util/concurrent/ExecutorCompletionService.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 {@link CompletionService} that uses a supplied {@link Executor}
 * to execute tasks.  This class arranges that submitted tasks are,
 * upon completion, placed on a queue accessible using {@code take}.
 * The class is lightweight enough to be suitable for transient use
 * when processing groups of tasks.
 *
 * <p>
 *
 * <b>Usage Examples.</b>
 *
 * Suppose you have a set of solvers for a certain problem, each
 * returning a value of some type {@code Result}, and would like to
 * run them concurrently, processing the results of each of them that
 * return a non-null value, in some method {@code use(Result r)}. You
 * could write this as:
 *
 * <pre> {@code
 * void solve(Executor e,
 *            Collection<Callable<Result>> solvers)
 *     throws InterruptedException, ExecutionException {
 *   CompletionService<Result> cs
 *       = new ExecutorCompletionService<>(e);
 *   solvers.forEach(cs::submit);
 *   for (int i = solvers.size(); i > 0; i--) {
 *     Result r = cs.take().get();
 *     if (r != null)
 *       use(r);
 *   }
 * }}</pre>
 *
 * Suppose instead that you would like to use the first non-null result
 * of the set of tasks, ignoring any that encounter exceptions,
 * and cancelling all other tasks when the first one is ready:
 *
 * <pre> {@code
 * void solve(Executor e,
 *            Collection<Callable<Result>> solvers)
 *     throws InterruptedException {
 *   CompletionService<Result> cs
 *       = new ExecutorCompletionService<>(e);
 *   int n = solvers.size();
 *   List<Future<Result>> futures = new ArrayList<>(n);
 *   Result result = null;
 *   try {
 *     solvers.forEach(solver -> futures.add(cs.submit(solver)));
 *     for (int i = n; i > 0; i--) {
 *       try {
 *         Result r = cs.take().get();
 *         if (r != null) {
 *           result = r;
 *           break;
 *         }
 *       } catch (ExecutionException ignore) {}
 *     }
 *   } finally {
 *     futures.forEach(future -> future.cancel(true));
 *   }
 *
 *   if (result != null)
 *     use(result);
 * }}</pre>
 */
public class ExecutorCompletionService<V> implements CompletionService<V> {
    private final Executor executor;
    private final AbstractExecutorService aes;
    private final BlockingQueue<Future<V>> completionQueue;

    /**
     * FutureTask extension to enqueue upon completion.
     */
    private static class QueueingFuture<V> extends FutureTask<Void> {
        QueueingFuture(RunnableFuture<V> task,
                       BlockingQueue<Future<V>> completionQueue) {
            super(task, null);
            this.task = task;
            this.completionQueue = completionQueue;
        }
        private final Future<V> task;
        private final BlockingQueue<Future<V>> completionQueue;
        protected void done() { completionQueue.add(task); }
    }

    private RunnableFuture<V> newTaskFor(Callable<V> task) {
        if (aes == null)
            return new FutureTask<V>(task);
        else
            return aes.newTaskFor(task);
    }

    private RunnableFuture<V> newTaskFor(Runnable task, V result) {
        if (aes == null)
            return new FutureTask<V>(task, result);
        else
            return aes.newTaskFor(task, result);
    }

    /**
     * Creates an ExecutorCompletionService using the supplied
     * executor for base task execution and a
     * {@link LinkedBlockingQueue} as a completion queue.
     *
     * @param executor the executor to use
     * @throws NullPointerException if executor is {@code null}
     */
    public ExecutorCompletionService(Executor executor) {
        if (executor == null)
            throw new NullPointerException();
        this.executor = executor;
        this.aes = (executor instanceof AbstractExecutorService) ?
            (AbstractExecutorService) executor : null;
        this.completionQueue = new LinkedBlockingQueue<Future<V>>();
    }

    /**
     * Creates an ExecutorCompletionService using the supplied
     * executor for base task execution and the supplied queue as its
     * completion queue.
     *
     * @param executor the executor to use
     * @param completionQueue the queue to use as the completion queue
     *        normally one dedicated for use by this service. This
     *        queue is treated as unbounded -- failed attempted
     *        {@code Queue.add} operations for completed tasks cause
     *        them not to be retrievable.
     * @throws NullPointerException if executor or completionQueue are {@code null}
     */
    public ExecutorCompletionService(Executor executor,
                                     BlockingQueue<Future<V>> completionQueue) {
        if (executor == null || completionQueue == null)
            throw new NullPointerException();
        this.executor = executor;
        this.aes = (executor instanceof AbstractExecutorService) ?
            (AbstractExecutorService) executor : null;
        this.completionQueue = completionQueue;
    }

