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(String target) throws InterruptedException {
 *     Callable<String> task = () -> searcher.search(target);
 *     Future<String> future = executor.submit(task);
 *     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<>(task);
 * 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 method has no
     * effect if the task is already completed or cancelled, or could
     * not be cancelled for some other reason.  Otherwise, if 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 (when known by the implementation)
     * is interrupted in an attempt to stop the task.
     *
     * <p>The return value from this method does not necessarily
     * indicate whether the task is now cancelled; use {@link
     * #isCancelled}.
     *Dale550

     * @param mayInterruptIfRunning {@code true} if the thread
     * executing this task should be interrupted (if the thread is
     * known to the implementation); otherwise, in-progress tasks are
     * allowed to complete
     * @return {@code false} if the task could not be cancelled,
     * typically because it has already completed; {@code true}
     * otherwise. If two or more threads cause a task to be cancelled,
     * then at least one of them returns {@code true}. Implementations
     * may provide stronger guarantees.
     */
    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:	1.43
Committed:	Fri Nov 27 17:42:00 2020 UTC (3 years, 6 months ago) by dl
Branch:	MAIN
Changes since 1.42:	+4 -3 lines
Log Message:	Incorporate snippets code improvements from Pavel Rappo
#	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	jsr166	1.41	* <p><b>Sample Usage</b> (Note that the following classes are all
25	jsr166	1.38	* made-up.)
26			*
27	jsr166	1.40	* <pre> {@code
28	dl	1.10	* interface ArchiveSearcher { String search(String target); }
29	tim	1.1	* class App {
30	dl	1.43	* ExecutorService executor = ...;
31			* ArchiveSearcher searcher = ...;
32	jsr166	1.41	* void showSearch(String target) throws InterruptedException {
33			* Callable<String> task = () -> searcher.search(target);
34			* Future<String> future = executor.submit(task);
35	dl	1.5	* displayOtherThings(); // do other things while searching
36	tim	1.1	* try {
37	dl	1.5	* displayText(future.get()); // use future
38	tim	1.8	* } catch (ExecutionException ex) { cleanup(); return; }
39	tim	1.1	* }
40	jsr166	1.34	* }}</pre>
41	tim	1.1	*
42	jsr166	1.37	* The {@link FutureTask} class is an implementation of {@code Future} that
43			* implements {@code Runnable}, and so may be executed by an {@code Executor}.
44			* For example, the above construction with {@code submit} could be replaced by:
45	jsr166	1.40	* <pre> {@code
46	jsr166	1.41	* FutureTask<String> future = new FutureTask<>(task);
47	jsr166	1.36	* executor.execute(future);}</pre>
48	brian	1.29	*
49	jsr166	1.31	* <p>Memory consistency effects: Actions taken by the asynchronous computation
50			* <a href="package-summary.html#MemoryVisibility"> <i>happen-before</i></a>
51			* actions following the corresponding {@code Future.get()} in another thread.
52	dl	1.30	*
53	tim	1.1	* @see FutureTask
54			* @see Executor
55	dl	1.12	* @since 1.5
56	dl	1.4	* @author Doug Lea
57	jsr166	1.37	* @param <V> The result type returned by this Future's {@code get} method
58	tim	1.1	*/
59	dl	1.17	public interface Future<V> {
60
61			/**
62	dl	1.42	* Attempts to cancel execution of this task. This method has no
63			* effect if the task is already completed or cancelled, or could
64			* not be cancelled for some other reason. Otherwise, if this
65			* task has not started when {@code cancel} is called, this task
66			* should never run. If the task has already started, then the
67			* {@code mayInterruptIfRunning} parameter determines whether the
68			* thread executing this task (when known by the implementation)
69			* is interrupted in an attempt to stop the task.
70			*
71			* <p>The return value from this method does not necessarily
72			* indicate whether the task is now cancelled; use {@link
73			* #isCancelled}.
74	dl	1.43	*Dale550
75
76	dl	1.42	* @param mayInterruptIfRunning {@code true} if the thread
77			* executing this task should be interrupted (if the thread is
78			* known to the implementation); otherwise, in-progress tasks are
79			* allowed to complete
80	jsr166	1.37	* @return {@code false} if the task could not be cancelled,
81	dl	1.42	* typically because it has already completed; {@code true}
82			* otherwise. If two or more threads cause a task to be cancelled,
83			* then at least one of them returns {@code true}. Implementations
84			* may provide stronger guarantees.
85	dl	1.17	*/
86			boolean cancel(boolean mayInterruptIfRunning);
87
88			/**
89	jsr166	1.37	* Returns {@code true} if this task was cancelled before it completed
90	dl	1.17	* normally.
91			*
92	jsr166	1.37	* @return {@code true} if this task was cancelled before it completed
93	dl	1.17	*/
94			boolean isCancelled();
95
96			/**
97	jsr166	1.37	* Returns {@code true} if this task completed.
98	dl	1.17	*
99			* Completion may be due to normal termination, an exception, or
100			* cancellation -- in all of these cases, this method will return
101	jsr166	1.37	* {@code true}.
102	jsr166	1.24	*
103	jsr166	1.37	* @return {@code true} if this task completed
104	dl	1.17	*/
105			boolean isDone();
106	tim	1.1
107			/**
108	dl	1.23	* Waits if necessary for the computation to complete, and then
109	tim	1.1	* retrieves its result.
110			*
111	dl	1.23	* @return the computed result
112			* @throws CancellationException if the computation was cancelled
113			* @throws ExecutionException if the computation threw an
114	dl	1.4	* exception
115	dl	1.23	* @throws InterruptedException if the current thread was interrupted
116	dl	1.4	* while waiting
117	tim	1.1	*/
118			V get() throws InterruptedException, ExecutionException;
119
120			/**
121			* Waits if necessary for at most the given time for the computation
122	dl	1.23	* to complete, and then retrieves its result, if available.
123	tim	1.1	*
124			* @param timeout the maximum time to wait
125	dl	1.14	* @param unit the time unit of the timeout argument
126	dl	1.23	* @return the computed result
127			* @throws CancellationException if the computation was cancelled
128			* @throws ExecutionException if the computation threw an
129	dl	1.4	* exception
130	dl	1.23	* @throws InterruptedException if the current thread was interrupted
131	dl	1.4	* while waiting
132	tim	1.1	* @throws TimeoutException if the wait timed out
133			*/
134	dl	1.14	V get(long timeout, TimeUnit unit)
135	tim	1.1	throws InterruptedException, ExecutionException, TimeoutException;
136			}