废话不多说，CompletionService是个接口，源码及其注释如下：
/*
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*/
/*
* 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;
/**
* A service that decouples the production of new asynchronous tasks
* from the consumption of the results of completed tasks. Producers
* {@code submit} tasks for execution. Consumers {@code take}
* completed tasks and process their results in the order they
* complete. A {@code CompletionService} can for example be used to
* manage asynchronous I/O, 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.
*
* 将生产异步的任务和消费完成的任务进行解耦。生产者提交任务执行。消费者消费完成的任务，
* 并且按照任务完成的顺序去处理结果。CompletionService可以用于管理异步IO。
* 执行读取的任务是在程序或系统的一部分提交的，当读取完成后，在系统不同的地方处理，很可能
* 处理的顺序也不是他们提交上来的时候的顺序
*
* <p>Typically, a {@code CompletionService} relies on a separate
* {@link Executor} to actually execute the tasks, in which case the
* {@code CompletionService} only manages an internal completion
* queue. The {@link ExecutorCompletionService} class provides an
* implementation of this approach.
*
* 典型的，CompletionService依赖于一个独立的Executor来执行任务，这样CompletionService就只需
* 管理内部的任务完成的队列。ExecutorCompletionService就实现了CompletionService
*
* <p>Memory consistency effects: Actions in a thread prior to
* submitting a task to a {@code CompletionService}
* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
* actions taken by that task, which in turn <i>happen-before</i>
* actions following a successful return from the corresponding {@code take()}.
*/
public interface CompletionService<V> {
/**
* 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.
*
* 提交一个带返回值的任务，返回一个Future对象代表预期的任务执行结果。
* 当执行结束后，这个任务执行结果会被take/poll取得
*
* @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<V> submit(Callable<V> 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Runnable任务去执行，返回一个Future对象代表此任务。
* 当执行结束后，这个任务执行结果会被take/poll取得
*
* @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 {@code 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<V> submit(Runnable task, V result);
/**
* Retrieves and removes the Future representing the next
* completed task, waiting if none are yet present.
*
* 遍历并删除下一个完成的任务的Future对象，如果没有完成的任务则等待
*
* @return the Future representing the next completed task
* @throws InterruptedException if interrupted while waiting
*/
Future<V> take() throws InterruptedException;
/**
* Retrieves and removes the Future representing the next
* completed task, or {@code null} if none are present.
*
* 遍历并删除下一个完成的任务的Future对象，如果没有完成的任务则等待
*
* @return the Future representing the next completed task, or
* {@code null} if none are present
*/
Future<V> 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.
*
* 遍历并删除下一个完成的任务的Future对象，如果没有完成的任务则等待指定的时间
*
* @param timeout how long to wait before giving up, in units of
* {@code unit}
* @param unit a {@code TimeUnit} determining how to interpret the
* {@code timeout} parameter
* @return the Future representing the next completed task or
* {@code null} if the specified waiting time elapses
* before one is present
* @throws InterruptedException if interrupted while waiting
*/
Future<V> poll(long timeout, TimeUnit unit) throws InterruptedException;
}