CountDownLatch

1.类声明：

//需要等待某个条件完成，即等待其他某些线程完成操作，才能继续执行的时候，可以使用此类。在计数器到达0之前，await方法会一直阻塞public class CountDownLatch{}

2.变量：

//同步器 private final Sync sync;

3.方法：

//初始化    public CountDownLatch(int count) {        if (count < 0) throw new IllegalArgumentException("count < 0");        this.sync = new Sync(count);    }    //自定义队列同步器，控制并发的访问    private static final class Sync extends AbstractQueuedSynchronizer {        private static final long serialVersionUID = 4982264981922014374L;        Sync(int count) {            setState(count);        }        int getCount() {            return getState();        }        protected int tryAcquireShared(int acquires) {            return (getState() == 0) ? 1 : -1;        }        protected boolean tryReleaseShared(int releases) {            // Decrement count; signal when transition to zero            for (;;) {                int c = getState();                if (c == 0)                    return false;                int nextc = c-1;                if (compareAndSetState(c, nextc))                    return nextc == 0;            }        }    }    //当前线程等待，知道计数器变为0        public void await() throws InterruptedException {        sync.acquireSharedInterruptibly(1);    }    //某个线程调用，计数器-1      public void countDown() {        sync.releaseShared(1);    }