AuthProvider.authenticate -- Full JID?

From the API docs:

AuthProvider
...
void **authenticate**(String username,
                  String password)
                  throws UnauthorizedException,
                         ConnectionException,
                         InternalUnauthenticatedException

username - the username or full JID.

I am implementing the AuthProvider interface. Every time authenticate() is called, it is called with just the username, and not the full jid. As the API docs specify “or full JID”, I suspect there is a condition under which the full JID will be sent as the username parameter.

I need to check the domain as well as the username in my code, so I need the full JID. Is there a config setting or something else that I can do to get it to send the full JID everytime?

Thanks