Which Transaction Manager is in Effect?

[Jayavardhan geepi]

Hi,
In a web app, I use Spring transaction management and using JTATransactionManager comes with Spring.
I'm using JBoss AS and I know that Jboss also has JTATransactionManager.
Apart from this, database itself will be having its own transaction management.
(Please correct me if I'm wrong)

I'm confused, which transaction manager takes effect , does any override another or all work together..
Please clarify.

Thanks,
Jay