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
#	Content
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	* A {@code Future} 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	* {@code get} when the computation has completed, blocking if
15	* necessary until it is ready. Cancellation is performed by the
16	* {@code cancel} 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	* If you would like to use a {@code Future} for the sake
20	* of cancellability but not provide a usable result, you can
21	* declare types of the form {@code Future<?>} and
22	* return {@code null} as a result of the underlying task.
23	*
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	* 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	* <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	* @param <V> The result type returned by this Future's {@code get} method
66	*/
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	* and this task has not started when {@code cancel} is called,
74	* this task should never run. If the task has already started,
75	* then the {@code mayInterruptIfRunning} parameter determines
76	* 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	* always return {@code true}. Subsequent calls to {@link #isCancelled}
81	* will always return {@code true} if this method returned {@code true}.
82	*
83	* @param mayInterruptIfRunning {@code true} if the thread executing this
84	* task should be interrupted; otherwise, in-progress tasks are allowed
85	* to complete
86	* @return {@code false} if the task could not be cancelled,
87	* typically because it has already completed normally;
88	* {@code true} otherwise
89	*/
90	boolean cancel(boolean mayInterruptIfRunning);
91
92	/**
93	* Returns {@code true} if this task was cancelled before it completed
94	* normally.
95	*
96	* @return {@code true} if this task was cancelled before it completed
97	*/
98	boolean isCancelled();
99
100	/**
101	* Returns {@code true} if this task completed.
102	*
103	* Completion may be due to normal termination, an exception, or
104	* cancellation -- in all of these cases, this method will return
105	* {@code true}.
106	*
107	* @return {@code true} if this task completed
108	*/
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	}