shutdown()
源碼說明如下：
/**
* Initiates an orderly shutdown in which previously submitted
* tasks are executed, but no new tasks will be accepted.
* Invocation has no additional effect if already shut down.
*
*This method does not wait for previously submitted tasks to
* complete execution. Use {@link #awaitTermination awaitTermination}
* to do that.
*
上面的意思是說：在先前提交的任務（就是run中跑的東西）被執行的時候，開始有序的關閉。新的任務不會被執行。如果已關閉，則調用沒有其他效果。該方法不會等待先前已提交的任務完全執行。
awaitTermination()
awaitTermination(long timeout, TimeUnit unit)
第一個參數timeout是等待關閉線程池執行器的時間大小，第二個參數TimeUnit是時間的粒度，也就是以多大的粒度（小時，分鐘，秒）來設置第一個參數的單位。
/**
* Returns true if this executor is in the process of terminating
* after {@link #shutdown} or {@link #shutdownNow} but has not
* completely terminated. This method may be useful for
* debugging.
A return of {@code true} reported a sufficient
* period after shutdown may indicate that submitted tasks have
* ignored or suppressed interruption, causing this executor not
* to properly terminate.
*
* @return {@code true} if terminating but not yet terminated
*/
等待執行器完全關閉，返回爲真。
- 總結
shutdown()關閉任務，awaitTermination關閉執行器（executor）。