how to extend LoginUrlAuthenticationEntryPoint or how to implement AuthenticationEntryPoint

Question

I'm trying to do this: Make spring security add the return to url in the query string for the login page , that is: get spring to tell the login page where I came from. I have some SSO integration.. so I'll send the url to them, or they'll append the referer for me, so I know that the user should be logged in and sent to /some/url. That's all dandy. The issue I'm having is in extending LoginUrlAuthenticationEntryPoint (unless you can tell me a good reason to implement AuthenticationEntryPoint instead). I need to only modify requests to the login page, like so:

RedirectStrategy redirectStrategy = new DefaultRedirectStrategy();

// only append returnto to '/login' urls
if(request.getServletPath() == "/login") {
    // indicates we want to return to this page after login
    redirectStrategy.sendRedirect(request, response, "/login?returnto=" + request.getServletPath());
}

How can I let the rest of the requests do their thing? This is incorrect (what I was just doing):

RequestDispatcher dispatcher = request.getRequestDispatcher("/login");

// just forward the request
dispatcher.forward(request, response);

because it puts us in a redirect loop. However it seems to be what spring does in its version of commence. I'm confused. What am I supposed to be doing with commence in my custom extension to LoginUrlAuthenticationEntryPoint?

Solution

Figured it out. Just needed to forward to the login page with 'returnto' appended. I found it confusing at first because LoginUrlAuthenticationEntryPoint didn't mean 'this redirects users to the login page' at first glance. This is all I needed:

@Override
public void commence(HttpServletRequest request, HttpServletResponse response, AuthenticationException authenticationException) throws IOException, ServletException {
    RedirectStrategy redirectStrategy = new DefaultRedirectStrategy();

    redirectStrategy.sendRedirect(request, response, "/login?returnto=" + request.getServletPath());
}

this was useful: LoginUrlAuthenticationEntryPoint.java on grepcode.com