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 <tt>take</tt>.
 * 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 <tt>Result</tt>, and would like to
 * run them concurrently, processing the results of each of them that
 * return a non-null value, in some method <tt>use(Result r)</tt>. You
 * could write this as:
 *
 * <pre>
 *   void solve(Executor e,
 *              Collection&lt;Callable&lt;Result&gt;&gt; solvers)
 *     throws InterruptedException, ExecutionException {
 *       CompletionService&lt;Result&gt; ecs
 *           = new ExecutorCompletionService&lt;Result&gt;(e);
 *       for (Callable&lt;Result&gt; s : solvers)
 *           ecs.submit(s);
 *       int n = solvers.size();
 *       for (int i = 0; i &lt; 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>
 *   void solve(Executor e,
 *              Collection&lt;Callable&lt;Result&gt;&gt; solvers)
 *     throws InterruptedException {
 *       CompletionService&lt;Result&gt; ecs
 *           = new ExecutorCompletionService&lt;Result&gt;(e);
 *       int n = solvers.size();
 *       List&lt;Future&lt;Result&gt;&gt; futures
 *           = new ArrayList&lt;Future&lt;Result&gt;&gt;(n);
 *       Result result = null;
 *       try {
 *           for (Callable&lt;Result&gt; s : solvers)
 *               futures.add(ecs.submit(s));
 *           for (int i = 0; i &lt; n; ++i) {
 *               try {
 *                   Result r = ecs.take().get();
 *                   if (r != null) {
 *                       result = r;
 *                       break;
 *                   }
 *               } catch (ExecutionException ignore) {}
 *           }
 *       }
 *       finally {
 *           for (Future&lt;Result&gt; 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 <tt>null</tt>
     */
    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 <tt>Queue.add</tt>
     * operations for completed taskes cause them not to be
     * retrievable.
     * @throws NullPointerException if executor or completionQueue are <tt>null</tt>
     */
    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:	1.17
Committed:	Sat Dec 2 20:53:11 2006 UTC (17 years, 6 months ago) by dl
Branch:	MAIN
Changes since 1.16:	+4 -1 lines
Log Message:	Clarify that queue should be unbounded
#	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			* upon completion, placed on a queue accessible using <tt>take</tt>.
13			* 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			* returning a value of some type <tt>Result</tt>, 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 <tt>use(Result r)</tt>. You
24			* could write this as:
25	dl	1.5	*
26			* <pre>
27	jsr166	1.15	* 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			* }
41	dl	1.5	* </pre>
42			*
43			* Suppose instead that you would like to use the first non-null result
44	dl	1.9	* of the set of tasks, ignoring any that encounter exceptions,
45	dl	1.6	* and cancelling all other tasks when the first one is ready:
46	dl	1.5	*
47			* <pre>
48	jsr166	1.15	* void solve(Executor e,
49			* Collection<Callable<Result>> solvers)
50			* throws InterruptedException {
51			* CompletionService<Result> ecs
52			* = new ExecutorCompletionService<Result>(e);
53			* int n = solvers.size();
54			* List<Future<Result>> futures
55			* = new ArrayList<Future<Result>>(n);
56			* Result result = null;
57			* try {
58			* for (Callable<Result> s : solvers)
59			* futures.add(ecs.submit(s));
60			* for (int i = 0; i < n; ++i) {
61			* try {
62			* Result r = ecs.take().get();
63			* if (r != null) {
64			* result = r;
65			* break;
66			* }
67			* } catch (ExecutionException ignore) {}
68			* }
69			* }
70			* finally {
71			* for (Future<Result> f : futures)
72			* f.cancel(true);
73			* }
74			*
75			* if (result != null)
76			* use(result);
77			* }
78	dl	1.5	* </pre>
79	dl	1.1	*/
80			public class ExecutorCompletionService<V> implements CompletionService<V> {
81	jozart	1.2	private final Executor executor;
82	dl	1.13	private final AbstractExecutorService aes;
83	dl	1.3	private final BlockingQueue<Future<V>> completionQueue;
84	dl	1.1
85			/**
86			* FutureTask extension to enqueue upon completion
87			*/
88	dl	1.13	private class QueueingFuture extends FutureTask<Void> {
89			QueueingFuture(RunnableFuture<V> task) {
90			super(task, null);
91			this.task = task;
92			}
93			protected void done() { completionQueue.add(task); }
94			private final Future<V> task;
95			}
96
97			private RunnableFuture<V> newTaskFor(Callable<V> task) {
98			if (aes == null)
99			return new FutureTask<V>(task);
100			else
101			return aes.newTaskFor(task);
102			}
103
104			private RunnableFuture<V> newTaskFor(Runnable task, V result) {
105			if (aes == null)
106			return new FutureTask<V>(task, result);
107			else
108			return aes.newTaskFor(task, result);
109	dl	1.1	}
110
111			/**
112			* Creates an ExecutorCompletionService using the supplied
113	dl	1.3	* executor for base task execution and a
114			* {@link LinkedBlockingQueue} as a completion queue.
