How to retrieve user credentials in apache tomcat and do manual authentication?

I have configured apache tomcat, but I want to do authentication manually. That means I need to handle authentication in the server, retrieve user credentials, and then do my authentication (instead of using tomcat-users tag).

Probably I can do it with spring framework, but I'm not sure how to do it. I have a simple servlet where I catch GET and POST requests, so I think it's possible also to catch authentication process.

Related docs/links/tutorials will also help.