Head First EJB

Things you must NOT do with BMT

1. A BMT bean must NOT start a transaction before ending the current transaction.

   

   Brain Power

   Imagine the implications of starting a new transaction before ending the current one. What might happen if you were allowed to do this?

   Note

   Nested transactions are not allowed in EJB!

   You’re expected to know what the term “nested transaction” means, and what it might look like in code.

2. A BMT stateless session or message-driven bean must NOT complete a transactional method without ending the transaction.

   

   Brain Power

   Why are stateful session beans allowed to end a method without ending the transaction?

   For a stateful bean, can you think of a scenario where you might want to do this (leave the transaction open)?

   What might go wrong if you do this?

   Note

   Only STATEFUL session beans can leave a transaction open at the end of a method.

BMT transactions are one way: they can propagate out to a CMT bean, but no other transaction can propagate in to a BMT bean

Both BMT and CMT bean transactions propagate into a CMT bean.

A CMT bean can run in transactions coming from both CMT and BMT beans. ...