/
100			boolean isCancelled();
101
102			/**
103	jsr166	1.24	* Returns <tt>true</tt> if this task completed.
104	dl	1.17	*
105			* Completion may be due to normal termination, an exception, or
106			* cancellation -- in all of these cases, this method will return
107			* <tt>true</tt>.
108	jsr166	1.24	*
109	jsr166	1.26	* @return <tt>true</tt> if this task completed
110	dl	1.17	*/
111			boolean isDone();
112	tim	1.1
113			/**
114	dl	1.23	* Waits if necessary for the computation to complete, and then
115	tim	1.1	* retrieves its result.
116			*
117	dl	1.23	* @return the computed result
118			* @throws CancellationException if the computation was cancelled
119			* @throws ExecutionException if the computation threw an
120	dl	1.4	* exception
121	dl	1.23	* @throws InterruptedException if the current thread was interrupted
122	dl	1.4	* while waiting
123	tim	1.1	*/
124			V get() throws InterruptedException, ExecutionException;
125
126			/**
127			* Waits if necessary for at most the given time for the computation
128	dl	1.23	* to complete, and then retrieves its result, if available.
129	tim	1.1	*
130			* @param timeout the maximum time to wait
131	dl	1.14	* @param unit the time unit of the timeout argument
132	dl	1.23	* @return the computed result
133			* @throws CancellationException if the computation was cancelled
134			* @throws ExecutionException if the computation threw an
135	dl	1.4	* exception
136	dl	1.23	* @throws InterruptedException if the current thread was interrupted
137	dl	1.4	* while waiting
138	tim	1.1	* @throws TimeoutException if the wait timed out
139			*/
140	dl	1.14	V get(long timeout, TimeUnit unit)
141	tim	1.1	throws InterruptedException, ExecutionException, TimeoutException;
142			}