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 a filter-map of a stream of
     * Future objects where the filter matches tasks that completed successfully.
     * {@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 99
     */
    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 99
     */
    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 99
     */
    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 99
     */
    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:	1.45
Committed:	Fri Mar 18 16:01:42 2022 UTC (2 years, 2 months ago) by dl
Branch:	MAIN
Changes since 1.44:	+139 -0 lines
Log Message:	jdk17+ suppressWarnings, FJ updates
#	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			* <p> This method is for cases where the caller knows that the task has
140			* already completed successfully, for example a filter-map of a stream of
141			* Future objects where the filter matches tasks that completed successfully.
142			* {@snippet lang=java :
143			* results = futures.stream()
144			* .filter(f -> f.state() == Future.State.SUCCESS)
145			* .map(Future::resultNow)
146			* .toList();
147			* }
148			*
149			* @implSpec
150			* The default implementation invokes {@code isDone()} to test if the task
151			* has completed. If done, it invokes {@code get()} to obtain the result.
152			*
153			* @return the computed result
154			* @throws IllegalStateException if the task has not completed or the task
155			* did not complete with a result
156			* @since 99
157			*/
158			default V resultNow() {
159			if (!isDone())
160			throw new IllegalStateException("Task has not completed");
161			boolean interrupted = false;
162			try {
163			while (true) {
164			try {
165			return get();
166			} catch (InterruptedException e) {
167			interrupted = true;
168			} catch (ExecutionException e) {
169			throw new IllegalStateException("Task completed with exception");
170			} catch (CancellationException e) {
171			throw new IllegalStateException("Task was cancelled");
172			}
173			}
174			} finally {
175			if (interrupted) Thread.currentThread().interrupt();
176			}
177			}
178
179			/**
180			* Returns the exception thrown by the task, without waiting.
181			*
182			* <p> This method is for cases where the caller knows that the task
183			* has already completed with an exception.
184			*
185			* @implSpec
186			* The default implementation invokes {@code isDone()} to test if the task
187			* has completed. If done and not cancelled, it invokes {@code get()} and
188			* catches the {@code ExecutionException} to obtain the exception.
189			*
190			* @return the exception thrown by the task
191			* @throws IllegalStateException if the task has not completed, the task
192			* completed normally, or the task was cancelled
193			* @since 99
194			*/
195			default Throwable exceptionNow() {
196			if (!isDone())
197			throw new IllegalStateException("Task has not completed");
198			if (isCancelled())
199			throw new IllegalStateException("Task was cancelled");
200			boolean interrupted = false;
201			try {
202			while (true) {
203			try {
204			get();
205			throw new IllegalStateException("Task completed with a result");
206			} catch (InterruptedException e) {
207			interrupted = true;
208			} catch (ExecutionException e) {
209			return e.getCause();
210			}
211			}
212			} finally {
213			if (interrupted) Thread.currentThread().interrupt();
214			}
215			}
216
217			/**
218			* Represents the computation state.
219			* @since 99
220			*/
221			enum State {
222			/**
223			* The task has not completed.
224			*/
225			RUNNING,
226			/**
227			* The task completed with a result.
228			* @see Future#resultNow()
229			*/
230			SUCCESS,
231			/**
232			* The task completed with an exception.
233			* @see Future#exceptionNow()
234			*/
235			FAILED,
236			/**
237			* The task was cancelled.
238			* @see #cancel(boolean)
239			*/
240			CANCELLED
241			}
242
243			/**
244			* {@return the computation state}
245			*
246			* @implSpec
247			* The default implementation uses {@code isDone()}, {@code isCancelled()},
248			* and {@code get()} to determine the state.
249			*
250			* @since 99
251			*/
252			default State state() {
253			if (!isDone())
254			return State.RUNNING;
255			if (isCancelled())
256			return State.CANCELLED;
257			boolean interrupted = false;
258			try {
259			while (true) {
260			try {
261			get(); // may throw InterruptedException when done
262			return State.SUCCESS;
263			} catch (InterruptedException e) {
264			interrupted = true;
265			} catch (ExecutionException e) {
266			return State.FAILED;
267			}
268			}
269			} finally {
270			if (interrupted) Thread.currentThread().interrupt();
271			}
272			}
273	tim	1.1	}
274	dl	1.45