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> 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<>(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 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(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:	1.22
Committed:	Sun May 25 02:41:29 2014 UTC (10 years ago) by jsr166
Branch:	MAIN
Changes since 1.21:	+32 -33 lines
Log Message:	use 2-space indent in code samples
#	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 {@link CompletionService} that uses a supplied {@link Executor}
11	* to execute tasks. This class arranges that submitted tasks are,
12	* upon completion, placed on a queue accessible using {@code take}.
13	* The class is lightweight enough to be suitable for transient use
14	* when processing groups of tasks.
15	*
16	* <p>
17	*
18	* <b>Usage Examples.</b>
19	*
20	* Suppose you have a set of solvers for a certain problem, each
21	* returning a value of some type {@code Result}, and would like to
22	* run them concurrently, processing the results of each of them that
23	* return a non-null value, in some method {@code use(Result r)}. You
24	* could write this as:
25	*
26	* <pre> {@code
27	* void solve(Executor e,
28	* Collection<Callable<Result>> solvers)
29	* throws InterruptedException, ExecutionException {
30	* 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	*
42	* Suppose instead that you would like to use the first non-null result
43	* of the set of tasks, ignoring any that encounter exceptions,
44	* and cancelling all other tasks when the first one is ready:
45	*
46	* <pre> {@code
47	* void solve(Executor e,
48	* Collection<Callable<Result>> solvers)
49	* throws InterruptedException {
50	* CompletionService<Result> ecs
51	* = new ExecutorCompletionService<Result>(e);
52	* int n = solvers.size();
53	* List<Future<Result>> futures = new ArrayList<>(n);
54	* Result result = null;
55	* try {
56	* for (Callable<Result> s : solvers)
57	* futures.add(ecs.submit(s));
58	* for (int i = 0; i < n; ++i) {
59	* try {
60	* Result r = ecs.take().get();
61	* if (r != null) {
62	* result = r;
63	* break;
64	* }
65	* } catch (ExecutionException ignore) {}
66	* }
67	* }
68	* finally {
69	* for (Future<Result> f : futures)
70	* f.cancel(true);
71	* }
72	*
73	* if (result != null)
74	* use(result);
75	* }}</pre>
76	*/
77	public class ExecutorCompletionService<V> implements CompletionService<V> {
78	private final Executor executor;
79	private final AbstractExecutorService aes;
80	private final BlockingQueue<Future<V>> completionQueue;
81
82	/**
83	* FutureTask extension to enqueue upon completion
84	*/
85	private class QueueingFuture extends FutureTask<Void> {
86	QueueingFuture(RunnableFuture<V> task) {
87	super(task, null);
88	this.task = task;
89	}
90	protected void done() { completionQueue.add(task); }
91	private final Future<V> task;
92	}
93
94	private RunnableFuture<V> newTaskFor(Callable<V> task) {
95	if (aes == null)
96	return new FutureTask<V>(task);
97	else
98	return aes.newTaskFor(task);
99	}
100
101	private RunnableFuture<V> newTaskFor(Runnable task, V result) {
102	if (aes == null)
103	return new FutureTask<V>(task, result);
104	else
105	return aes.newTaskFor(task, result);
106	}
107
108	/**
109	* Creates an ExecutorCompletionService using the supplied
110	* executor for base task execution and a
111	* {@link LinkedBlockingQueue} as a completion queue.
112	*
113	* @param executor the executor to use
114	* @throws NullPointerException if executor is {@code null}
115	*/
116	public ExecutorCompletionService(Executor executor) {
117	if (executor == null)
118	throw new NullPointerException();
119	this.executor = executor;
120	this.aes = (executor instanceof AbstractExecutorService) ?
121	(AbstractExecutorService) executor : null;
122	this.completionQueue = new LinkedBlockingQueue<Future<V>>();
123	}
124
125	/**
126	* Creates an ExecutorCompletionService using the supplied
127	* executor for base task execution and the supplied queue as its
128	* completion queue.
129	*
130	* @param executor the executor to use
131	* @param completionQueue the queue to use as the completion queue
132	* normally one dedicated for use by this service. This
133	* queue is treated as unbounded -- failed attempted
134	* {@code Queue.add} operations for completed tasks cause
135	* them not to be retrievable.
136	* @throws NullPointerException if executor or completionQueue are {@code null}
137	*/
138	public ExecutorCompletionService(Executor executor,
139	BlockingQueue<Future<V>> completionQueue) {
140	if (executor == null \|\| completionQueue == null)
141	throw new NullPointerException();
142	this.executor = executor;
143	this.aes = (executor instanceof AbstractExecutorService) ?
144	(AbstractExecutorService) executor : null;
145	this.completionQueue = completionQueue;
146	}
147
148	public Future<V> submit(Callable<V> task) {
149	if (task == null) throw new NullPointerException();
150	RunnableFuture<V> f = newTaskFor(task);
151	executor.execute(new QueueingFuture(f));
152	return f;
153	}
154
155	public Future<V> submit(Runnable task, V result) {
156	if (task == null) throw new NullPointerException();
157	RunnableFuture<V> f = newTaskFor(task, result);
158	executor.execute(new QueueingFuture(f));
159	return f;
160	}
161
162	public Future<V> take() throws InterruptedException {
163	return completionQueue.take();
164	}
165
166	public Future<V> poll() {
167	return completionQueue.poll();
168	}
169
170	public Future<V> poll(long timeout, TimeUnit unit)
171	throws InterruptedException {
172	return completionQueue.poll(timeout, unit);
173	}
174
175	}