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 <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 {@code Future<?>} 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> {@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 <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> {@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 <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:	1.36
Committed:	Wed May 9 18:56:54 2012 UTC (12 years ago) by jsr166
Branch:	MAIN
Changes since 1.35:	+6 -6 lines
Log Message:	code snippet whitespace
#	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	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	jsr166	1.34	* declare types of the form {@code Future<?>} and
22	dl	1.21	* 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	jsr166	1.34	* <pre> {@code
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	jsr166	1.34	* 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	jsr166	1.34	* }}</pre>
45	tim	1.1	*
46	jsr166	1.24	* The {@link FutureTask} class is an implementation of <tt>Future</tt> that
47			* implements <tt>Runnable</tt>, and so may be executed by an <tt>Executor</tt>.
48	tim	1.18	* For example, the above construction with <tt>submit</tt> could be replaced by:
49	jsr166	1.34	* <pre> {@code
50	jsr166	1.36	* 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	brian	1.29	*
57	jsr166	1.31	* <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	dl	1.30	*
61	tim	1.1	* @see FutureTask
62			* @see Executor
63	dl	1.12	* @since 1.5
64	dl	1.4	* @author Doug Lea
65	dl	1.16	* @param <V> The result type returned by this Future's <tt>get</tt> method
66	tim	1.1	*/
67	dl	1.17	public interface Future<V> {
68
69			/**
70			* Attempts to cancel execution of this task. This attempt will
71	jsr166	1.33	* fail if the task has already completed, has already been cancelled,
72	dl	1.17	* or could not be cancelled for some other reason. If successful,
73			* and this task has not started when <tt>cancel</tt> is called,
74			* this task should never run. If the task has already started,
75	dl	1.23	* then the <tt>mayInterruptIfRunning</tt> parameter determines
76	dl	1.17	* whether the thread executing this task should be interrupted in
77			* an attempt to stop the task.
78			*
79	jsr166	1.27	* <p>After this method returns, subsequent calls to {@link #isDone} will
80			* always return <tt>true</tt>. Subsequent calls to {@link #isCancelled}
81			* will always return <tt>true</tt> if this method returned <tt>true</tt>.
82			*
83	dl	1.17	* @param mayInterruptIfRunning <tt>true</tt> if the thread executing this
84			* task should be interrupted; otherwise, in-progress tasks are allowed
85			* to complete
86			* @return <tt>false</tt> if the task could not be cancelled,
87	jozart	1.19	* typically because it has already completed normally;
88	dl	1.17	* <tt>true</tt> otherwise
89			*/
90			boolean cancel(boolean mayInterruptIfRunning);
91
92			/**
93			* Returns <tt>true</tt> if this task was cancelled before it completed
94			* normally.
95			*
96	jsr166	1.28	* @return <tt>true</tt> if this task was cancelled before it completed
97	dl	1.17	*/
98			boolean isCancelled();
99
100			/**
101	jsr166	1.24	* Returns <tt>true</tt> if this task completed.
102	dl	1.17	*
103			* Completion may be due to normal termination, an exception, or
104			* cancellation -- in all of these cases, this method will return
105			* <tt>true</tt>.
106	jsr166	1.24	*
107	jsr166	1.26	* @return <tt>true</tt> if this task completed
108	dl	1.17	*/
109			boolean isDone();
110	tim	1.1
111			/**
112	dl	1.23	* Waits if necessary for the computation to complete, and then
113	tim	1.1	* retrieves its result.
114			*
115	dl	1.23	* @return the computed result
116			* @throws CancellationException if the computation was cancelled
117			* @throws ExecutionException if the computation threw an
118	dl	1.4	* exception
119	dl	1.23	* @throws InterruptedException if the current thread was interrupted
120	dl	1.4	* while waiting
121	tim	1.1	*/
122			V get() throws InterruptedException, ExecutionException;
123
124			/**
125			* Waits if necessary for at most the given time for the computation
126	dl	1.23	* to complete, and then retrieves its result, if available.
127	tim	1.1	*
128			* @param timeout the maximum time to wait
129	dl	1.14	* @param unit the time unit of the timeout argument
130	dl	1.23	* @return the computed result
131			* @throws CancellationException if the computation was cancelled
132			* @throws ExecutionException if the computation threw an
133	dl	1.4	* exception
134	dl	1.23	* @throws InterruptedException if the current thread was interrupted
135	dl	1.4	* while waiting
136	tim	1.1	* @throws TimeoutException if the wait timed out
137			*/
138	dl	1.14	V get(long timeout, TimeUnit unit)
139	tim	1.1	throws InterruptedException, ExecutionException, TimeoutException;
140			}