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(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<>(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:	1.1
Committed:	Sat Mar 26 06:22:50 2016 UTC (8 years, 1 month ago) by jsr166
Branch:	MAIN
CVS Tags:	HEAD
Log Message:	fork jdk8 maintenance branch for source and jtreg tests
#	User	Rev	Content
1	jsr166	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			* 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.)
27			*
28			* <pre> {@code
29			* interface ArchiveSearcher { String search(String target); }
30			* class App {
31			* ExecutorService executor = ...
32			* ArchiveSearcher searcher = ...
33			* void showSearch(final String target)
34			* throws InterruptedException {
35			* Future<String> future
36			* = executor.submit(new Callable<String>() {
37			* public String call() {
38			* return searcher.search(target);
39			* }});
40			* displayOtherThings(); // do other things while searching
41			* try {
42			* displayText(future.get()); // use future
43			* } catch (ExecutionException ex) { cleanup(); return; }
44			* }
45			* }}</pre>
46			*
47			* The {@link FutureTask} class is an implementation of {@code Future} that
48			* implements {@code Runnable}, and so may be executed by an {@code Executor}.
49			* For example, the above construction with {@code submit} could be replaced by:
50			* <pre> {@code
51			* FutureTask<String> future =
52			* new FutureTask<>(new Callable<String>() {
53			* public String call() {
54			* return searcher.search(target);
55			* }});
56			* executor.execute(future);}</pre>
57			*
58			* <p>Memory consistency effects: Actions taken by the asynchronous computation
59			* <a href="package-summary.html#MemoryVisibility"> <i>happen-before</i></a>
60			* actions following the corresponding {@code Future.get()} in another thread.
61			*
62			* @see FutureTask
63			* @see Executor
64			* @since 1.5
65			* @author Doug Lea
66			* @param <V> The result type returned by this Future's {@code get} method
67			*/
68			public interface Future<V> {
69
70			/**
71			* Attempts to cancel execution of this task. This attempt will
72			* fail if the task has already completed, has already been cancelled,
73			* or could not be cancelled for some other reason. If successful,
74			* and this task has not started when {@code cancel} is called,
75			* this task should never run. If the task has already started,
76			* then the {@code mayInterruptIfRunning} parameter determines
77			* whether the thread executing this task should be interrupted in
78			* an attempt to stop the task.
79			*
80			* <p>After this method returns, subsequent calls to {@link #isDone} will
81			* always return {@code true}. Subsequent calls to {@link #isCancelled}
82			* will always return {@code true} if this method returned {@code true}.
83			*
84			* @param mayInterruptIfRunning {@code true} if the thread executing this
85			* task should be interrupted; otherwise, in-progress tasks are allowed
86			* to complete
87			* @return {@code false} if the task could not be cancelled,
88			* typically because it has already completed normally;
89			* {@code true} otherwise
90			*/
91			boolean cancel(boolean mayInterruptIfRunning);
92
93			/**
94			* Returns {@code true} if this task was cancelled before it completed
95			* normally.
96			*
97			* @return {@code true} if this task was cancelled before it completed
98			*/
99			boolean isCancelled();
100
101			/**
102			* Returns {@code true} if this task completed.
103			*
104			* Completion may be due to normal termination, an exception, or
105			* cancellation -- in all of these cases, this method will return
106			* {@code true}.
107			*
108			* @return {@code true} if this task completed
109			*/
110			boolean isDone();
111
112			/**
113			* Waits if necessary for the computation to complete, and then
114			* retrieves its result.
115			*
116			* @return the computed result
117			* @throws CancellationException if the computation was cancelled
118			* @throws ExecutionException if the computation threw an
119			* exception
120			* @throws InterruptedException if the current thread was interrupted
121			* while waiting
122			*/
123			V get() throws InterruptedException, ExecutionException;
124
125			/**
126			* Waits if necessary for at most the given time for the computation
127			* to complete, and then retrieves its result, if available.
128			*
129			* @param timeout the maximum time to wait
130			* @param unit the time unit of the timeout argument
131			* @return the computed result
132			* @throws CancellationException if the computation was cancelled
133			* @throws ExecutionException if the computation threw an
134			* exception
135			* @throws InterruptedException if the current thread was interrupted
136			* while waiting
137			* @throws TimeoutException if the wait timed out
138			*/
139			V get(long timeout, TimeUnit unit)
140			throws InterruptedException, ExecutionException, TimeoutException;
141			}