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
package java.util.concurrent;
/**
* A service that decouples the production of new asynchronous tasks
* from the consumption of the results of completed tasks. Producers
* <tt>submit</tt> tasks for execution. Consumers <tt>take</tt>
* completed tasks and process their results in the order they
* complete. A <tt>CompletionService</tt> 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。
* <p>Typically, a <tt>CompletionService</tt> relies on a separate
* {@link Executor} to actually execute the tasks, in which case the
* <tt>CompletionService</tt> 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。
* <p>Memory consistency effects: Actions in a thread prior to
* submitting a task to a {@code CompletionService}
* [url=package-summary.html#MemoryVisibility]<i>happen-before</i>[/url]
* 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.
*
提交一个有返回结果的Callable任务到执行器， Future<V>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<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.
*
提交一个有返回结果的Runnable任务到执行器， Future<V>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 <tt>get()</tt> 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.
*
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<V> take() throws InterruptedException;
/**
* Retrieves and removes the Future representing the next
* completed task or <tt>null</tt> 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
* <tt>null</tt> 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.
*
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
* <tt>unit</tt>
* @param unit a <tt>TimeUnit</tt> determining how to interpret the
* <tt>timeout</tt> parameter
* @return the Future representing the next completed task or
* <tt>null</tt> 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;
}