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: State changes to a <tt>Future</tt>
 * that <a href="package-summary.html#MemoryVisibility">
 * <i>happen-before</i></a> it is made available
 * <i>happen-before</i> access via <tt>Future.get()</tt>.
 *
 * @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:	1.30
Committed:	Thu Sep 8 00:04:00 2005 UTC (18 years, 8 months ago) by dl
Branch:	MAIN
Changes since 1.29:	+5 -0 lines
Log Message:	Edit pass for happens-before descriptions
#	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	import java.util.concurrent.*; // for javadoc (till 6280605 is fixed)
9
10	/**
11	* 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	* 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	* 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	*
25	* <p>
26	* <b>Sample Usage</b> (Note that the following classes are all
27	* made-up.) <p>
28	* <pre>
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	* }
46	* </pre>
47	*
48	* 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	* For example, the above construction with <tt>submit</tt> could be replaced by:
51	* <pre>
52	* 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	* </pre>
59	*
60	* <p> Memory consistency effects: State changes to a <tt>Future</tt>
61	* that <a href="package-summary.html#MemoryVisibility">
62	* <i>happen-before</i></a> it is made available
63	* <i>happen-before</i> access via <tt>Future.get()</tt>.
64	*
65	* @see FutureTask
66	* @see Executor
67	* @since 1.5
68	* @author Doug Lea
69	* @param <V> The result type returned by this Future's <tt>get</tt> method
70	*/
71	public interface Future<V> {
72
73	/**
74	* Attempts to cancel execution of this task. This attempt will
75	* fail if the task has already completed, already been cancelled,
76	* or could not be cancelled for some other reason. If successful,
77	* and this task has not started when <tt>cancel</tt> is called,
78	* this task should never run. If the task has already started,
79	* then the <tt>mayInterruptIfRunning</tt> parameter determines
80	* whether the thread executing this task should be interrupted in
81	* an attempt to stop the task.
82	*
83	* <p>After this method returns, subsequent calls to {@link #isDone} will
84	* always return <tt>true</tt>. Subsequent calls to {@link #isCancelled}
85	* will always return <tt>true</tt> if this method returned <tt>true</tt>.
86	*
87	* @param mayInterruptIfRunning <tt>true</tt> if the thread executing this
88	* task should be interrupted; otherwise, in-progress tasks are allowed
89	* to complete
90	* @return <tt>false</tt> if the task could not be cancelled,
91	* typically because it has already completed normally;
92	* <tt>true</tt> otherwise
93	*/
94	boolean cancel(boolean mayInterruptIfRunning);
95
96	/**
97	* Returns <tt>true</tt> if this task was cancelled before it completed
98	* normally.
99	*
100	* @return <tt>true</tt> if this task was cancelled before it completed
101	*/
102	boolean isCancelled();
103
104	/**
105	* Returns <tt>true</tt> if this task completed.
106	*
107	* Completion may be due to normal termination, an exception, or
108	* cancellation -- in all of these cases, this method will return
109	* <tt>true</tt>.
110	*
111	* @return <tt>true</tt> if this task completed
112	*/
113	boolean isDone();
114
115	/**
116	* Waits if necessary for the computation to complete, and then
117	* retrieves its result.
118	*
119	* @return the computed result
120	* @throws CancellationException if the computation was cancelled
121	* @throws ExecutionException if the computation threw an
122	* exception
123	* @throws InterruptedException if the current thread was interrupted
124	* while waiting
125	*/
126	V get() throws InterruptedException, ExecutionException;
127
128	/**
129	* Waits if necessary for at most the given time for the computation
130	* to complete, and then retrieves its result, if available.
131	*
132	* @param timeout the maximum time to wait
133	* @param unit the time unit of the timeout argument
134	* @return the computed result
135	* @throws CancellationException if the computation was cancelled
136	* @throws ExecutionException if the computation threw an
137	* exception
138	* @throws InterruptedException if the current thread was interrupted
139	* while waiting
140	* @throws TimeoutException if the wait timed out
141	*/
142	V get(long timeout, TimeUnit unit)
143	throws InterruptedException, ExecutionException, TimeoutException;
144	}