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/*

 * 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<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>

 *   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 <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

     * @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);

    }

}