并发工具类 CountDownLatch

 

 

在多线程中，如果需要一个线程，在其他的线程执行完之后再执行，则需要我们本篇将的并发工具类：CountDownLatch类：

CountDownLatch类的执行原理是：

     我们设定的被等待的线程的个数会被记录在底层，当我们的一个线程执行完毕之后调用CountDownLatch类的countDown();方法

那么底层被记录的数字机会减1，当所有的被等待的线程都执行完之后被记录的数字就会变成0，这个时候，等待线程，也就是被CountDownLatch类的await();

方法修饰的线程就会被唤醒。

下面用代码解释一下：（妈妈等待三个孩子吃饺子，三个都吃完再洗刷碗筷）

这是主方法

    public static void main(String[] args) {
        CountDownLatch countDownLatch = new CountDownLatch(3);
        Child1 child1 = new Child1(countDownLatch);
        Child2 child2 = new Child2(countDownLatch);
        Child3 child3 = new Child3(countDownLatch);
        child1.setName("小明");
        child2.setName("小红");
        child3.setName("小刚");
        mather mom = new mather(countDownLatch);
        child1.start();
        child2.start();
        child3.start();
        mom.start();


    }

这是孩子1

public class Child1 extends Thread {
    CountDownLatch countDownLatch;

    public Child1(CountDownLatch countDownLatch) {
        this.countDownLatch = countDownLatch;
    }

    @Override
    public void run() {

        for (int i = 0; i < 20; i++) {
            System.out.println(Thread.currentThread().getName() + "吃了第" + i + "个饺子");
        }


        countDownLatch.countDown();

    }
}

这是孩子2

public class Child2 extends Thread {
    CountDownLatch countDownLatch;

    public Child2(CountDownLatch countDownLatch) {
        this.countDownLatch = countDownLatch;
    }

    @Override
    public void run() {

        for (int i = 0; i < 20; i++) {
            System.out.println(Thread.currentThread().getName() + "吃了第" + i + "个饺子");
        }


        countDownLatch.countDown();

    }
}

这是孩子3

public class Child3 extends Thread {
    CountDownLatch countDownLatch;

    public Child3(CountDownLatch countDownLatch) {
        this.countDownLatch = countDownLatch;
    }

    @Override
    public void run() {

        for (int i = 0; i < 20; i++) {
            System.out.println(Thread.currentThread().getName() + "吃了第" + i + "个饺子");
        }

        countDownLatch.countDown();


    }
}

这是妈妈：

public class mather extends Thread {
    CountDownLatch countDownLatch;

    public mather(CountDownLatch countDownLatch) {
        this.countDownLatch = countDownLatch;
    }

    @Override
    public void run() {

        try {
            countDownLatch.await();//注意要放在执行语句之前
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("开始收拾碗筷");


    }
}

代码很简单，不做过多解释