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.

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