public void join() {
	    synchronized (this) {
	     isClosed = true;
	      notifyAll();                    //ѻgetTask()еȴĹ߳
	    }
	    Thread[] threads = new Thread[activeCount()];
	    //enumerate()̳ThreadGroup࣬߳еǰлŵĹ߳
	    int count = enumerate(threads);  
	    for (int i=0; i<count; i++) {    //ȴй߳н
	      try {
	        threads[i].join();           //ȴ߳н
	      }catch(InterruptedException ex) { }
	    }
	  }