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有人可以帮我理解Java CountDownLatch是什么以及何时使用它？

我对这个程序的工作原理并不十分清楚.据我所知,所有三个线程立即启动,每个线程将在3000ms后调用CountDownLatch.倒数会逐一减少.在锁存器变为零之后,程序打印"已完成".也许我理解的方式不正确.

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 …