java并发编程JUC CountDownLatch线程同步

import java.util.concurrent.CountDownLatch;

public class Tester {
   public static void main(String args[]) {
      //设置计数器 counter = 4 ，等于线程数
      CountDownLatch countDownLatch = new CountDownLatch(4);

      Thread app1 = new Thread(new Application("App1",  countDownLatch));
      Thread app2 = new Thread(new Application("App2",  countDownLatch));          
      Thread app3 = new Thread(new Application("App3",  countDownLatch));
      Thread app4 = new Thread(new Application("App4",  countDownLatch));  
 
      // 启动多线程去检查其他四个程序的可用状态
      app1.start();
      app2.start();
      app3.start();
      app4.start();

      try {
         //主线程调用await进行等待，等待上述四个线程正常完成
         countDownLatch.await();            
         //上述四个线程检查的应用程序启动正常之后, 打印如下信息
         System.out.println("All applications are up and running.");
      } catch(InterruptedException e) {
         System.out.println(e.getMessage());
      }        
   }
}

class Application implements Runnable {
   private String name; //应用程序名称
   private CountDownLatch countDownLatch; 

   public Application(String name, CountDownLatch countDownLatch) {
      this.name = name;
      this.countDownLatch = countDownLatch;
   }

   public void run() {
      try {
         System.out.println(name + " started. ");
         Thread.sleep(1000);
      } catch (InterruptedException e) {
         System.out.println(e.getMessage());
      }
      System.out.println( name + " is Up and running.");
      //将countDownLatch计数器的值减1
      countDownLatch.countDown();    
   }
}

App2 started.  
App3 started.  
App1 started.  
App4 started.  
App1 is Up and running.
App3 is Up and running.
App4 is Up and running.
App2 is Up and running.
All applications are up and running.