    public Future<V> submit(Callable<V> task) {
        if (task == null) throw new NullPointerException();
        RunnableFuture<V> f = newTaskFor(task);
        executor.execute(new QueueingFuture<V>(f, completionQueue));
        return f;
    }

    public Future<V> submit(Runnable task, V result) {
        if (task == null) throw new NullPointerException();
        RunnableFuture<V> f = newTaskFor(task, result);
        executor.execute(new QueueingFuture<V>(f, completionQueue));
        return f;
    }

    public Future<V> take() throws InterruptedException {
        return completionQueue.take();
    }

    public Future<V> poll() {
        return completionQueue.poll();
    }

    public Future<V> poll(long timeout, TimeUnit unit)
            throws InterruptedException {
        return completionQueue.poll(timeout, unit);
    }

}
Revision:	1.28
Committed:	Sun Nov 6 22:15:01 2016 UTC (7 years, 6 months ago) by jsr166
Branch:	MAIN
Changes since 1.27:	+2 -2 lines
Log Message:	elide parens in unary lambdas
#	User	Rev	Content
1	dl	1.1	/*
2			* Written by Doug Lea with assistance from members of JCP JSR-166
3	dl	1.8	* Expert Group and released to the public domain, as explained at
4	jsr166	1.20	* http://creativecommons.org/publicdomain/zero/1.0/
5	dl	1.1	*/
6
7			package java.util.concurrent;
8
9			/**
10	jozart	1.2	* A {@link CompletionService} that uses a supplied {@link Executor}
11	dl	1.7	* to execute tasks. This class arranges that submitted tasks are,
12	jsr166	1.18	* upon completion, placed on a queue accessible using {@code take}.
13	dl	1.7	* The class is lightweight enough to be suitable for transient use
14			* when processing groups of tasks.
15	dl	1.5	*
16			* <p>
17			*
18			* <b>Usage Examples.</b>
19	dl	1.9	*
20			* Suppose you have a set of solvers for a certain problem, each
21	jsr166	1.18	* returning a value of some type {@code Result}, and would like to
22	dl	1.9	* run them concurrently, processing the results of each of them that
23	jsr166	1.18	* return a non-null value, in some method {@code use(Result r)}. You
24	dl	1.9	* could write this as:
25	dl	1.5	*
26	jsr166	1.18	* <pre> {@code
27			* void solve(Executor e,
28			* Collection<Callable<Result>> solvers)
29	jsr166	1.15	* throws InterruptedException, ExecutionException {
30	jsr166	1.26	* CompletionService<Result> cs
31	jsr166	1.25	* = new ExecutorCompletionService<>(e);
32	jsr166	1.27	* solvers.forEach(cs::submit);
33	jsr166	1.26	* for (int i = solvers.size(); i > 0; i--) {
34			* Result r = cs.take().get();
35	jsr166	1.22	* if (r != null)
36			* use(r);
37			* }
38	jsr166	1.18	* }}</pre>
39	dl	1.5	*
40			* Suppose instead that you would like to use the first non-null result
41	dl	1.9	* of the set of tasks, ignoring any that encounter exceptions,
42	dl	1.6	* and cancelling all other tasks when the first one is ready:
43	dl	1.5	*
44	jsr166	1.18	* <pre> {@code
45			* void solve(Executor e,
46			* Collection<Callable<Result>> solvers)
47	jsr166	1.15	* throws InterruptedException {
48	jsr166	1.26	* CompletionService<Result> cs
49	jsr166	1.25	* = new ExecutorCompletionService<>(e);
50	jsr166	1.22	* int n = solvers.size();
51			* List<Future<Result>> futures = new ArrayList<>(n);
52			* Result result = null;
53			* try {
54	jsr166	1.28	* solvers.forEach(solver -> futures.add(cs.submit(solver)));
55	jsr166	1.26	* for (int i = n; i > 0; i--) {
56	jsr166	1.22	* try {
57	jsr166	1.26	* Result r = cs.take().get();
58	jsr166	1.22	* if (r != null) {
59			* result = r;
60			* break;
61	jsr166	1.18	* }
62	jsr166	1.22	* } catch (ExecutionException ignore) {}
63	jsr166	1.18	* }
64	jsr166	1.26	* } finally {
65	jsr166	1.28	* futures.forEach(future -> future.cancel(true));
66	jsr166	1.22	* }
67	jsr166	1.18	*
68	jsr166	1.22	* if (result != null)
69			* use(result);
70	jsr166	1.18	* }}</pre>
71	dl	1.1	*/
72			public class ExecutorCompletionService<V> implements CompletionService<V> {
