java countDownLatch 线程辅助类详解编程语言

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 
        } 
    } 
}