java 后台线层也叫守护线层

import java.util.concurrent.locks.*;
class Do9
{
    public static void main(String[] args)
    {
        Ds d=new Ds();
        Thread t1=new Thread(d);
        Thread t2=new Thread(d);
        Thread t3=new Thread(d);
        t1.start();
        t2.start();
        t3.setDaemon(true);//变成后台线层，要在start()之前
        t3.start();
 
        for (int i=0; i<9999;i++ )
        {
            if(i==9997)
            {
            t1.interrupt();
            t2.interrupt();
            break;
            }
            System.out.println(i);
        }
    }
}
class Ds implements     Runnable
{
 
  private boolean flag=true;
  public synchronized void run()
  {
    while(flag)
      {
    try{wait();}catch(InterruptedException e){System.out.println(Thread.currentThread().getName()+"..."+e);}
    System.out.println(Thread.currentThread().getName()+"哈哈");
    break;
      }
 
  }
  public void setFlag()
    {
    flag=false;
    }
}