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
#	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><b>Sample Usage</b> (Note that the following classes are all
25	* made-up.)
26	*
27	* <pre> {@code
28	* interface ArchiveSearcher { String search(String target); }
29	* class App {
30	* ExecutorService executor = ...;
31	* ArchiveSearcher searcher = ...;
32	* void showSearch(String target) throws InterruptedException {
33	* Callable<String> task = () -> searcher.search(target);
34	* Future<String> future = executor.submit(task);
35	* displayOtherThings(); // do other things while searching
36	* try {
37	* displayText(future.get()); // use future
38	* } catch (ExecutionException ex) { cleanup(); return; }
39	* }
40	* }}</pre>
41	*
42	* 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	* <pre> {@code
46	* FutureTask<String> future = new FutureTask<>(task);
47	* executor.execute(future);}</pre>
48	*
49	* <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	*
53	* @see FutureTask
54	* @see Executor
55	* @since 1.5
56	* @author Doug Lea
57	* @param <V> The result type returned by this Future's {@code get} method
58	*/
59	public interface Future<V> {
60
61	/**
62	* 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	*Dale550
75
76	* @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	* @return {@code false} if the task could not be cancelled,
81	* 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	*/
86	boolean cancel(boolean mayInterruptIfRunning);
87
88	/**
89	* Returns {@code true} if this task was cancelled before it completed
90	* normally.
91	*
92	* @return {@code true} if this task was cancelled before it completed
93	*/
94	boolean isCancelled();
95
96	/**
97	* Returns {@code true} if this task completed.
98	*
99	* Completion may be due to normal termination, an exception, or
100	* cancellation -- in all of these cases, this method will return
101	* {@code true}.
102	*
103	* @return {@code true} if this task completed
104	*/
105	boolean isDone();
106
107	/**
108	* Waits if necessary for the computation to complete, and then
109	* retrieves its result.
110	*
111	* @return the computed result
112	* @throws CancellationException if the computation was cancelled
113	* @throws ExecutionException if the computation threw an
114	* exception
115	* @throws InterruptedException if the current thread was interrupted
116	* while waiting
117	*/
118	V get() throws InterruptedException, ExecutionException;
119
120	/**
121	* Waits if necessary for at most the given time for the computation
122	* to complete, and then retrieves its result, if available.
123	*
124	* @param timeout the maximum time to wait
125	* @param unit the time unit of the timeout argument
126	* @return the computed result
127	* @throws CancellationException if the computation was cancelled
128	* @throws ExecutionException if the computation threw an
129	* exception
130	* @throws InterruptedException if the current thread was interrupted
131	* while waiting
132	* @throws TimeoutException if the wait timed out
133	*/
134	V get(long timeout, TimeUnit unit)
135	throws InterruptedException, ExecutionException, TimeoutException;
136	}