public static void main(String[] args) throws Exception {
		System.out.println("JoinTest start "+new Date());	
		MyThread myThread = new MyThread();
		myThread.start();
		// 主线程等待 myThread 执行完成
		myThread.join();				
		System.out.println("JoinTest done "+new Date());
	}
		
    private static class MyThread extends Thread{
    	public void run(){
    		// sleep 2000 ms
    		Utils.doSleep(2000);
    	}
    }
    
    
 调用目标线程的join方法，当前线程会被阻塞，等待目标线程执行完成
 
 join使用 wait 与 notifyAll 实现  典型的等待通知机制
 
   public final synchronized void join(long millis)
    throws InterruptedException {
        long base = System.currentTimeMillis();
        long now = 0;

        if (millis < 0) {
            throw new IllegalArgumentException("timeout value is negative");
        }
        
        if (millis == 0) {
            while (isAlive()) {
                // 无限等待
                wait(0);
            }
        } else {
            while (isAlive()) {
                long delay = millis - now;
                if (delay <= 0) {
                    break;
                }
                // 超时等待
                wait(delay);
                now = System.currentTimeMillis() - base;
            }
        }
    }
    
    
    join 方法 使用  synchronized  线程对象作为锁对象，符合使用wait的经典套路
    
    synchronized(obj) {
         while(!condition) {
              obj.wait();
         }
         doSomething();
     }
    
    notifyAll 由 jvm 调用  ， 具体可以看jvm 的 c++ 代码
    void JavaThread::exit(bool destroy_vm, ExitType exit_type)
  
    static void ensure_join(JavaThread* thread) {
        Handle threadObj(thread, thread->threadObj());
        ObjectLocker lock(threadObj, thread);
        thread->clear_pending_exception();
        java_lang_Thread::set_thread_status(threadObj(), java_lang_Thread::TERMINATED);
        java_lang_Thread::set_thread(threadObj(), NULL);

        //  notify_all !!!!!
        lock.notify_all(thread);
        thread->clear_pending_exception();
    }