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}.
     *
     * @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;

    /**
     * Returns the computed result, without waiting.
     *
     * <p>This method is for cases where the caller knows that the task has
     * already completed successfully, for example when filtering a stream
     * of Future objects for the successful tasks and using a mapping
     * operation to obtain a stream of results.
     * {@snippet lang=java :
     *     results = futures.stream()
     *                .filter(f -> f.state() == Future.State.SUCCESS)
     *                .map(Future::resultNow)
     *                .toList();
     * }
     *
     * @implSpec
     * The default implementation invokes {@code isDone()} to test if the task
     * has completed. If done, it invokes {@code get()} to obtain the result.
     *
     * @return the computed result
     * @throws IllegalStateException if the task has not completed or the task
     * did not complete with a result
     * @since 19
     */
    default V resultNow() {
        if (!isDone())
            throw new IllegalStateException("Task has not completed");
        boolean interrupted = false;
        try {
            while (true) {
                try {
                    return get();
                } catch (InterruptedException e) {
                    interrupted = true;
                } catch (ExecutionException e) {
                    throw new IllegalStateException("Task completed with exception");
                } catch (CancellationException e) {
                    throw new IllegalStateException("Task was cancelled");
                }
            }
        } finally {
            if (interrupted) Thread.currentThread().interrupt();
        }
    }

    /**
     * Returns the exception thrown by the task, without waiting.
     *
     * <p>This method is for cases where the caller knows that the task
     * has already completed with an exception.
     *
     * @implSpec
     * The default implementation invokes {@code isDone()} to test if the task
     * has completed. If done and not cancelled, it invokes {@code get()} and
     * catches the {@code ExecutionException} to obtain the exception.
     *
     * @return the exception thrown by the task
     * @throws IllegalStateException if the task has not completed, the task
     * completed normally, or the task was cancelled
     * @since 19
     */
    default Throwable exceptionNow() {
        if (!isDone())
            throw new IllegalStateException("Task has not completed");
        if (isCancelled())
            throw new IllegalStateException("Task was cancelled");
        boolean interrupted = false;
        try {
            while (true) {
                try {
                    get();
                    throw new IllegalStateException("Task completed with a result");
                } catch (InterruptedException e) {
                    interrupted = true;
                } catch (ExecutionException e) {
                    return e.getCause();
                }
            }
        } finally {
            if (interrupted) Thread.currentThread().interrupt();
        }
    }

    /**
     * Represents the computation state.
     * @since 19
     */
    enum State {
        /**
         * The task has not completed.
         */
        RUNNING,
        /**
         * The task completed with a result.
         * @see Future#resultNow()
         */
        SUCCESS,
        /**
         * The task completed with an exception.
         * @see Future#exceptionNow()
         */
        FAILED,
        /**
         * The task was cancelled.
         * @see #cancel(boolean)
         */
        CANCELLED
    }

