Java多线程_同步工具CountDownLatch

概念：
CountDownLatch是多线程里面一个类似于计数器的高级同步工具，它的初始值代表线程的数量，当一个线程完成了任务后，CountDownLatch的值就减1，当值为0的时候，代表所有线程完成了任务，然后等待的线程可以开始自己的任务。

方法：
这里主要涉及到两个核心的方法：

• await():线程响应中断，进入等待状态。
• countDown():减1操作，CountDownLatch的值减1。

示例：

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

/*
 * 模拟考试过程：学生等待老师发卷，老师发完卷学生答卷，老师等待学生交卷，等到学生全部交卷，老师离开
 */
public class CountDownLatchDemo implements Runnable {
    static CountDownLatch teacher = new CountDownLatch(1);
    static CountDownLatch student = new CountDownLatch(3);
    static ExecutorService service = Executors.newCachedThreadPool();

    @Override
    public void run() {
        System.out.println(Thread.currentThread().getName() + "等待发卷");
        try {
            teacher.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(Thread.currentThread().getName() + "开始答卷");
        try {
            Thread.sleep(new Random().nextInt(1000));
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(Thread.currentThread().getName() + "交卷");
        student.countDown();
    }

    public static void main(String[] args) {
        CountDownLatchDemo cdld = new CountDownLatchDemo();
        for (int i = 0; i < 3; i++) {
            service.execute(cdld);
        }
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e1) {
            e1.printStackTrace();
        }
        System.out.println(Thread.currentThread().getName() + "开始发卷");
        try {
            Thread.sleep(200);
        } catch (InterruptedException e1) {
            e1.printStackTrace();
        }
        System.out.println(Thread.currentThread().getName() + "等待交卷");
        teacher.countDown();
        try {
            student.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(Thread.currentThread().getName() + "收齐试卷，准备离开");
        service.shutdown();
    }
}

下面，我们来看一看结果：