翻译javadoc系列文章之：ExecutorService
/**
* An {@link Executor} that provides methods to manage termination and
* methods that can produce a {@link Future} for tracking progress of
* one or more asynchronous tasks.
*
* <p> An <tt>ExecutorService</tt> can be shut down, which will cause
* it to reject new tasks. Two different methods are provided for
* shutting down an <tt>ExecutorService</tt>. The {@link #shutdown}
* method will allow previously submitted tasks to execute before
* terminating, while the {@link #shutdownNow} method prevents waiting
* tasks from starting and attempts to stop currently executing tasks.
* Upon termination, an executor has no tasks actively executing, no
* tasks awaiting execution, and no new tasks can be submitted. An
* unused <tt>ExecutorService</tt> should be shut down to allow
* reclamation of its resources.
*
* <p> Method <tt>submit</tt> extends base method {@link
* Executor#execute} by creating and returning a {@link Future} that
* can be used to cancel execution and/or wait for completion.
* Methods <tt>invokeAny</tt> and <tt>invokeAll</tt> perform the most
* commonly useful forms of bulk execution, executing a collection of
* tasks and then waiting for at least one, or all, to
* complete. (Class {@link ExecutorCompletionService} can be used to
* write customized variants of these methods.)
*
* <p>The {@link Executors} class provides factory methods for the
* executor services provided in this package.
ExecutorService 接口是一个提供管理终止（termination）和能够生成跟踪一个或多个异步任务进程结果的方法的接口。
介绍ExecutorService接口的功能：
1:提供管理终止功能（拒绝新任务进入功能）；
2:提供方法来生成能够追踪一个或多个任务执行结果的Future；
一个ExecutorService可以被关闭，即使其拒绝接受新的任务。
它提供了两种不同的方法来关闭一个ExecutorService。
方法1（shutdown方法）：允许之前已经提交的任务继续执行到结束。
方法2（shutdownNow方法）：会阻止等待开始的