前面两章，我们介绍了CAS以及ReentrantLock的基本知识；现在我们先了解下threadPool的几种状态，以及几种默认实现（newFixedThreadPool,newCacheThreadPool,newSingleThreadPool,newScheduleThreadPool）；
threadPool常见的几种状态
* RUNNING: Accept new tasks and process queued tasks(接受新的任务，并且能够处理队列的中任务)
* SHUTDOWN: Don't accept new tasks, but process queued tasks(不能接受接受新任务，等待队列的任务处理完毕)
* STOP: Don't accept new tasks, don't process queued tasks,
* and interrupt in-progress tasks（不再接受新的任务，不再处理队列的任务，并且中断正在执行的任务）
* TIDYING: All tasks have terminated, workerCount is zero,（所有的任务已经中止，队列中任务数为零）
* the thread transitioning to state TIDYING
* will run the terminated() hook method
* TERMINATED: terminated() has completed（所有的任务处理完毕）
threadPool中几种任务之间的转换 ；特别注意状态转换的先后顺序；
- RUNNING -> SHUTDOWN
On invocation of shutdown(), perhaps implicitly in finalize()
当调用shutdown方法以及显式的执行 finalize()方法；- (RUNNING or SHUTDOWN) -> STOP
On invocation of shutdownNow()
当调用shutdownNow()方法- SHUTDOWN -> TIDYING
When both queue and pool are empty
队列和pool中为空时- STOP -> TIDYING
When pool is empty
pool 为空- TIDYING -> TERMINATED
When the terminated() hook method has completed
terminated方法调用完成之后
ThreadPool中的常量解释
首先ThreadPoolExecutor，int整型中32位的前3位用来表示线程池状态，后3位表示线程池中有效的线程数。
- COUNT_BITS =29，前三位为线程池状态,后面位数为线程池的个数
private static final int COUNT_BITS = Integer.SIZE - 3;
- CAPACITY = pow(2,29)-1
private static final int CAPACITY = (1 << COUNT_BITS) - 1;
- 特别注意到状态位左移之后的数值状态,同时注意几个状态之间大小比较 running<shutdown<stop<tidying<terminated
// runState is stored in the high-order bits
private static final int RUNNING = -1 << COUNT_BITS;//前三位为111，后面全是零
private static final int SHUTDOWN = 0 << COUNT_BITS;//前三位000.后面全是零
private static final int STOP = 1 << COUNT_BITS;//前三位001，后面全是零
private static final int TIDYING = 2 << COUNT_BITS;//前面为010.后面全是零
private static final int TERMINATED = 3 << COUNT_BITS;//前面为011，后面全是零
- 根据上面的几个常量，我们清楚线程状态由int高三位决定，数目由低29位决定；通过下面的两个方法workerCountOf（）, workerCountOf();能够获取到int对应高三位，以及低29位的值；即int对应的状态位，以及线程池的容量大小；
// 得到线程数;CAPACITY bit数据位具体为 00011111111111111111111111111111。
//与操作的话前面3位肯定为0，相当于直接取c的后29位的值
private static int workerCountOf(int c) {
return c & CAPACITY;
}
// 得到状态; CAPACITY的非操作得到的二进制位11100000000000000000000000000000，然后做在一个与操作，相当于直接取前3位的的值
private static int runStateOf(int c) {
return c & ~CAPACITY;
}
-初始化int，即线程池在运行，但是count为0的时候；
private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
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