User - Controller - Unique usernames/emails

[GoogleCodeExporter]

Trying to register a new user with the same username or email as an
existing one currently throws an exception. It shouldn't.

I don't really know how to solve this. But I guess there are two ways: 

1. Catch the exception and parse it for clues as to why something went
wrong. If it was due to a "Integrity Contstraint Violation", the username
or email wasn't unique.

More: http://java.sun.com/javase/7/docs/api/

2. Somehow do it as a transaction, first selecting any users with the same
username or email, and if the selection is empty, insert. But what if the
entire transaction fails, either due to someone else doing stuff, or
because of bad sql?

More: http://java.sun.com/docs/books/tutorial/jdbc/basics/transactions.html

Original issue reported on code.google.com by tgwizard on 9 Apr 2010 at 5:36