Jav中CountDownLatch是什么

public CountDownLatch(int count)

// 使当前线程在锁存器倒计数至零之前一直等待，除非线程被中断。
void await()
// 使当前线程在锁存器倒计数至零之前一直等待，除非线程被中断或超出了指定的等待时间。
boolean await(long timeout, TimeUnit unit)
// 递减锁存器的计数，如果计数到达零，则释放所有等待的线程。
void countDown()
// 返回当前计数。
long getCount()

public class TestCountDownLatch {
  private static final int RUNNER_NUMBER = 5; // 运动员个数
  private static final Random RANDOM = new Random();
  public static void main(String[] args) {
    // 用于判断发令之前运动员是否已经完全进入准备状态，需要等待5个运动员，所以参数为5
    CountDownLatch readyLatch = new CountDownLatch(RUNNER_NUMBER);
    // 用于判断裁判是否已经发令，只需要等待一个裁判，所以参数为1
    CountDownLatch startLatch = new CountDownLatch(1);
    for (int i = 0; i < RUNNER_NUMBER; i++) {
      Thread t = new Thread(new Runner((i + 1) + "号运动员", readyLatch, startLatch));
      t.start();
    }
    try {
      readyLatch.await();
    } catch (InterruptedException e) {
      e.printStackTrace();
    }
    startLatch.countDown();
    System.out.println("裁判：所有运动员准备完毕，开始...");
  }
  static class Runner implements Runnable {
    private CountDownLatch readyLatch;
    private CountDownLatch startLatch;
    private String name;
    public Runner(String name, CountDownLatch readyLatch, CountDownLatch startLatch) {
      this.name = name;
      this.readyLatch = readyLatch;
      this.startLatch = startLatch;
    }
    public void run() {
      int readyTime = RANDOM.nextInt(1000);
      System.out.println(name + "：我需要" + readyTime + "秒时间准备.");
      try {
        Thread.sleep(readyTime);
      } catch (InterruptedException e) {
        e.printStackTrace();
      }
      System.out.println(name + "：我已经准备完毕.");
      readyLatch.countDown();
      try {
        startLatch.await(); // 等待裁判发开始命令
      } catch (InterruptedException e) {
        e.printStackTrace();
      }
      System.out.println(name + "：开跑...");
    }
  }
}