10.3 CountDownLatch-Java语言基础8-Java多线程 - 码小课

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CountDownLatch 的应用场景：让某一条线程等待其它线程执行完毕之后再执行。
构造方法：
```bash
// count表示等待线程数量，底层定义了一个计数器
public CountDownLatch(int count) {}
```

让线程等待：
```bash
// 当计数器为0的时候，会唤醒等待的线程
public void await() throws InterruptedException {}
```

当前线程执行完毕：

```bash
// 线程执行完毕时调用，会将计数器-1
public void countDown() {}
```

示例：模拟妈妈等待孩子吃完饭之后才收拾碗筷

```bash
package com.github.demo19;

import java.util.concurrent.CountDownLatch;

/**
 * @author maxiaoke.com
 * @version 1.0
 * 
 */
public class MotherThread extends Thread {
    private CountDownLatch countDownLatch;

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

@Override
    public void run() {
        // 等待
        try {
            this.countDownLatch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        // 收拾碗筷
        System.out.println(Thread.currentThread().getName() + "收拾碗筷");
    }
}
```

```bash
package com.github.demo19;

import java.util.concurrent.CountDownLatch;

/**
 * @author maxiaoke.com
 * @version 1.0
 * 
 */
public class SonThread1 extends Thread {

private CountDownLatch countDownLatch;

public SonThread1(CountDownLatch countDownLatch) {
        this.countDownLatch = countDownLatch;
    }

@Override
    public void run() {
        // 吃饭
        for (int i = 0; i < 10; i++) {
            System.out.println(Thread.currentThread().getName() + "吃了第" + (i + 1) + "个饺子");
        }
        // 告诉妈妈吃完了
        this.countDownLatch.countDown();
    }
}
```

```bash
package com.github.demo19;

import java.util.concurrent.CountDownLatch;

/**
 * @author maxiaoke.com
 * @version 1.0
 * 
 */
public class SonThread2 extends Thread {
    private CountDownLatch countDownLatch;

public SonThread2(CountDownLatch countDownLatch) {
        this.countDownLatch = countDownLatch;
    }

@Override
    public void run() {
        // 吃饭
        for (int i = 0; i < 20; i++) {
            System.out.println(Thread.currentThread().getName() + "吃了第" + (i + 1) + "个饺子");
        }
        // 告诉妈妈吃完了
        this.countDownLatch.countDown();
    }
}
```

```bash
package com.github.demo19;

import java.util.concurrent.CountDownLatch;

/**
 * @author maxiaoke.com
 * @version 1.0
 * 
 */
public class SonThread3 extends Thread {
    private CountDownLatch countDownLatch;

public SonThread3(CountDownLatch countDownLatch) {
        this.countDownLatch = countDownLatch;
    }

@Override
    public void run() {
        // 吃饭
        for (int i = 0; i < 30; i++) {
            System.out.println(Thread.currentThread().getName() + "吃了第" + (i + 1) + "个饺子");
        }
        // 告诉妈妈吃完了
        this.countDownLatch.countDown();
    }
}
```

```bash
package com.github.demo19;

import java.util.concurrent.CountDownLatch;

/**
 * @author maxiaoke.com
 * @version 1.0
 * 
 */
public class Test {
    public static void main(String[] args) {
        // CountDownLatch的对象，需要传递给四个线程
        CountDownLatch countDownLatch = new CountDownLatch(3);
        // 创建四个线程对象并开启它们
        MotherThread motherThread = new MotherThread(countDownLatch);
        motherThread.setName("妈妈");
        SonThread1 sonThread1 = new SonThread1(countDownLatch);
        sonThread1.setName("孩子1");
        SonThread2 sonThread2 = new SonThread2(countDownLatch);
        sonThread2.setName("孩子2");
        SonThread3 sonThread3 = new SonThread3(countDownLatch);
        sonThread3.setName("孩子3");
        // 启动线程
        motherThread.start();
        sonThread2.start();
        sonThread3.start();
        sonThread1.start();
    }
}
```

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