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ThreadPoolExecutor继承AbstractExecutorService，是一个线程池的具体的实现
1. ctl
private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0)); private static final int COUNT_BITS = Integer.SIZE - 3; private static final int CAPACITY = (1 << COUNT_BITS) - 1; // 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;
AtomicInteger类型的私有变量ctl，使用高3bit表示线程池状态runState，低29bit来表示线程池线程总数。可见，Executor线程数量是有限制的，最多(2^29)-1个(about 500 million)。
前3bit表示线程池生命周期的5个状态，RNUNING，SHUTDOWN，STOP，TIDYING，TERMINATED。注释中给出了线程池所有可能的状态转化，随着生命周期的变更，几个状态从小到大赋值。
2. workQueue
private final BlockingQueue<Runnable> workQueue;
存储待执行tasks的任务队列。
3. workers
private final HashSet<Worker> workers = new HashSet<Worker>();
工作线程集合。
4. mainLock
private final ReentrantLock mainLock = new ReentrantLock();
访问workers时必须持有的锁。
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原文地址：http://www.cnblogs.com/qquan/p/5616373.html
