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/licenses/publicdomain
 */

package java.util.concurrent;
import java.util.concurrent.*; // for javadoc (till 6280605 is fixed)

/**
 * 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 <tt>Future&lt;?&gt;</tt> 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>
 * interface ArchiveSearcher { String search(String target); }
 * class App {
 *   ExecutorService executor = ...
 *   ArchiveSearcher searcher = ...
 *   void showSearch(final String target)
 *       throws InterruptedException {
 *     Future&lt;String&gt; future
 *       = executor.submit(new Callable&lt;String&gt;() {
 *         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>
 *     FutureTask&lt;String&gt; future =
 *       new FutureTask&lt;String&gt;(new Callable&lt;String&gt;() {
 *         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, 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:	1.31
Committed:	Wed Sep 14 21:45:12 2005 UTC (18 years, 8 months ago) by jsr166
Branch:	MAIN
Changes since 1.30:	+3 -4 lines
Log Message:	happens-before
#	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			* http://creativecommons.org/licenses/publicdomain
5	tim	1.1	*/
6
7			package java.util.concurrent;
8	jsr166	1.25	import java.util.concurrent.*; // for javadoc (till 6280605 is fixed)
9	tim	1.1
10			/**
11	dl	1.12	* A <tt>Future</tt> represents the result of an asynchronous
12			* computation. Methods are provided to check if the computation is
13			* complete, to wait for its completion, and to retrieve the result of
14			* the computation. The result can only be retrieved using method
15			* <tt>get</tt> when the computation has completed, blocking if
16	dl	1.17	* necessary until it is ready. Cancellation is performed by the
17			* <tt>cancel</tt> method. Additional methods are provided to
18			* determine if the task completed normally or was cancelled. Once a
19			* computation has completed, the computation cannot be cancelled.
20	dl	1.21	* If you would like to use a <tt>Future</tt> for the sake
21			* of cancellability but not provide a usable result, you can
22			* declare types of the form <tt>Future<?></tt> and
23			* return <tt>null</tt> as a result of the underlying task.
24	tim	1.1	*
25			* <p>
26	dl	1.5	* <b>Sample Usage</b> (Note that the following classes are all
27			* made-up.) <p>
28	tim	1.1	* <pre>
29	dl	1.10	* interface ArchiveSearcher { String search(String target); }
30	tim	1.1	* class App {
31	tim	1.18	* ExecutorService executor = ...
32	dl	1.5	* ArchiveSearcher searcher = ...
33	jsr166	1.26	* void showSearch(final String target)
34			* throws InterruptedException {
35			* Future<String> future
36			* = executor.submit(new Callable<String>() {
37	jsr166	1.27	* public String call() {
38			* return searcher.search(target);
39			* }});
40	dl	1.5	* displayOtherThings(); // do other things while searching
41	tim	1.1	* try {
42	dl	1.5	* displayText(future.get()); // use future
43	tim	1.8	* } catch (ExecutionException ex) { cleanup(); return; }
44	tim	1.1	* }
45			* }
46			* </pre>
47			*
48	jsr166	1.24	* The {@link FutureTask} class is an implementation of <tt>Future</tt> that
49			* implements <tt>Runnable</tt>, and so may be executed by an <tt>Executor</tt>.
50	tim	1.18	* For example, the above construction with <tt>submit</tt> could be replaced by:
51	dl	1.10	* <pre>
52	tim	1.18	* FutureTask<String> future =
53			* new FutureTask<String>(new Callable<String>() {
54			* public String call() {
55			* return searcher.search(target);
56			* }});
57			* executor.execute(future);
58	dl	1.10	* </pre>
59	brian	1.29	*
60	jsr166	1.31	* <p>Memory consistency effects: Actions taken by the asynchronous computation
61			* <a href="package-summary.html#MemoryVisibility"> <i>happen-before</i></a>
62			* actions following the corresponding {@code Future.get()} in another thread.
63	dl	1.30	*
64	tim	1.1	* @see FutureTask
65			* @see Executor
66	dl	1.12	* @since 1.5
67	dl	1.4	* @author Doug Lea
68	dl	1.16	* @param <V> The result type returned by this Future's <tt>get</tt> method
69	tim	1.1	*/
70	dl	1.17	public interface Future<V> {
71
72			/**
73			* Attempts to cancel execution of this task. This attempt will
74			* fail if the task has already completed, already been cancelled,
75			* or could not be cancelled for some other reason. If successful,
76			* and this task has not started when <tt>cancel</tt> is called,
77			* this task should never run. If the task has already started,
78	dl	1.23	* then the <tt>mayInterruptIfRunning</tt> parameter determines
79	dl	1.17	* whether the thread executing this task should be interrupted in
80			* an attempt to stop the task.
81			*
82	jsr166	1.27	* <p>After this method returns, subsequent calls to {@link #isDone} will
83			* always return <tt>true</tt>. Subsequent calls to {@link #isCancelled}
84			* will always return <tt>true</tt> if this method returned <tt>true</tt>.
85			*
86	dl	1.17	* @param mayInterruptIfRunning <tt>true</tt> if the thread executing this
87			* task should be interrupted; otherwise, in-progress tasks are allowed
88			* to complete
89			* @return <tt>false</tt> if the task could not be cancelled,
90	jozart	1.19	* typically because it has already completed normally;
91	dl	1.17	* <tt>true</tt> otherwise
92			*/
93			boolean cancel(boolean mayInterruptIfRunning);
94
95			/**
96			* Returns <tt>true</tt> if this task was cancelled before it completed
97			* normally.
98			*
99	jsr166	1.28	* @return <tt>true</tt> if this task was cancelled before it completed
100	dl	1.17	*/
101			boolean isCancelled();
102
103			/**
104	jsr166	1.24	* Returns <tt>true</tt> if this task completed.
105	dl	1.17	*
106			* Completion may be due to normal termination, an exception, or
107			* cancellation -- in all of these cases, this method will return
108			* <tt>true</tt>.
109	jsr166	1.24	*
110	jsr166	1.26	* @return <tt>true</tt> if this task completed
111	dl	1.17	*/
112			boolean isDone();
113	tim	1.1
114			/**
115	dl	1.23	* Waits if necessary for the computation to complete, and then
116	tim	1.1	* retrieves its result.
117			*
118	dl	1.23	* @return the computed result
119			* @throws CancellationException if the computation was cancelled
120			* @throws ExecutionException if the computation threw an
121	dl	1.4	* exception
122	dl	1.23	* @throws InterruptedException if the current thread was interrupted
123	dl	1.4	* while waiting
124	tim	1.1	*/
125			V get() throws InterruptedException, ExecutionException;
126
127			/**
128			* Waits if necessary for at most the given time for the computation
129	dl	1.23	* to complete, and then retrieves its result, if available.
130	tim	1.1	*
131			* @param timeout the maximum time to wait
132	dl	1.14	* @param unit the time unit of the timeout argument
133	dl	1.23	* @return the computed result
134			* @throws CancellationException if the computation was cancelled
135			* @throws ExecutionException if the computation threw an
136	dl	1.4	* exception
137	dl	1.23	* @throws InterruptedException if the current thread was interrupted
138	dl	1.4	* while waiting
139	tim	1.1	* @throws TimeoutException if the wait timed out
140			*/
141	dl	1.14	V get(long timeout, TimeUnit unit)
142	tim	1.1	throws InterruptedException, ExecutionException, TimeoutException;
143			}