CountDownLatch 强调的是一个线程（或多个）需要等待另外的 n 个线程干完某件事情之后才能继续执行。

测试案例

package com.personal.test.countdownlatch;
import java.util.Random;
import java.util.concurrent.CountDownLatch;
public class Test20201225162321 {
    public static void test20201225162329() {
        CountDownLatch latch = new CountDownLatch(2);
        Test20201225162416 t1 = new Test20201225162416(latch);
        Test20201225162416 t2 = new Test20201225162416(latch);
        new Thread(t1).start();
        new Thread(t2).start();
    }
    public static void main(String[] args) {
        Test20201225162321.test20201225162329();
    }
}
class Test20201225162416 implements Runnable {
    private final CountDownLatch latch;
    public Test20201225162416(CountDownLatch latch) {
        this.latch = latch;
    }
    @lombok.SneakyThrows
    @Override
    public void run() {
        System.out.println(Thread.currentThread().getName());
        Thread.sleep(new Random().nextInt(1) * 1000L);
        latch.countDown();
    }
}

参考资料

https://www.cnblogs.com/csu_xajy/p/4338357.html
https://blog.csdn.net/yujin753/article/details/46125283