115	jsr166	1.15	*
116	dl	1.5	* @param executor the executor to use
117	dl	1.10	* @throws NullPointerException if executor is <tt>null</tt>
118	dl	1.1	*/
119	jozart	1.2	public ExecutorCompletionService(Executor executor) {
120	jsr166	1.12	if (executor == null)
121	dl	1.1	throw new NullPointerException();
122			this.executor = executor;
123	dl	1.13	this.aes = (executor instanceof AbstractExecutorService) ?
124			(AbstractExecutorService) executor : null;
125	dl	1.3	this.completionQueue = new LinkedBlockingQueue<Future<V>>();
126	dl	1.1	}
127
128			/**
129	dl	1.3	* Creates an ExecutorCompletionService using the supplied
130			* executor for base task execution and the supplied queue as its
131			* completion queue.
132	jsr166	1.15	*
133	dl	1.5	* @param executor the executor to use
134	dl	1.7	* @param completionQueue the queue to use as the completion queue
135	dl	1.17	* normally one dedicated for use by this service. This queue is
136			* treated as unbounded -- failed attempted <tt>Queue.add</tt>
137			* operations for completed taskes cause them not to be
138			* retrievable.
139	dl	1.10	* @throws NullPointerException if executor or completionQueue are <tt>null</tt>
140	dl	1.1	*/
141	dl	1.3	public ExecutorCompletionService(Executor executor,
142			BlockingQueue<Future<V>> completionQueue) {
143	jsr166	1.12	if (executor == null \|\| completionQueue == null)
144	dl	1.3	throw new NullPointerException();
145			this.executor = executor;
146	dl	1.13	this.aes = (executor instanceof AbstractExecutorService) ?
147			(AbstractExecutorService) executor : null;
148	dl	1.3	this.completionQueue = completionQueue;
149	dl	1.1	}
150
151			public Future<V> submit(Callable<V> task) {
152	dl	1.7	if (task == null) throw new NullPointerException();
153	dl	1.13	RunnableFuture<V> f = newTaskFor(task);
154			executor.execute(new QueueingFuture(f));
155	dl	1.1	return f;
156			}
157
158			public Future<V> submit(Runnable task, V result) {
159	dl	1.7	if (task == null) throw new NullPointerException();
160	dl	1.13	RunnableFuture<V> f = newTaskFor(task, result);
161			executor.execute(new QueueingFuture(f));
162	dl	1.1	return f;
163			}
164
165			public Future<V> take() throws InterruptedException {
166	dl	1.3	return completionQueue.take();
167	dl	1.1	}
168
169			public Future<V> poll() {
170	dl	1.3	return completionQueue.poll();
171	dl	1.1	}
172
173			public Future<V> poll(long timeout, TimeUnit unit) throws InterruptedException {
174	dl	1.3	return completionQueue.poll(timeout, unit);
175	dl	1.1	}
176	dl	1.5
177	dl	1.1	}