实现多人即时聊天，服务器向各客户端发送信息,如果给当前不在线用户发送消息，该用户上线后能看到消息，服务器向客户端发送的消息也是如此-People realize real-time chat, server, send a message to the client, if not currently online users to send messages, the user can see on-line message, the