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Deprecated Actors Library for Scala
There is a newer version: 2.11.2
/*
 * 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 scala.actors.threadpool;
import scala.actors.threadpool.*; // for javadoc (till 6280605 is fixed)

/**
 * 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 take.
 * The class is lightweight enough to be suitable for transient use
 * when processing groups of tasks.
 *
 * 
 *
 * Usage Examples.
 *
 * Suppose you have a set of solvers for a certain problem, each
 * returning a value of some type Result, and would like to
 * run them concurrently, processing the results of each of them that
 * return a non-null value, in some method use(Result r). You
 * could write this as:
 *
 * 
 *   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);
 *       }
 *   }
 * 
 *
 * 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:
 *
 *  *   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);
 *   }
 * 
 */
public class ExecutorCompletionService implements CompletionService {
    private final Executor executor;
    private final AbstractExecutorService aes;
    private final BlockingQueue completionQueue;

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

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

    private RunnableFuture newTaskFor(Runnable task, Object result) {
        if (aes == null)
            return new FutureTask(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 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();
    }

    /**
     * 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 Queue.add
     * operations for completed taskes cause them not to be
     * retrievable.
     * @throws NullPointerException if executor or completionQueue are null
     */
    public ExecutorCompletionService(Executor executor,
                                     BlockingQueue 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 submit(Callable task) {
        if (task == null) throw new NullPointerException();
        RunnableFuture f = newTaskFor(task);
        executor.execute(new QueueingFuture(f));
        return f;
    }

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

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

    public Future poll() {
        return (Future)completionQueue.poll();
    }

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

}