join的源码

long base = System.currentTimeMills();

 long now = 0;

if(millis < 0){

 throw new IllegalArgumentException("timeout value is negative");

}

 if(millis == 0){

      while(isAlive()){

      wait(0);

      }

 }else{

      while(isAlive()){

           long delay = millis-now;

           if(delay <= 0){

               break;

           }

           wait(delay);

           now  = System.currentTimeMillis()-base;

      }

 }