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 (18 months, 1 week ago) by jsr166
Branch:	MAIN
CVS Tags:	HEAD
Changes since 1.46:	+2 -2 lines
Log Message:	whitespace
#	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	jsr166	1.35	* http://creativecommons.org/publicdomain/zero/1.0/
5	tim	1.1	*/
6
7			package java.util.concurrent;
8
9			/**
10	jsr166	1.37	* A {@code Future} represents the result of an asynchronous
11	dl	1.12	* 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	jsr166	1.37	* {@code get} when the computation has completed, blocking if
15	dl	1.17	* necessary until it is ready. Cancellation is performed by the
16	jsr166	1.37	* {@code cancel} method. Additional methods are provided to
17	dl	1.17	* determine if the task completed normally or was cancelled. Once a
18			* computation has completed, the computation cannot be cancelled.
19	jsr166	1.37	* If you would like to use a {@code Future} for the sake
20	dl	1.21	* of cancellability but not provide a usable result, you can
21	jsr166	1.34	* declare types of the form {@code Future<?>} and
22	jsr166	1.37	* return {@code null} as a result of the underlying task.
23	tim	1.1	*
24	jsr166	1.41	* <p><b>Sample Usage</b> (Note that the following classes are all
25	jsr166	1.38	* made-up.)
26			*
27	jsr166	1.40	* <pre> {@code
28	dl	1.10	* interface ArchiveSearcher { String search(String target); }
29	tim	1.1	* class App {
30	dl	1.43	* ExecutorService executor = ...;
31			* ArchiveSearcher searcher = ...;
32	jsr166	1.41	* void showSearch(String target) throws InterruptedException {
33			* Callable<String> task = () -> searcher.search(target);
34			* Future<String> future = executor.submit(task);
35	dl	1.5	* displayOtherThings(); // do other things while searching
36	tim	1.1	* try {
37	dl	1.5	* displayText(future.get()); // use future
38	tim	1.8	* } catch (ExecutionException ex) { cleanup(); return; }
39	tim	1.1	* }
40	jsr166	1.34	* }}</pre>
41	tim	1.1	*
42	jsr166	1.37	* 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	jsr166	1.40	* <pre> {@code
46	jsr166	1.41	* FutureTask<String> future = new FutureTask<>(task);
47	jsr166	1.36	* executor.execute(future);}</pre>
48	brian	1.29	*
49	jsr166	1.31	* <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	dl	1.30	*
53	tim	1.1	* @see FutureTask
54			* @see Executor
55	dl	1.12	* @since 1.5
56	dl	1.4	* @author Doug Lea
57	jsr166	1.37	* @param <V> The result type returned by this Future's {@code get} method
58	tim	1.1	*/
59	dl	1.17	public interface Future<V> {
60
61			/**
62	dl	1.42	* 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	dl	1.44	*
75	dl	1.42	* @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	jsr166	1.37	* @return {@code false} if the task could not be cancelled,
80	dl	1.42	* 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	dl	1.17	*/
85			boolean cancel(boolean mayInterruptIfRunning);
86
87			/**
88	jsr166	1.37	* Returns {@code true} if this task was cancelled before it completed
89	dl	1.17	* normally.
90			*
91	jsr166	1.37	* @return {@code true} if this task was cancelled before it completed
92	dl	1.17	*/
93			boolean isCancelled();
94
95			/**
96	jsr166	1.37	* Returns {@code true} if this task completed.
97	dl	1.17	*
98			* Completion may be due to normal termination, an exception, or
99			* cancellation -- in all of these cases, this method will return
100	jsr166	1.37	* {@code true}.
101	jsr166	1.24	*
102	jsr166	1.37	* @return {@code true} if this task completed
103	dl	1.17	*/
104			boolean isDone();
105	tim	1.1
106			/**
107	dl	1.23	* Waits if necessary for the computation to complete, and then
108	tim	1.1	* retrieves its result.
109			*
110	dl	1.23	* @return the computed result
111			* @throws CancellationException if the computation was cancelled
112			* @throws ExecutionException if the computation threw an
113	dl	1.4	* exception
114	dl	1.23	* @throws InterruptedException if the current thread was interrupted
115	dl	1.4	* while waiting
116	tim	1.1	*/
117			V get() throws InterruptedException, ExecutionException;
118
119			/**
120			* Waits if necessary for at most the given time for the computation
121	dl	1.23	* to complete, and then retrieves its result, if available.
122	tim	1.1	*
123			* @param timeout the maximum time to wait
124	dl	1.14	* @param unit the time unit of the timeout argument
125	dl	1.23	* @return the computed result
126			* @throws CancellationException if the computation was cancelled
127			* @throws ExecutionException if the computation threw an
128	dl	1.4	* exception
129	dl	1.23	* @throws InterruptedException if the current thread was interrupted
130	dl	1.4	* while waiting
131	tim	1.1	* @throws TimeoutException if the wait timed out
132			*/
133	dl	1.14	V get(long timeout, TimeUnit unit)
134	tim	1.1	throws InterruptedException, ExecutionException, TimeoutException;
135	dl	1.45
136			/**
137			* Returns the computed result, without waiting.
138			*
139	jsr166	1.47	* <p>This method is for cases where the caller knows that the task has
140	dl	1.46	* 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	dl	1.45	* {@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	dl	1.46	* @since 19
158	dl	1.45	*/
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	jsr166	1.47	* <p>This method is for cases where the caller knows that the task
184	dl	1.45	* 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	dl	1.46	* @since 19
195	dl	1.45	*/
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	dl	1.46	* @since 19
221	dl	1.45	*/
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	dl	1.46	* @since 19
252	dl	1.45	*/
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	tim	1.1	}