线程池状态：
线程池的5种状态：RUNNING、SHUTDOWN、STOP、TIDYING、TERMINATED。
见 ThreadPoolExecutor 源码
// runState is stored in the high-order bitsprivate static final int RUNNING= -1 <<COUNT_BITS;private static final int SHUTDOWN =0 <<COUNT_BITS;private static final int STOP =1 <<COUNT_BITS;private static final int TIDYING=2 <<COUNT_BITS;private static final int TERMINATED =3 <<COUNT_BITS;1. RUNNING：线程池一旦被创建，就处于RUNNING 状态，任务数为 0，能够接收新任务，对已排队的任务进行处理。
2. SHUTDOWN：不接收新任务，但能处理已排队的任务。调用线程池的 shutdown() 方法，线程池由 RUNNING 转变为 SHUTDOWN 状态。
3. STOP：不接收新任务，不处理已排队的任务，并且会中断正在处理的任务。调用线程池的 shutdownNow() 方法，线程池由(RUNNING 或 SHUTDOWN ) 转变为 STOP 状态。
4. TIDYING：
- SHUTDOWN 状态下，任务数为 0， 其他所有任务已终止，线程池会变为 TIDYING 状态，会执行 terminated() 方法。线程池中的 terminated() 方法是空实现，可以重写该方法进行相应的处理。
- 线程池在 SHUTDOWN 状态，任务队列为空且执行中任务为空，线程池就会由 SHUTDOWN 转变为 TIDYING 状态。
- 线程池在 STOP 状态，线程池中执行中任务为空时，就会由 STOP 转变为 TIDYING 状态。
5. TERMINATED：线程池彻底终止。线程池在 TIDYING 状态执行完 terminated() 方法就会由 TIDYING 转变为 TERMINATED 状态。
状态转换如图
JDK 源码中的解释如下
状态：
The runState provides the main lifecyle control, taking on values:RUNNING:Accept new tasks and process queued tasksSHUTDOWN: Don't accept new tasks, but process queued tasksSTOP: Don't accept new tasks, don't process queued tasks,and interrupt in-progress tasksTIDYING:All tasks have terminated, workerCount is zero,the thread transitioning to state TIDYINGwill run the terminated() hook methodTERMINATED: terminated() has completed状态间的变化
RUNNING -> SHUTDOWN On invocation of shutdown(), perhaps implicitly in finalize()(RUNNING or SHUTDOWN) -> STOP On invocation of shutdownNow()SHUTDOWN -> TIDYING When both queue and pool are emptySTOP -> TIDYING When pool is emptyTIDYING -> TERMINATED When the terminated() hook method has completedThreads waiting in awaitTermination() will return when thestate reaches TERMINATED.
