Actually I am able to receive the parameters in SessionInit() method only for the first time when the Session is Initialized.
If I want to receive the parameters each time, I had to override the service() method
@Override
protected void service(HttpServletRequest request,
HttpServletResponse response) throws ServletException, IOException {
super.service(request, response);
String appName= request.getParameter("app_name");
if(appName!=null){
getService().getServlet().getServletContext().setAttribute("app_name", appName);
}
}