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;

/**
 * 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:	1.32
Committed:	Tue Feb 7 20:54:24 2006 UTC (18 years, 3 months ago) by jsr166
Branch:	MAIN
Changes since 1.31:	+0 -1 lines
Log Message:	6378729: Remove workaround for 6280605
#	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/licenses/publicdomain
5	*/
6
7	package java.util.concurrent;
8
9	/**
10	* A <tt>Future</tt> 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	* <tt>get</tt> when the computation has completed, blocking if
15	* necessary until it is ready. Cancellation is performed by the
16	* <tt>cancel</tt> 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 <tt>Future</tt> for the sake
20	* of cancellability but not provide a usable result, you can
21	* declare types of the form <tt>Future<?></tt> and
22	* return <tt>null</tt> 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>
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	* }
45	* </pre>
46	*
47	* The {@link FutureTask} class is an implementation of <tt>Future</tt> that
48	* implements <tt>Runnable</tt>, and so may be executed by an <tt>Executor</tt>.
49	* For example, the above construction with <tt>submit</tt> could be replaced by:
50	* <pre>
51	* FutureTask<String> future =
52	* new FutureTask<String>(new Callable<String>() {
53	* public String call() {
54	* return searcher.search(target);
55	* }});
56	* executor.execute(future);
57	* </pre>
58	*
59	* <p>Memory consistency effects: Actions taken by the asynchronous computation
60	* <a href="package-summary.html#MemoryVisibility"> <i>happen-before</i></a>
61	* actions following the corresponding {@code Future.get()} in another thread.
62	*
63	* @see FutureTask
64	* @see Executor
65	* @since 1.5
66	* @author Doug Lea
67	* @param <V> The result type returned by this Future's <tt>get</tt> method
68	*/
69	public interface Future<V> {
70
71	/**
72	* Attempts to cancel execution of this task. This attempt will
73	* fail if the task has already completed, already been cancelled,
74	* or could not be cancelled for some other reason. If successful,
75	* and this task has not started when <tt>cancel</tt> is called,
76	* this task should never run. If the task has already started,
77	* then the <tt>mayInterruptIfRunning</tt> parameter determines
78	* whether the thread executing this task should be interrupted in
79	* an attempt to stop the task.
80	*
81	* <p>After this method returns, subsequent calls to {@link #isDone} will
82	* always return <tt>true</tt>. Subsequent calls to {@link #isCancelled}
83	* will always return <tt>true</tt> if this method returned <tt>true</tt>.
84	*
85	* @param mayInterruptIfRunning <tt>true</tt> if the thread executing this
86	* task should be interrupted; otherwise, in-progress tasks are allowed
87	* to complete
88	* @return <tt>false</tt> if the task could not be cancelled,
89	* typically because it has already completed normally;
90	* <tt>true</tt> otherwise
91	*/
92	boolean cancel(boolean mayInterruptIfRunning);
93
94	/**
95	* Returns <tt>true</tt> if this task was cancelled before it completed
96	* normally.
97	*
98	* @return <tt>true</tt> if this task was cancelled before it completed
99	*/
100	boolean isCancelled();
101
102	/**
103	* Returns <tt>true</tt> if this task completed.
104	*
105	* Completion may be due to normal termination, an exception, or
106	* cancellation -- in all of these cases, this method will return
107	* <tt>true</tt>.
108	*
109	* @return <tt>true</tt> if this task completed
110	*/
111	boolean isDone();
112
113	/**
114	* Waits if necessary for the computation to complete, and then
115	* retrieves its result.
116	*
117	* @return the computed result
118	* @throws CancellationException if the computation was cancelled
119	* @throws ExecutionException if the computation threw an
120	* exception
121	* @throws InterruptedException if the current thread was interrupted
122	* while waiting
123	*/
124	V get() throws InterruptedException, ExecutionException;
125
126	/**
127	* Waits if necessary for at most the given time for the computation
128	* to complete, and then retrieves its result, if available.
129	*
130	* @param timeout the maximum time to wait
131	* @param unit the time unit of the timeout argument
132	* @return the computed result
133	* @throws CancellationException if the computation was cancelled
134	* @throws ExecutionException if the computation threw an
135	* exception
136	* @throws InterruptedException if the current thread was interrupted
137	* while waiting
138	* @throws TimeoutException if the wait timed out
139	*/
140	V get(long timeout, TimeUnit unit)
141	throws InterruptedException, ExecutionException, TimeoutException;
142	}