当多个终端连接到服务器后，将它们都注册到服务器的一个Hashtable中：

Hashtable<String,SocketChannel> clientlist=new Hashtable<String,SocketChannel>();
String 相当于终端的用户名，SocketChannel 是各个终端的通道。

现在，如果一个终端关闭，服务器捕获这个异常后，如何确定是哪个终端呢？
 if (key.isReadable()) {
                        int count = 0;
                           SocketChannel channel = (SocketChannel) key.channel();
                           buffer.clear();
                           while ((count = channel.read(buffer)) > 0) {
                               buffer.flip();
                               //while (buffer.hasRemaining()) {
                                String receiveText = String.valueOf(cs.decode(buffer).array());
                                   System.out.println(receiveText);
                                   dispatch(channel, receiveText); 
                               //}
                                   channel.register(selector, SelectionKey.OP_READ);
                           }
                           if (count < 0) {
                               channel.close();
                               System.out.println("断开。。。"+PORT_NUMBER);
                           }
                    }
                }
我是这样写的，判断是哪个终端断开的
register(Selector sel, int ops, Object att) 的时候
在 Object att 里面存 终端的用户名
异常的时候，通过 SelectionKey.attachment()，把 终端的用户名 取出来