Explanation/Reference:
When a JTA application-managed entity manager is used, if the entity manager is created outside the scope of the current JTA transaction, it is the responsibility of the application to associate the entity manager with the transaction (if desired) by calling EntityManager.joinTransaction.
Reference: Identify correct and incorrect statements or examples about application-managed persistence contexts.
http://java.boot.by/scbcd5-guide/ch07s03.html