Can a single EntityManager (Hibernate) participate in multiple concurrent transactions?

Question

Is there a way one EntityManager can participate smoothely in multiple concurrent transactions?

Well, not that concurrent. Something like:

1. Start tx 1
2. Do stuff in tx 1
3. Start tx 2
4. Do stuff in tx 2
5. Commit tx 2
6. Join tx 1 back
7. Do stuff in tx 1
8. Commit tx 1

with steps followed one by one not overlapping.

Answer 1

Separate transactions? No, it can not.

Attempting to call EntityTransaction.begin() on a currently active transaction (which is what you would have to do, diretly or indirectly) will result in IllegalStateException being thrown.

Unit of work / transaction scope concepts are explained in detail in the Hibernate EntityManager manual.