JpaLocalTxnInterceptor incorrectly assumes that @Transactional methods never handle exceptions

[Vogel612]

When reading through JpaLocalTxnInterceptor#invoke we can see the following comment:

   //everything was normal so commit the txn (do not move into try block above as it
    //  interferes with the advised method's throwing semantics)
    try {
      txn.commit();
    } finally {
      //close the em if necessary
      if (null != didWeStartWork.get() ) {
        didWeStartWork.remove();
        unitOfWork.end();
      }
    }

This is a patently false assumption when considering persistence methods that follow another schema. Example:

@Transactional
public NiceResponse<EntityType> create(EntityType instance) {
  try{
    try {
      entityManager.persist(instance);
      entityManager.flush();
      return NiceResponse.success(instance);
    } catch (PersistenceException ex) {
       // unwrap to find actual cause and use that to find out what happened
      entityManager.getTransaction().rollback(); // this is theoretically optional
      thow ex.getCause();
    }
  } catch (ConstraintViolationException ex) {
    return NiceResponse.failure("Some useful error message", ex);
  }
  // possibly further catch blocks
}

Commiting the transaction after this method will throw an Exception all error cases, simply because the transaction has been rolled back already, but there's no Exception thrown to early-exit invoke.

[Vogel612]

Assuming I'd be assigned, it seems that checking for the transaction's status before trying to commit it would be appropriate.

I'd suggest using the following alternative:

try {
  if (txn.isRollbackOnly()) {
    txn.rollback();
  } else {
    txn.commit();
  }
}

I'm not terribly experienced with the detailed semantics of JPA Transactions though, so I might be missing something.