I have implemented UserDetailsService, it returns an instance of MyUser (which implements UserDetails)
public MyUser l
If your MyUser object implements Principal and you place it in the http session on successful login, you could get it from the session and cast it.
You could implement the AuthenticationSuccessHandler inject your UserDetailsService in there and place it in the session.
Then you could inject your AuthenticationSuccessHandler into the UsernamePasswordauthenticationFilter which handles the element.
If you give me details about your security context config I could give you more details.