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>
 * @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.
     *
     * @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 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:	1.24
Committed:	Tue Apr 26 01:17:18 2005 UTC (19 years, 1 month ago) by jsr166
Branch:	MAIN
Changes since 1.23:	+4 -7 lines
Log Message:	doc fixes
#	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
9			/**
10	dl	1.12	* 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	dl	1.17	* 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	dl	1.21	* 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	tim	1.1	*
24			* <p>
25	dl	1.5	* <b>Sample Usage</b> (Note that the following classes are all
26			* made-up.) <p>
27	tim	1.1	* <pre>
28	dl	1.10	* interface ArchiveSearcher { String search(String target); }
29	tim	1.1	* class App {
30	tim	1.18	* ExecutorService executor = ...
31	dl	1.5	* ArchiveSearcher searcher = ...
32			* void showSearch(final String target) throws InterruptedException {
33	tim	1.18	* Future<String> future = executor.submit(new Callable<String>() {
34			* public String call() { return searcher.search(target); }
35			* });
36	dl	1.5	* displayOtherThings(); // do other things while searching
37	tim	1.1	* try {
38	dl	1.5	* displayText(future.get()); // use future
39	tim	1.8	* } catch (ExecutionException ex) { cleanup(); return; }
40	tim	1.1	* }
41			* }
42			* </pre>
43			*
44	jsr166	1.24	* The {@link FutureTask} class is an implementation of <tt>Future</tt> that
45			* implements <tt>Runnable</tt>, and so may be executed by an <tt>Executor</tt>.
46	tim	1.18	* For example, the above construction with <tt>submit</tt> could be replaced by:
47	dl	1.10	* <pre>
48	tim	1.18	* FutureTask<String> future =
49			* new FutureTask<String>(new Callable<String>() {
50			* public String call() {
51			* return searcher.search(target);
52			* }});
53			* executor.execute(future);
54	dl	1.10	* </pre>
55	tim	1.1	* @see FutureTask
56			* @see Executor
57	dl	1.12	* @since 1.5
58	dl	1.4	* @author Doug Lea
59	dl	1.16	* @param <V> The result type returned by this Future's <tt>get</tt> method
60	tim	1.1	*/
61	dl	1.17	public interface Future<V> {
62
63			/**
64			* Attempts to cancel execution of this task. This attempt will
65			* fail if the task has already completed, already been cancelled,
66			* or could not be cancelled for some other reason. If successful,
67			* and this task has not started when <tt>cancel</tt> is called,
68			* this task should never run. If the task has already started,
69	dl	1.23	* then the <tt>mayInterruptIfRunning</tt> parameter determines
70	dl	1.17	* whether the thread executing this task should be interrupted in
71			* an attempt to stop the task.
72			*
73			* @param mayInterruptIfRunning <tt>true</tt> if the thread executing this
74			* task should be interrupted; otherwise, in-progress tasks are allowed
75			* to complete
76			* @return <tt>false</tt> if the task could not be cancelled,
77	jozart	1.19	* typically because it has already completed normally;
78	dl	1.17	* <tt>true</tt> otherwise
79			*/
80			boolean cancel(boolean mayInterruptIfRunning);
81
82			/**
83			* Returns <tt>true</tt> if this task was cancelled before it completed
84			* normally.
85			*
86			* @return <tt>true</tt> if task was cancelled before it completed
87			*/
88			boolean isCancelled();
89
90			/**
91	jsr166	1.24	* Returns <tt>true</tt> if this task completed.
92	dl	1.17	*
93			* Completion may be due to normal termination, an exception, or
94			* cancellation -- in all of these cases, this method will return
95			* <tt>true</tt>.
96	jsr166	1.24	*
97	dl	1.17	* @return <tt>true</tt> if this task completed.
98			*/
99			boolean isDone();
100	tim	1.1
101			/**
102	dl	1.23	* Waits if necessary for the computation to complete, and then
103	tim	1.1	* retrieves its result.
104			*
105	dl	1.23	* @return the computed result
106			* @throws CancellationException if the computation was cancelled
107			* @throws ExecutionException if the computation threw an
108	dl	1.4	* exception
109	dl	1.23	* @throws InterruptedException if the current thread was interrupted
110	dl	1.4	* while waiting
111	tim	1.1	*/
112			V get() throws InterruptedException, ExecutionException;
113
114			/**
115			* Waits if necessary for at most the given time for the computation
116	dl	1.23	* to complete, and then retrieves its result, if available.
117	tim	1.1	*
118			* @param timeout the maximum time to wait
119	dl	1.14	* @param unit the time unit of the timeout argument
120	dl	1.23	* @return the computed result
121			* @throws CancellationException if the computation was cancelled
122			* @throws ExecutionException if the computation threw an
123	dl	1.4	* exception
124	dl	1.23	* @throws InterruptedException if the current thread was interrupted
125	dl	1.4	* while waiting
126	tim	1.1	* @throws TimeoutException if the wait timed out
127			*/
128	dl	1.14	V get(long timeout, TimeUnit unit)
129	tim	1.1	throws InterruptedException, ExecutionException, TimeoutException;
130			}