countDownLatch 线程辅助类，是一个线程计数类，用来线程同步，每当一个线程执行完成后，就是加1，当达到计数，就算执行完成。初始值必须小于等于要执行的线程数，为0 是，不其作用，大于0，小于等于线程数时，为同步线程数，当大于线程数时，为死循环（可能不对）反正就是一直等待。粘贴上代码，我也是粘贴别人的。自己手 敲一边。实现机制没看太懂，不过没看到他同步里用锁，代码看到Unsafe，就进不去了。

import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.concurrent.CountDownLatch;
 
public class T1 {
    final static SimpleDateFormat sdf = new SimpleDateFormat(
            "yyyy-MM-dd HH:mm:ss");
 
    public static void main(String[] args) throws Exception {
        CountDownLatch countDownLatch = new CountDownLatch(3);
        Worker w1 = new Worker("w1", 2000, countDownLatch);
        Worker w2 = new Worker("w2", 5000, countDownLatch);
        w1.start();//
        w2.start();//
        countDownLatch.await();// 等待所有工人完成工作
        System.out.println("all work done at " + sdf.format(new Date()));
    }
 
    static class Worker extends Thread {
        String workerName;
        int workTime;
        CountDownLatch latch;
 
        public Worker(String workerName, int workTime, CountDownLatch latch) {
            this.workerName = workerName;
            this.workTime = workTime;
            this.latch = latch;
        }
 
        @Override
        public void run() {
            System.out.println("Worker " + workerName + " do work begin at "
                    + sdf.format(new Date()));
            try {
                Thread.sleep(workTime);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("Worker " + workerName + " do work complete at "
                    + sdf.format(new Date()));
            latch.countDown();// 工人完成工作，计数器减一
        }
    }
}