线程同步工具之CountDownLatch

CountDownLatch,一个同步辅助类,在完成一组其他线程汇总执行的操作前,它允许一个或多个线程一直等待

主要方法:

     public CountDownLatch(int count); 构造方法参数指定了计数的次数

     public void countDowm():     当前线程调用此方法,则计数器减1

     public void await();     调用此方法会一致阻塞当前线程,直到计数器的值为0

示例中,

package demo1;

import java.text.SimpleDateFormat;

import java.util.Date;

import java.util.concurrent.CountDownLatch;

import java.util.concurrent.Executor;

import java.util.concurrent.Executors;

public class CountDownLatchDemo {

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

     public static void main(String[] args) throws InterruptedException {

         CountDownLatch latch = new CountDownLatch(2);// 两个工人的协作

         Executor executor = Executors.newFixedThreadPool(2);

         executor.execute(new Worker("zhang san", 2000, latch));

         executor.execute(new Worker("li si", 3000, latch));

         /**

          * 当主线程执行到这的时候,现在一共是有3个线程在同时执行

          * 当主线程执行了latch.await()方法的时候,就会阻塞主线程,只到CountDownLatch的计数器变成0

          * 而当其他线程执行了latch.countDown()方法后,才会把计数器减一.

          * 所以这个时候,主线程就会等待计数器变成0后再执行

          */

         latch.await();

         System.out.println("all work done at " + sdf.format(new Date()));

     }

     /**

      * 匿名内部类写法

      * @author edgewalk

      * @date  2017年6月8日

      */

     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;

         }

         public void run() {

              try {

                  System.out.println("Worker " + workerName + " do work begin at " + sdf.format(new Date()));

                  doWork();// 工作了

                  System.out.println("Worker " + workerName + " do work complete at " + sdf.format(new Date()));

              } catch (Exception e) {

                  e.printStackTrace();

              }finally {

                   if(latch != null) {

                        latch.countDown();

                     }

              }

         }

         private void doWork() {

              try {

                  Thread.sleep(workTime);

              } catch (InterruptedException e) {

                  e.printStackTrace();

              }

         }

     }

}

打印结果:

     

Worker li si do work begin at 2017-06-08 23:11:38

Worker zhang san do work begin at 2017-06-08 23:11:38

Worker zhang san do work complete at 2017-06-08 23:11:40

Worker li si do work complete at 2017-06-08 23:11:41

all work done at 2017-06-08 23:11:41

补充:

     如果我们不使用CountDownLatch的await()方法,那么all work done at ...这句话就不能保证在最后打印了

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