CountDownLatch

允许 count 个线程阻塞在一个地方，直至所有线程的任务都执行完毕。

1. new CountDownLatch(int count)
   • count → AbstractQueuedSynchronizer 的 state
2. countDown() → state-1
   • If the current count equals zero then nothing happens.
   • 减少到 0 不会再减
3. await() 阻塞

CountDownLatch latch = new CountDownLatch(5); 
latch.countDown();
latch.countDown();
// ..........
latch.await();