在Java多线程中如何使用CountDownLatch？

有人可以帮助我了解什么是Java CountDownLatch以及何时使用它吗？

对于这个程序的工作方式，我没有一个很清楚的想法。据我了解，所有三个线程同时启动，每个线程将在3000ms之后调用CountDownLatch。因此，递减计数将逐一递减。锁存器变为零后，程序将打印“ Completed”。也许我的理解方式不正确。

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

class Processor implements Runnable {
    private CountDownLatch latch;

    public Processor(CountDownLatch latch) {
        this.latch = latch;
    }

    public void run() {
        System.out.println("Started.");

        try {
            Thread.sleep(3000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        latch.countDown();
    }
}

// ------------------------------------------------ -----

public class App {

    public static void main(String[] args) {

        CountDownLatch latch = new CountDownLatch(3); // coundown from 3 to 0

        ExecutorService executor = Executors.newFixedThreadPool(3); // 3 Threads in pool

        for(int i=0; i < 3; i++) {
            executor.submit(new Processor(latch)); // ref to latch. each time call new Processes latch will count down by 1
        }

        try {
            latch.await();  // wait until latch counted down to 0
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println("Completed.");
    }

}