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

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

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

null

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