73	jozart	1.2	private final Executor executor;
74	dl	1.13	private final AbstractExecutorService aes;
75	dl	1.3	private final BlockingQueue<Future<V>> completionQueue;
76	dl	1.1
77			/**
78	jsr166	1.24	* FutureTask extension to enqueue upon completion.
79	dl	1.1	*/
80	jsr166	1.23	private static class QueueingFuture<V> extends FutureTask<Void> {
81			QueueingFuture(RunnableFuture<V> task,
82			BlockingQueue<Future<V>> completionQueue) {
83	dl	1.13	super(task, null);
84			this.task = task;
85	jsr166	1.23	this.completionQueue = completionQueue;
86	dl	1.13	}
87	jsr166	1.23	private final Future<V> task;
88			private final BlockingQueue<Future<V>> completionQueue;
89	dl	1.13	protected void done() { completionQueue.add(task); }
90			}
91
92			private RunnableFuture<V> newTaskFor(Callable<V> task) {
93			if (aes == null)
94			return new FutureTask<V>(task);
95			else
96			return aes.newTaskFor(task);
97			}
98
99			private RunnableFuture<V> newTaskFor(Runnable task, V result) {
100			if (aes == null)
101			return new FutureTask<V>(task, result);
102			else
103			return aes.newTaskFor(task, result);
104	dl	1.1	}
105
106			/**
107			* Creates an ExecutorCompletionService using the supplied
108	dl	1.3	* executor for base task execution and a
109			* {@link LinkedBlockingQueue} as a completion queue.
110	jsr166	1.15	*
111	dl	1.5	* @param executor the executor to use
112	jsr166	1.18	* @throws NullPointerException if executor is {@code null}
113	dl	1.1	*/
114	jozart	1.2	public ExecutorCompletionService(Executor executor) {
115	jsr166	1.12	if (executor == null)
116	dl	1.1	throw new NullPointerException();
117			this.executor = executor;
118	dl	1.13	this.aes = (executor instanceof AbstractExecutorService) ?
119			(AbstractExecutorService) executor : null;
120	dl	1.3	this.completionQueue = new LinkedBlockingQueue<Future<V>>();
121	dl	1.1	}
122
123			/**
124	dl	1.3	* Creates an ExecutorCompletionService using the supplied
125			* executor for base task execution and the supplied queue as its
126			* completion queue.
127	jsr166	1.15	*
128	dl	1.5	* @param executor the executor to use
129	dl	1.7	* @param completionQueue the queue to use as the completion queue
130	jsr166	1.18	* normally one dedicated for use by this service. This
131			* queue is treated as unbounded -- failed attempted
132	jsr166	1.21	* {@code Queue.add} operations for completed tasks cause
133	jsr166	1.18	* them not to be retrievable.
134			* @throws NullPointerException if executor or completionQueue are {@code null}
135	dl	1.1	*/
136	dl	1.3	public ExecutorCompletionService(Executor executor,
137			BlockingQueue<Future<V>> completionQueue) {
138	jsr166	1.12	if (executor == null \|\| completionQueue == null)
139	dl	1.3	throw new NullPointerException();
140			this.executor = executor;
141	dl	1.13	this.aes = (executor instanceof AbstractExecutorService) ?
142			(AbstractExecutorService) executor : null;
143	dl	1.3	this.completionQueue = completionQueue;
144	dl	1.1	}
145
146			public Future<V> submit(Callable<V> task) {
147	dl	1.7	if (task == null) throw new NullPointerException();
148	dl	1.13	RunnableFuture<V> f = newTaskFor(task);
149	jsr166	1.23	executor.execute(new QueueingFuture<V>(f, completionQueue));
150	dl	1.1	return f;
151			}
152
153			public Future<V> submit(Runnable task, V result) {
154	dl	1.7	if (task == null) throw new NullPointerException();
155	dl	1.13	RunnableFuture<V> f = newTaskFor(task, result);
156	jsr166	1.23	executor.execute(new QueueingFuture<V>(f, completionQueue));
157	dl	1.1	return f;
158			}
159
160			public Future<V> take() throws InterruptedException {
161	dl	1.3	return completionQueue.take();
162	dl	1.1	}
163
164			public Future<V> poll() {
165	dl	1.3	return completionQueue.poll();
166	dl	1.1	}
167
168	jsr166	1.19	public Future<V> poll(long timeout, TimeUnit unit)
169			throws InterruptedException {
170	dl	1.3	return completionQueue.poll(timeout, unit);
171	dl	1.1	}
172	dl	1.5
173	dl	1.1	}