well if you mean the userid of database it's easy but if you mean the userid of the application I dont have a clue!

in first case if you want to some kind of auditing what we do here is add a column named user_id then add a default value to the column

like

alter table xxx add (user_id default user not null)

then you would have to write triggers to update that column whenever there is an DML

I dont know if this what you are lokking for but that's how we set up out Web Application with WebLogic