src/java.base/share/classes/java/util/concurrent/ExecutorCompletionService.java

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 * This code is free software; you can redistribute it and/or modify it
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 * published by the Free Software Foundation.  Oracle designates this
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 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
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 * accompanied this code).
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/*
 * This file is available under and governed by the GNU General Public
 * License version 2 only, as published by the Free Software Foundation.
 * However, the following notice accompanied the original version of this
 * file:
 *
 * 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;

import java.util.Objects;

/**
 * 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> cs
 *       = new ExecutorCompletionService<>(e);
 *   solvers.forEach(cs::submit);
 *   for (int i = solvers.size(); i > 0; i--) {
 *     Result r = cs.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> cs
 *       = new ExecutorCompletionService<>(e);
 *   int n = solvers.size();
 *   List<Future<Result>> futures = new ArrayList<>(n);
 *   Result result = null;
 *   try {
 *     solvers.forEach(solver -> futures.add(cs.submit(solver)));
 *     for (int i = n; i > 0; i--) {
 *       try {
 *         Result r = cs.take().get();
 *         if (r != null) {
 *           result = r;
 *           break;
 *         }
 *       } catch (ExecutionException ignore) {}
 *     }
 *   } finally {
 *     futures.forEach(future -> future.cancel(true));
 *   }
 *
 *   if (result != null)
 *     use(result);
 * }}</pre>
 *
 * @param <V> the type of values the tasks of this service produce and consume
 *
 * @since 1.5
 */
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 static class QueueingFuture<V> extends FutureTask<Void> {
        QueueingFuture(RunnableFuture<V> task,
                       BlockingQueue<Future<V>> completionQueue) {
            super(task, null);
            this.task = task;
            this.completionQueue = completionQueue;
        }
        private final Future<V> task;
        private final BlockingQueue<Future<V>> completionQueue;
        protected void done() { completionQueue.add(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) {
        Objects.requireNonNull(executor, "executor");
        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) {
        Objects.requireNonNull(executor, "executor");
        Objects.requireNonNull(completionQueue, "completionQueue");
        this.executor = executor;
        this.aes = (executor instanceof AbstractExecutorService) ?
            (AbstractExecutorService) executor : null;
        this.completionQueue = completionQueue;
    }

    /**
     * @throws RejectedExecutionException {@inheritDoc}
     * @throws NullPointerException       {@inheritDoc}
     */
    public Future<V> submit(Callable<V> task) {
        Objects.requireNonNull(task, "task");
        RunnableFuture<V> f = newTaskFor(task);
        executor.execute(new QueueingFuture<V>(f, completionQueue));
        return f;
    }

    /**
     * @throws RejectedExecutionException {@inheritDoc}
     * @throws NullPointerException       {@inheritDoc}
     */
    public Future<V> submit(Runnable task, V result) {
        Objects.requireNonNull(task, "task");
        RunnableFuture<V> f = newTaskFor(task, result);
        executor.execute(new QueueingFuture<V>(f, completionQueue));
        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);
    }

}