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/licenses/publicdomain
 */

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> ecs
 *         = new ExecutorCompletionService<Result>(e);
 *     for (Callable<Result> s : solvers)
 *         ecs.submit(s);
 *     int n = solvers.size();
 *     for (int i = 0; i < n; ++i) {
 *         Result r = ecs.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> ecs
 *         = new ExecutorCompletionService<Result>(e);
 *     int n = solvers.size();
 *     List<Future<Result>> futures
 *         = new ArrayList<Future<Result>>(n);
 *     Result result = null;
 *     try {
 *         for (Callable<Result> s : solvers)
 *             futures.add(ecs.submit(s));
 *         for (int i = 0; i < n; ++i) {
 *             try {
 *                 Result r = ecs.take().get();
 *                 if (r != null) {
 *                     result = r;
 *                     break;
 *                 }
 *             } catch (ExecutionException ignore) {}
 *         }
 *     }
 *     finally {
 *         for (Future<Result> f : futures)
 *             f.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 class QueueingFuture extends FutureTask<Void> {
        QueueingFuture(RunnableFuture<V> task) {
            super(task, null);
            this.task = task;
        }
        protected void done() { completionQueue.add(task); }
        private final Future<V> 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 taskes 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(f));
        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(f));
        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:	1.19
Committed:	Fri Oct 22 05:49:04 2010 UTC (13 years, 7 months ago) by jsr166
Branch:	MAIN
Changes since 1.18:	+2 -1 lines
Log Message:	80 cols
#	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			* http://creativecommons.org/licenses/publicdomain
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	1.18	* CompletionService<Result> ecs
31			* = new ExecutorCompletionService<Result>(e);
32			* for (Callable<Result> s : solvers)
33			* ecs.submit(s);
34			* int n = solvers.size();
35			* for (int i = 0; i < n; ++i) {
36			* Result r = ecs.take().get();
37			* if (r != null)
38			* use(r);
39			* }
40			* }}</pre>
41	dl	1.5	*
42			* Suppose instead that you would like to use the first non-null result
43	dl	1.9	* of the set of tasks, ignoring any that encounter exceptions,
44	dl	1.6	* and cancelling all other tasks when the first one is ready:
45	dl	1.5	*
46	jsr166	1.18	* <pre> {@code
47			* void solve(Executor e,
48			* Collection<Callable<Result>> solvers)
49	jsr166	1.15	* throws InterruptedException {
50	jsr166	1.18	* CompletionService<Result> ecs
51			* = new ExecutorCompletionService<Result>(e);
52			* int n = solvers.size();
53			* List<Future<Result>> futures
54			* = new ArrayList<Future<Result>>(n);
55			* Result result = null;
56			* try {
57			* for (Callable<Result> s : solvers)
58			* futures.add(ecs.submit(s));
59			* for (int i = 0; i < n; ++i) {
60			* try {
61			* Result r = ecs.take().get();
62			* if (r != null) {
63			* result = r;
64			* break;
65			* }
66			* } catch (ExecutionException ignore) {}
67			* }
68			* }
69			* finally {
70			* for (Future<Result> f : futures)
71			* f.cancel(true);
72			* }
73			*
74			* if (result != null)
75			* use(result);
76			* }}</pre>
77	dl	1.1	*/
78			public class ExecutorCompletionService<V> implements CompletionService<V> {
79	jozart	1.2	private final Executor executor;
80	dl	1.13	private final AbstractExecutorService aes;
81	dl	1.3	private final BlockingQueue<Future<V>> completionQueue;
82	dl	1.1
83			/**
84			* FutureTask extension to enqueue upon completion
85			*/
86	dl	1.13	private class QueueingFuture extends FutureTask<Void> {
87			QueueingFuture(RunnableFuture<V> task) {
88			super(task, null);
89			this.task = task;
90			}
91			protected void done() { completionQueue.add(task); }
92			private final Future<V> task;
93			}
94
95			private RunnableFuture<V> newTaskFor(Callable<V> task) {
96			if (aes == null)
97			return new FutureTask<V>(task);
98			else
99			return aes.newTaskFor(task);
100			}
101
102			private RunnableFuture<V> newTaskFor(Runnable task, V result) {
103			if (aes == null)
104			return new FutureTask<V>(task, result);
105			else
106			return aes.newTaskFor(task, result);
107	dl	1.1	}
108
109			/**
110			* Creates an ExecutorCompletionService using the supplied
111	dl	1.3	* executor for base task execution and a
112			* {@link LinkedBlockingQueue} as a completion queue.
113	jsr166	1.15	*
114	dl	1.5	* @param executor the executor to use
115	jsr166	1.18	* @throws NullPointerException if executor is {@code null}
116	dl	1.1	*/
117	jozart	1.2	public ExecutorCompletionService(Executor executor) {
118	jsr166	1.12	if (executor == null)
119	dl	1.1	throw new NullPointerException();
120			this.executor = executor;
121	dl	1.13	this.aes = (executor instanceof AbstractExecutorService) ?
122			(AbstractExecutorService) executor : null;
123	dl	1.3	this.completionQueue = new LinkedBlockingQueue<Future<V>>();
124	dl	1.1	}
125
126			/**
127	dl	1.3	* Creates an ExecutorCompletionService using the supplied
128			* executor for base task execution and the supplied queue as its
129			* completion queue.
130	jsr166	1.15	*
131	dl	1.5	* @param executor the executor to use
132	dl	1.7	* @param completionQueue the queue to use as the completion queue
133	jsr166	1.18	* normally one dedicated for use by this service. This
134			* queue is treated as unbounded -- failed attempted
135			* {@code Queue.add} operations for completed taskes cause
136			* them not to be retrievable.
137			* @throws NullPointerException if executor or completionQueue are {@code null}
138	dl	1.1	*/
139	dl	1.3	public ExecutorCompletionService(Executor executor,
140			BlockingQueue<Future<V>> completionQueue) {
141	jsr166	1.12	if (executor == null \|\| completionQueue == null)
142	dl	1.3	throw new NullPointerException();
143			this.executor = executor;
144	dl	1.13	this.aes = (executor instanceof AbstractExecutorService) ?
145			(AbstractExecutorService) executor : null;
146	dl	1.3	this.completionQueue = completionQueue;
147	dl	1.1	}
148
149			public Future<V> submit(Callable<V> task) {
150	dl	1.7	if (task == null) throw new NullPointerException();
151	dl	1.13	RunnableFuture<V> f = newTaskFor(task);
152			executor.execute(new QueueingFuture(f));
153	dl	1.1	return f;
154			}
155
156			public Future<V> submit(Runnable task, V result) {
157	dl	1.7	if (task == null) throw new NullPointerException();
158	dl	1.13	RunnableFuture<V> f = newTaskFor(task, result);
159			executor.execute(new QueueingFuture(f));
160	dl	1.1	return f;
161			}
162
163			public Future<V> take() throws InterruptedException {
164	dl	1.3	return completionQueue.take();
165	dl	1.1	}
166
167			public Future<V> poll() {
168	dl	1.3	return completionQueue.poll();
169	dl	1.1	}
170
171	jsr166	1.19	public Future<V> poll(long timeout, TimeUnit unit)
172			throws InterruptedException {
173	dl	1.3	return completionQueue.poll(timeout, unit);
174	dl	1.1	}
175	dl	1.5
176	dl	1.1	}