public <R> R doInTransaction(final Function<EntityManager, R> whatToDo) {
    final EntityManager em = emf.createEntityManager();
    final R result;
    try {
      try {
        em.getTransaction().begin();
        result = whatToDo.apply(em);
        em.getTransaction().commit();
      } finally {
        if (em.getTransaction().isActive()) {
          em.getTransaction().rollback();
        }
      }
    } finally {
      em.close();
    }
    return result;
  }

em.unwrap(Session.class).doWork(connection -> {
  // stored procedures are called here directly over JDBC
});