There's more...

Another important piece of code is the following one:

@Stateful
public class UserBean {

    @PersistenceContext(unitName = "ch02-jta-pu", 
                        type = PersistenceContextType.EXTENDED)
    private EntityManager em;

    ...
}

Here, we are defining PersistenceContext as EXTENDED. This means that this persistence context is bound to the @Stateful bean until it is removed from the container.

The other option is TRANSACTION, which means the persistence context will only live for the duration of the transaction.