CompletionService接口定义

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

package java.util.concurrent;

/**
 * A service that decouples the production of new asynchronous tasks
 * from the consumption of the results of completed tasks.  Producers
 * submit tasks for execution. Consumers take
 * completed tasks and process their results in the order they
 * complete.  A CompletionService can for example be used to
 * manage asynchronous IO, in which tasks that perform reads are
 * submitted in one part of a program or system, and then acted upon
 * in a different part of the program when the reads complete,
 * possibly in a different order than they were requested.
 *
CompletionService提供了生产异步任务和消费任务结果的解耦服务。生产者提交任务执行。
消费者以任务完成的顺序，消费完成的任务结果。CompletionService可以用于管理异步的IO
操作，比如应用或系统提交一些reads子任务操作，然后reads子任务可能以非他们请求的顺序完成
任务，执行操作。
 * 
Typically, a CompletionService relies on a separate
 * {@link Executor} to actually execute the tasks, in which case the
 * CompletionService only manages an internal completion
 * queue. The {@link ExecutorCompletionService} class provides an
 * implementation of this approach.
 *
CompletionService实际上依赖于一个独立的Executor用于执行任务，CompletionService仅
用于管理内部的任务完成队列。ExecutorCompletionService提供了进一步的实现。
 * 
Memory consistency effects: Actions in a thread prior to
 * submitting a task to a {@code CompletionService}
 * [url=package-summary.html#MemoryVisibility]happen-before[/url]
 * actions taken by that task, which in turn happen-before
 * actions following a successful return from the corresponding {@code take()}.
 *
 */
public interface CompletionService {
    /**
     * Submits a value-returning task for execution and returns a Future
     * representing the pending results of the task.  Upon completion,
     * this task may be taken or polled.
     *
     提交一个有返回结果的Callable任务到执行器， Future代表任务的结果，在任务完成时，
     任务将会被take或poll
     * @param task the task to submit
     * @return a Future representing pending completion of the task
     * @throws RejectedExecutionException if the task cannot be
     *         scheduled for execution
     * @throws NullPointerException if the task is null
     */
    Future submit(Callable task);

    /**
     * Submits a Runnable task for execution and returns a Future
     * representing that task.  Upon completion, this task may be
     * taken or polled.
     *
     提交一个有返回结果的Runnable任务到执行器， Future代表任务的结果，在任务完成时，
     任务将会被take或poll
     * @param task the task to submit
     * @param result the result to return upon successful completion
     * @return a Future representing pending completion of the task,
     *         and whose get() method will return the given
     *         result value upon completion
     * @throws RejectedExecutionException if the task cannot be
     *         scheduled for execution
     * @throws NullPointerException if the task is null
     */
    Future submit(Runnable task, V result);

    /**
     * Retrieves and removes the Future representing the next
     * completed task, waiting if none are yet present.
     *
     从完成任务队列头取出任务结果，如果任务未完成，则等待
     * @return the Future representing the next completed task
     * @throws InterruptedException if interrupted while waiting
     */
    Future take() throws InterruptedException;


    /**
     * Retrieves and removes the Future representing the next
     * completed task or null if none are present.
     *
     与take不同的是，如果没有完成的任务，返回为null
     * @return the Future representing the next completed task, or
     *         null if none are present
     */
    Future poll();

    /**
     * Retrieves and removes the Future representing the next
     * completed task, waiting if necessary up to the specified wait
     * time if none are yet present.
     *
     与非超时poll的区别是，如果没有完成的任务，则等待直至超时，如果超时等待期间还没有完成任务，返回为null
     * @param timeout how long to wait before giving up, in units of
     *        unit
     * @param unit a TimeUnit determining how to interpret the
     *        timeout parameter
     * @return the Future representing the next completed task or
     *         null if the specified waiting time elapses
     *         before one is present
     * @throws InterruptedException if interrupted while waiting
     */
    Future poll(long timeout, TimeUnit unit) throws InterruptedException;
}