I did some more research on the problem stated above. It doesn't really matter if the finderMethod throws an exception or produces a result, the transaction is always commited, as soon as I call a finderMethod!!!
And setting transaction attributes for the CMP Entity Bean doesn't help either. I set "Required" for all methods but the problem keeps the same.
Might this be a bug in JBoss?
Ok, it was one of the beloved "read thefuckingmanual"-Problems.
For all who might run into the same, here is the solution:
By Default JBoss commits current transactions when a find, select or remove Method is called!
This (in my opinion stupid) behaviour can be changed by setting <sync-on-commit-only> in the jboss.xml in tag <container-configuration> to "true".
Some more details can be found in the jboss_3_2.dtd.