Java多线程--主线程等待所有子线程执行完毕

参考： http://blog.csdn.net/nms312/article/details/30115055

本文介绍两种主线程等待子线程的实现方式：

1、使用Thread的join()方法，join()方法会阻塞主线程继续向下执行。

2、使用java.util.concurrent中的CountDownLatch，是一个倒数计数器。初始化时先设置一个倒数计数初始值，每调用一次countDown()方法，倒数值减一，他的await()方法会阻塞当前进程，直到倒数至0。

public class Threads {
	 public  static  void  main (String[]  args ) {
	        SubThread thread  =  new  SubThread () ;
	        thread.start () ;
	        //主线程处理其他工作,让子线程异步去执行.
	         mainThreadOtherWork () ;
	         System.out.println ( "now waiting sub thread done." ) ;
	        //主线程其他工作完毕,等待子线程的结束, 调用join系列的方法即可(可以设置超时时间)
	         try  {
	            thread.join () ;
	        }  catch  ( InterruptedException e) {
	            e.printStackTrace () ;
	        }
	        System.out.println ("now all done.") ;
	}

private  static  void  mainThreadOtherWork () {
         System.out.println ("main thread work start") ;
          try  {
              Thread.sleep ( 3000L ) ;
         }  catch  ( InterruptedException e) {
             e.printStackTrace () ;
         }
          System.out.println ("main thread work done.") ;
    }
}

package classstudy;

public class  SubThread  extends  Thread{
    @Override
    public  void  run () {
        working () ;
   }

    private  void  working () {
        System.out.println ("sub thread start working." ) ;
        busy () ;
        System.out.println ("sub thread stop working." ) ;
   }

    private  void  busy () {
        try  {
             sleep ( 5000L ) ;
       }  catch  ( InterruptedException e) {
            e.printStackTrace () ;
       }
   }
   
}

运行结果：

main thread work start
sub thread start working.
main thread work done.
now waiting sub thread done.
sub thread stop working.
now all done.

package classstudy;

import java.util.concurrent.CountDownLatch;  

public class Thread1 extends Thread  
{  
  
    private CountDownLatch count;  
  
    public Thread1(CountDownLatch count,String name)  
    {  
        this.count = count;  
        this.setName(name);  
    }  
  
    @Override  
    public void run()  
    {  
        System.out.println(this.getName() + " staring...");  
  
        System.out.println(this.getName() + " end...");  
        this.count.countDown();  
    }  
  
    /** 
     * @param args 
     */  
    public static void main(String[] args)  
    {  
        System.out.println("main thread starting...");  
  
        CountDownLatch count = new CountDownLatch(5);  
  
        for (int i = 1; i <= 5; i++)  
        {  
        	Thread1 my = new Thread1(count, "Thread " + i);  
            my.start();  
        }  
  
        try  
        {  
            count.await();  
        }  
        catch (InterruptedException e)  
        {  
            e.printStackTrace();  
        }  
  
        System.out.println("main thread end...");  
  
    }  
  
}  

运行结果：

main thread starting...
Thread 1 staring...
Thread 1 end...
Thread 3 staring...
Thread 3 end...
Thread 2 staring...
Thread 2 end...
Thread 4 staring...
Thread 4 end...
Thread 5 staring...
Thread 5 end...
main thread end...

实际应用场景，Map myDynamicData, 这个动态变化的数据消失发送邮件。这样会存在一个问题，主线程等待子线程执行完毕，如果子线程耗费时间很长，将阻碍主线程继续向下执行，直观现象就是一直卡在那儿。

解决办法：启一个线程去监控myDynamicData里面的数据。