difference between userTransaction.rollback() & userTransaction timeout

Hi all,
I am using stateful session bean and BMT to manage my transaction. I am keeping an open transaction in one of my method. I am expecting the user to call a second method to commit/rollback the transaction. Now, If the user doesn't call the second method, I am setting the transaction timeout. I want the transaction to be discarded if the user doesn't call the second method by 5 seconds. I am expecting the same behavior as the rollback(). Now, though my transaction times out, it doesn't release the locks on db / other resources like it does in case of roll back. Now, What is the difference between timeout and rollback? How can I discard my transaction if user doesn't call the second method??(should i use timer service!!)

regards,
Sajid

Please don't post the same question in multiple forums. Since there is nothing JBoss specific in your question I'm closing this one.