说明：  实例化HashMap类型的一个对象用来存放每一个客户对应的套接字和通道，实例化线程通道选择器等，当由用户请求时，接受，加入选择项，注册感兴趣事件，另一线程处理输入流，在前面加echo:，发给客户端。
(Instantiate an object of type HashMap to store the corresponding socket of each client and channel, select the instance of the thread channel, etc., when the user request, receive, add select items up for events of interest, the other thread input stream, with the addition echo:, to the client.)