CountDownLatch的用法

1.

package com.test.frame.test.countDownLatch;

import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.concurrent.CountDownLatch;

/**
 * created by guanguan on 2017/10/26
 **/
public class CountDownLatchDemo {


    final static SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");


    static class Worker extends Thread {

        private String workName;
        private int workTime;
        CountDownLatch latch;

        public Worker(String workName, int workTime, CountDownLatch latch) {
            this.workName = workName;
            this.workTime = workTime;
            this.latch = latch;
        }

        public Worker(Worker worker) {
        }

        @Override
        public void run() {

            System.out.println("worker : "+workName +"start begin at "+sdf.format(new Date()));
            doWork();
            System.out.println("worker : "+workName +"start complete at "+sdf.format(new Date()));



        }

        private void doWork() {

            try {
                Thread.sleep(workTime);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }finally {
                latch.countDown();//工人完成工做，計數器減一

            }
        }
    }



    public static void main(String[] args) throws InterruptedException {
         /**
         * 初始化CountDownLatch，將其值置爲2，latch計數器的操做是原子性的
         */
        CountDownLatch latch=new CountDownLatch(2);

        Worker worker1= new Worker("WORK-1",5000,latch);
        Worker worker2= new Worker("WORK-2",2000,latch);
        Worker worker3= new Worker("WORK-3",1000,latch);
        worker1.start();
        latch.await();      //此時將永遠阻塞，由於只有一個線程開啓，完成任務
        worker2.start();
        worker3.start();


    }
}

運行結果是：
worker : WORK-1start begin at 2017-10-26 14:32:54
worker : WORK-1start complete at 2017-10-26 14:32:59

此時因爲latch.await()的緣由，則線程將永遠阻塞，由於latch仍是1，沒有置爲0，則線程2，線程3將沒法進行


若是將 latch.await();      //此時將永遠阻塞，由於只有一個線程開啓，完成任務
        worker2.start(); 這兩行對換，


 public static void main(String[] args) throws InterruptedException {
        CountDownLatch latch=new CountDownLatch(2);//兩個工人的協做

        Worker worker1= new Worker("WORK-1",5000,latch);
        Worker worker2= new Worker("WORK-2",2000,latch);
        Worker worker3= new Worker("WORK-3",1000,latch);
        worker1.start();
        worker2.start();

        latch.await();       
        worker3.start();


    }


運行結果以下:

worker : WORK-2start begin at 2017-10-26 14:34:35
worker : WORK-1start begin at 2017-10-26 14:34:35
worker : WORK-2start complete at 2017-10-26 14:34:37
worker : WORK-1start complete at 2017-10-26 14:34:40
worker : WORK-3start begin at 2017-10-26 14:34:40
worker : WORK-3start complete at 2017-10-26 14:34:41

Process finished with exit code 0

代表線程1，線程2同時運行，並阻塞，而後1，2所有完成以後，線程3纔開始運行