线程及线程池状态

java中定义了线程状态有6种，源码在Thread类中定义了枚举类 State：

•NEW
 A thread that has not yet started is in this state. 

•RUNNABLE
 A thread executing in the Java virtual machine is in this state. 

•BLOCKED
 A thread that is blocked waiting for a monitor lock is in this state. 

•WAITING
 A thread that is waiting indefinitely for another thread to perform a particular action is in this state. 

•TIMED_WAITING
 A thread that is waiting for another thread to perform an action for up to a specified waiting time is in this state. 

•TERMINATED
 A thread that has exited is in this state. 

线程池状态有五种，使用int的高3位表示：

1、RUNNING

(1) 状态说明：线程池处在RUNNING状态时，能够接收新任务，以及对已添加的任务进行处理。 
(02) 状态切换：线程池的初始化状态是RUNNING。换句话说，线程池被一旦被创建，就处于RUNNING状态，并且线程池中的任务数为0！

private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
ThreadPoolExecutor中定义如下：

　　// 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、 SHUTDOWN

(1) 状态说明：线程池处在SHUTDOWN状态时，不接收新任务，但能处理已添加的任务。 
(2) 状态切换：调用线程池的shutdown()接口时，线程池由RUNNING -> SHUTDOWN。

3、STOP

(1) 状态说明：线程池处在STOP状态时，不接收新任务，不处理已添加的任务，并且会中断正在处理的任务。 
(2) 状态切换：调用线程池的shutdownNow()接口时，线程池由(RUNNING or SHUTDOWN ) -> STOP。

4、TIDYING

(1) 状态说明：当所有的任务已终止，ctl记录的”任务数量”为0，线程池会变为TIDYING状态。当线程池变为TIDYING状态时，会执行钩子函数terminated()。terminated()在ThreadPoolExecutor类中是空的，若用户想在线程池变为TIDYING时，进行相应的处理；可以通过重载terminated()函数来实现。 
(2) 状态切换：当线程池在SHUTDOWN状态下，阻塞队列为空并且线程池中执行的任务也为空时，就会由 SHUTDOWN -> TIDYING。 
当线程池在STOP状态下，线程池中执行的任务为空时，就会由STOP -> TIDYING。

5、 TERMINATED

(1) 状态说明：线程池彻底终止，就变成TERMINATED状态。 
(2) 状态切换：线程池处在TIDYING状态时，执行完terminated()之后，就会由 TIDYING -> TERMINATED。