CountDownLatch

 

 

 构造方法：  （还是设置 state 的值！）

（ 当state的值为0的时候，才会执行! 否则阻塞！！！）

    public CountDownLatch(int count) {
        if (count < 0) throw new IllegalArgumentException("count < 0");
        this.sync = new Sync(count);
    }

await 方法： 

 public void await() throws InterruptedException {
        sync.acquireSharedInterruptibly(1);
    }

 

  public final void acquireSharedInterruptibly(int arg)
            throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        if (tryAcquireShared(arg) < 0)
            doAcquireSharedInterruptibly(arg);
    }

 

protected int tryAcquireShared(int acquires) {
            return (getState() == 0) ? 1 : -1;   // 如果不是0  就阻塞
        }

countDown 方法：

public void countDown() {
        sync.releaseShared(1);
    }

 

public final boolean releaseShared(int arg) {
        if (tryReleaseShared(arg)) {
            doReleaseShared();
            return true;
        }
        return false;
    }

 

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;        // 将state的值减1
                if (compareAndSetState(c, nextc))
                    return nextc == 0;
            }
        }

 

CountDownLatch 的应用场景：

　　由前面可以看出，当state的值为0的时候，线程才会正常执行，当state大于1的时候，线程进入等待状态！！！

( 开始执行线程前，等待 n 个线程完成任务！！！ )