Need to login and redirect to a url

Hi all,

I'm trying to execute this code.

IAuthentication Authen = UMFactory.getAuthenticator();

HttpServletRequest req = request.getServletRequest();

HttpServletResponse res = request.getServletResponse(true);

Authen.logout(req,res);

req.setAttribute(ILoginConstants.LOGON_UID_ALIAS,"user1");

req.setAttribute(ILoginConstants.LOGON_PWD_ALIAS,"pass1");

res.sendRedirect("/irj/servlet/prt/portal/prtroot/com.sap.portal.navigation.portallauncher.default");

I'm trying to logout currently login user and want to redirect to the above url with new user user1 and password pass1.

Code is executing but giving me a blank screen. Thus want to know is anything that I'm missing?

Thanks in advance.

Regards,

Shiva