线程池包含哪些状态

参考答案

1.   线程池5种状态

1.1   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。

1.2  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。

1.3   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。

1.4  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。

1.5   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。

ThreadPoolExecutor 源码:

// runState is stored in the high-order bits
private 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;

2. 线程池状态转换

3. JDK 源码中的解释

状态：

The runState provides the main lifecyle control, taking on values:

RUNNING:  Accept new tasks and process queued tasks
SHUTDOWN: Don't accept new tasks, but process queued tasks
STOP:     Don't accept new tasks, don't process queued tasks,
            and interrupt in-progress tasks
TIDYING:  All tasks have terminated, workerCount is zero,
            the thread transitioning to state TIDYING
            will run the terminated() hook method
TERMINATED: 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 empty
STOP -> TIDYING
   When pool is empty
TIDYING -> TERMINATED
   When the terminated() hook method has completed

Threads waiting in awaitTermination() will return when the
state reaches TERMINATED.

以上，是Java面试题【线程池包含哪些状态】的参考答案。

输出，是最好的学习方法。

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