ExecutorCompletionService解析

Executor接口的定义：http://donald-draper.iteye.com/blog/2365625
ExecutorService接口定义：http://donald-draper.iteye.com/blog/2365738
Future接口定义：http://donald-draper.iteye.com/blog/2365798
FutureTask解析：http://donald-draper.iteye.com/blog/2365980
CompletionService接口定义：http://donald-draper.iteye.com/blog/2366239

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.
 *
 ExecutorCompletionService是执行器的补充或者说是辅助，用于执行任务。
 任务提交到ExecutorCompletionService执行后，如果任务执行完，则添加到完成
 队列，我们可以通过take，取回完成任务的结果。ExecutorCompletionService
 是处理集合任务的一个轻量级的实现。

 * <p>
 *
 * <b>Usage Examples.</b>
 *
 实例：
 * 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:
 *
假设将一个确定的问题，分成n个部分，这n个部分可以并发执行，每个部分返回一个非null，结果
在其他一些方法中，使用这些结果。
 * <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:
 *
 假设我们仅想用任务集合中第一个完成任务的结果，并忽略其他任务遇到的异常，
 当第一个完成任务的结果可用时，取消其他任务。
 * <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<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;//执行器
    private final AbstractExecutorService aes;//执行器服务
    private final BlockingQueue<Future<V>> completionQueue;//任务完成队列

    /**
     * FutureTask extension to enqueue upon completion
     扩展FutureTask的队列完成任务
     */
    private class QueueingFuture extends FutureTask<Void> {
        QueueingFuture(RunnableFuture<V> task) {
            super(task, null);
            this.task = task;
        }
	//这个是关键，在FutureTask那篇文章中，我们有讲，及当任务完成时调用done方法，
	//done方法为抽象方法，待子类扩展
        protected void done() { 
	        //当任务任务执行结束时，添加到完成队列
		completionQueue.add(task); 
	}
        private final Future<V> task;
    }
    //根据Callable，创建
    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.
     *
     根据执行器构建ExecutorCompletionService，完成队列默认为LinkedBlockingQueue
     * @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;
	//将完成任务放在LinkedBlockingQueue中
        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.
     *
     根据执行器和完成队列构建ExecutorCompletionService
     * @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 taskes 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;
    }
   //提交执行Callable任务
    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;
    }
   //提交执行Runnable任务
    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;
    }
    //从完成队列take完成任务的结果，没有则阻塞，直到有任务完成
    public Future<V> take() throws InterruptedException {
        return completionQueue.take();
    }
    //从完成队列获取完成任务的结果，没有则返回null
    public Future<V> poll() {
        return completionQueue.poll();
    }
    //从完成队列获取完成任务的结果，没有则超时等待，如果超时等待期间还没有完成任务，返回为null
    public Future<V> poll(long timeout, TimeUnit unit)
            throws InterruptedException {
        return completionQueue.poll(timeout, unit);
    }
}

总结：
ExecutorCompletionService内部关联一个执行器AbstractExecutorService和
一个阻塞的任务完成队列，默认为LinkedBlockingQueue。当提交任务，则包装成QueueingFuture，QueueingFuture
扩展了FutureTask，重写done方法，即在任务执行结束时，添加任务执行结果到完成队列。
而take，poll，超时poll直接委托给完成队列。