    /**
     * {@return the computation state}
     *
     * @implSpec
     * The default implementation uses {@code isDone()}, {@code isCancelled()},
     * and {@code get()} to determine the state.
     *
     * @since 19
     */
    default State state() {
        if (!isDone())
            return State.RUNNING;
        if (isCancelled())
            return State.CANCELLED;
        boolean interrupted = false;
        try {
            while (true) {
                try {
                    get();  // may throw InterruptedException when done
                    return State.SUCCESS;
                } catch (InterruptedException e) {
                    interrupted = true;
                } catch (ExecutionException e) {
                    return State.FAILED;
                }
            }
        } finally {
            if (interrupted) Thread.currentThread().interrupt();
        }
    }
}
Revision:	1.47
Committed:	Fri Nov 25 16:39:48 2022 UTC (17 months, 3 weeks ago) by jsr166
Branch:	MAIN
CVS Tags:	HEAD
Changes since 1.46:	+2 -2 lines
Log Message:	whitespace
#	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	*
75	* @param mayInterruptIfRunning {@code true} if the thread
76	* executing this task should be interrupted (if the thread is
77	* known to the implementation); otherwise, in-progress tasks are
78	* allowed to complete
79	* @return {@code false} if the task could not be cancelled,
80	* typically because it has already completed; {@code true}
81	* otherwise. If two or more threads cause a task to be cancelled,
82	* then at least one of them returns {@code true}. Implementations
83	* may provide stronger guarantees.
84	*/
85	boolean cancel(boolean mayInterruptIfRunning);
86
87	/**
88	* Returns {@code true} if this task was cancelled before it completed
89	* normally.
90	*
91	* @return {@code true} if this task was cancelled before it completed
92	*/
93	boolean isCancelled();
94
95	/**
96	* Returns {@code true} if this task completed.
97	*
98	* Completion may be due to normal termination, an exception, or
99	* cancellation -- in all of these cases, this method will return
100	* {@code true}.
101	*
102	* @return {@code true} if this task completed
103	*/
104	boolean isDone();
105
106	/**
107	* Waits if necessary for the computation to complete, and then
108	* retrieves its result.
109	*
110	* @return the computed result
111	* @throws CancellationException if the computation was cancelled
112	* @throws ExecutionException if the computation threw an
113	* exception
114	* @throws InterruptedException if the current thread was interrupted
115	* while waiting
116	*/
117	V get() throws InterruptedException, ExecutionException;
118
119	/**
120	* Waits if necessary for at most the given time for the computation
121	* to complete, and then retrieves its result, if available.
122	*
123	* @param timeout the maximum time to wait
124	* @param unit the time unit of the timeout argument
125	* @return the computed result
126	* @throws CancellationException if the computation was cancelled
127	* @throws ExecutionException if the computation threw an
128	* exception
129	* @throws InterruptedException if the current thread was interrupted
130	* while waiting
131	* @throws TimeoutException if the wait timed out
132	*/
133	V get(long timeout, TimeUnit unit)
134	throws InterruptedException, ExecutionException, TimeoutException;
135
136	/**
137	* Returns the computed result, without waiting.
138	*
139	* <p>This method is for cases where the caller knows that the task has
140	* already completed successfully, for example when filtering a stream
141	* of Future objects for the successful tasks and using a mapping
142	* operation to obtain a stream of results.
143	* {@snippet lang=java :
144	* results = futures.stream()
145	* .filter(f -> f.state() == Future.State.SUCCESS)
146	* .map(Future::resultNow)
147	* .toList();
148	* }
149	*
150	* @implSpec
151	* The default implementation invokes {@code isDone()} to test if the task
152	* has completed. If done, it invokes {@code get()} to obtain the result.
153	*
154	* @return the computed result
155	* @throws IllegalStateException if the task has not completed or the task
156	* did not complete with a result
157	* @since 19
158	*/
159	default V resultNow() {
160	if (!isDone())
161	throw new IllegalStateException("Task has not completed");
162	boolean interrupted = false;
163	try {
164	while (true) {
165	try {
166	return get();
167	} catch (InterruptedException e) {
168	interrupted = true;
169	} catch (ExecutionException e) {
170	throw new IllegalStateException("Task completed with exception");
171	} catch (CancellationException e) {
172	throw new IllegalStateException("Task was cancelled");
173	}
174	}
175	} finally {
176	if (interrupted) Thread.currentThread().interrupt();
177	}
178	}
179
180	/**
181	* Returns the exception thrown by the task, without waiting.
182	*
183	* <p>This method is for cases where the caller knows that the task
184	* has already completed with an exception.
185	*
186	* @implSpec
187	* The default implementation invokes {@code isDone()} to test if the task
188	* has completed. If done and not cancelled, it invokes {@code get()} and
189	* catches the {@code ExecutionException} to obtain the exception.
190	*
191	* @return the exception thrown by the task
192	* @throws IllegalStateException if the task has not completed, the task
193	* completed normally, or the task was cancelled
194	* @since 19
195	*/
196	default Throwable exceptionNow() {
197	if (!isDone())
198	throw new IllegalStateException("Task has not completed");
199	if (isCancelled())
200	throw new IllegalStateException("Task was cancelled");
201	boolean interrupted = false;
202	try {
203	while (true) {
204	try {
205	get();
206	throw new IllegalStateException("Task completed with a result");
207	} catch (InterruptedException e) {
208	interrupted = true;
209	} catch (ExecutionException e) {
210	return e.getCause();
211	}
212	}
213	} finally {
214	if (interrupted) Thread.currentThread().interrupt();
215	}
216	}
217
218	/**
219	* Represents the computation state.
220	* @since 19
221	*/
222	enum State {
223	/**
224	* The task has not completed.
225	*/
226	RUNNING,
227	/**
228	* The task completed with a result.
229	* @see Future#resultNow()
230	*/
231	SUCCESS,
232	/**
233	* The task completed with an exception.
234	* @see Future#exceptionNow()
235	*/
236	FAILED,
237	/**
238	* The task was cancelled.
239	* @see #cancel(boolean)
240	*/
241	CANCELLED
242	}
243
244	/**
245	* {@return the computation state}
246	*
247	* @implSpec
248	* The default implementation uses {@code isDone()}, {@code isCancelled()},
249	* and {@code get()} to determine the state.
250	*
251	* @since 19
252	*/
253	default State state() {
254	if (!isDone())
255	return State.RUNNING;
256	if (isCancelled())
257	return State.CANCELLED;
258	boolean interrupted = false;
259	try {
260	while (true) {
261	try {
262	get(); // may throw InterruptedException when done
263	return State.SUCCESS;
264	} catch (InterruptedException e) {
265	interrupted = true;
266	} catch (ExecutionException e) {
267	return State.FAILED;
268	}
269	}
270	} finally {
271	if (interrupted) Thread.currentThread().interrupt();
272	}
273	}
274	}