How to implement security at the container (form) level?

Question

Java

I want to implement user authorization at the form level on sevlets, using user roles, but I did not find a good example, help me.
Is this generally used in public sites (I mean sites not of some kind of organizations or a site only for company employees, for example) ?? Or everything is done normally, he himself wrote the registration form for entering the exit and that's it ?!
Is it used in general Authorization of the user at the form level + use of roles or is it better to write the form yourself and prescribe roles in web.xml?
Is it possible to specify the roles in the application itself and not to register them in the server file, and if they are registered in the server file, how will this affect other applications deployed on this server?
How to learn on a servlet under what role the person comes?