public static void transaction(EntityManagerFactory emf, Consumer<EntityManager> operation) {
    var em = emf.createEntityManager();
    try {
	em.getTransaction().begin();
	operation.accept(em);
	em.getTransaction().commit();
    } catch (Throwable t1) {
	try {
	    em.getTransaction().rollback();
	} catch (Throwable t2) {
	    t2.printStackTrace();
	}
	throw t1;
    } finally {
	em.close();
    }
}
	
public static <E> E transaction(EntityManagerFactory emf, Function<EntityManager, E> operation) {
    var em = emf.createEntityManager();
    try {
	em.getTransaction().begin();
	E result = operation.apply(em);
	em.getTransaction().commit();
	return result;
    } catch (Throwable t1) {
	try {
	    em.getTransaction().rollback();
	} catch (Throwable t2) {
	    t2.printStackTrace();
	}
	throw t1;
    } finally {
	em.close();
    }
}

public E update(E entity) {
    return transaction(emf, (EntityManager em) -> em.merge(entity));
}
	
public void save(E entity) {
    transaction(emf, (EntityManager em) -> em.persist(entity));
}