Java多线程中如何使用CountDownLatch

有没有人能帮我理解一下JavaCountDownLatch是什么，什么时候用？

我不太清楚这个程序是如何工作的.据我所知，所有三个线程同时启动，每个线程将在3000毫秒后调用CountDownLatch.所以倒计时会一个接一个递减.闩锁归零后，程序打印"已完成".也许我理解的方式是错误的.

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.");
    }

}