Executor类实际上充当着一个提供Executor实现实例的共同位置，但它只有new方法（例如用于创建新线程池）；它没有预先创建实例。
The Executor class does serve as a common place to get Executor-implementing instances , but it only has new methods ( to create a new thread pool , for example ); it doesn 't have precreated instances .
程池