spring security - adding parameters to request for json login not working

Question

I have been following this post on how to create an entry point into my spring mvc 3.1 web application for someone to login using a json request.

Spring Security and JSON Authentication

I've got a question about the code below. Inside attemptAuthentication I am adding extra request parameters which are json specific. And then I try to access those parameters in obtainUsername and obtainPassword but the parameters are not there.

public Authentication attemptAuthentication(HttpServletRequest request, HttpServletResponse response)
        throws AuthenticationException {

    if ("application/json".equals(request.getHeader("Content-Type"))) {
        StringBuffer sb = new StringBuffer();
        String line = null;

        BufferedReader reader;
        try {
            reader = request.getReader();
            while ((line = reader.readLine()) != null){
                sb.append(line);
            }                                   

            //json transformation
            ObjectMapper mapper = new ObjectMapper();
            JsonLoginRequest loginRequest = mapper.readValue(sb.toString(), JsonLoginRequest.class);

            String jsonUsername = loginRequest.getJ_username();
            request.setAttribute("jsonUsername", jsonUsername);

            String jsonPassword = loginRequest.getJ_password();
            request.setAttribute("jsonPassword", jsonPassword);

            String jsonStore = loginRequest.getJ_store();
            request.setAttribute("jsonStore", jsonStore);             

        }
        catch (JsonParseException e) {

            e.printStackTrace();
        } catch (JsonMappingException e) {

            e.printStackTrace();
        } 
        catch (IOException e) {

            e.printStackTrace();
        }                       
    }

    String usernameParameter = obtainUsername(request);
    String password = obtainPassword(request);

When I do this jsonUsername and jsonStore don't exist even though I added them above.

@Override
protected String obtainUsername(HttpServletRequest request) {
    String combinedUsername = null;
    if ("application/json".equals(request.getHeader("Content-Type"))) {
        String jsonUsername = request.getParameter("jsonUsername");
        String jsonStore = request.getParameter("jsonStore");            
        combinedUsername = 
                jsonUsername + 
                SecurityConstants.TWO_FACTOR_AUTHENTICTION_DELIM + 
                jsonStore;      

    }else {
        String username = super.obtainUsername(request);
        String store = request.getParameter(SecurityConstants.STORE_PARAM);
        String hiddenStore = request.getParameter(SecurityConstants.HIDDEN_STORE_PARAM);
        combinedUsername = 
                username + 
                SecurityConstants.TWO_FACTOR_AUTHENTICTION_DELIM + 
                store + 
                SecurityConstants.TWO_FACTOR_AUTHENTICTION_DELIM +
                hiddenStore;            
    }   
    return combinedUsername;
}

Can someone help me with what is wrong? thanks