How to verify that a user has been defined in weblogic in Java code?

Hi everyone. Here's my problem: some users have been defined with a certain group in weblogic security realms. How can I understand in my Java code, that a given username& password is correct (it has been defined in weblogic) and it belongs to that group? In other way, how can I verify that a user has been defined in weblogic with a certain group? Any advice and suggestions will be greatly appreciated.