//握手前,读取用户名，并赋值给websocket的session
    @Override
    public boolean beforeHandshake(ServerHttpRequest request,
            ServerHttpResponse response, WebSocketHandler handler,
            Map<String, Object> attr) throws Exception {        
         if (request instanceof ServletServerHttpRequest) {
                ServletServerHttpRequest servletRequest = (ServletServerHttpRequest) request;
                HttpSession session = servletRequest.getServletRequest().getSession(false);
                if (session != null) {
                    String userName = (String) session.getAttribute(Constants.SESSION_USERNAME);
                    if(userName == null){
                        userName = "WEBSOCKET_USERNAME_IS_NULL";
                    }
                    attr.put(Constants.WEBSOCKET_USERNAME,userName);
                }
            }
        return true;
    }