17.CountDownLatch

假设一个主线程要等待5个 Worker 线程执行完才能退出，可以使用CountDownLatch来实现

public static void main(String[] args) throws InterruptedException { 
    CountDownLatch latch = new CountDownLatch(5); 
    new MyThread("线程1", latch).start(); 
    new MyThread("线程2", latch).start();
    new MyThread("线程3", latch).start(); 
    new MyThread("线程4", latch).start(); 
    // new MyThread("线程5", latch).start(); 
    // 当前线程等待 
    latch.await(); 
    System.out.println("程序运行结束"); 
}



public void run() { 
    try {
        Thread.sleep(random.nextInt(2000)); 
    } catch (InterruptedException e) { 
        e.printStackTrace(); 
    }
    System.out.println(Thread.currentThread().getName() + "运行结束"); 
    latch.countDown(); 
}

CountDownLatch原理和Semaphore原理类似，同样是基于AQS，不过没有公平和非公平之分

继承关系图：

image.png