Finally, I could fix the issue by using createTupleQuery():

    val criteriaBuilder = entityManager.criteriaBuilder
    val criteriaQuery = criteriaBuilder.createTupleQuery()
    val root = criteriaQuery.from(User::class.java)

    val predicate = spec.toPredicate(root, criteriaBuilder.createQuery(), criteriaBuilder)
    criteriaQuery.where(predicate)

    val selections = listOf("userName", "displayName").map{
        root.get(it)
    }
    criteriaQuery.multiselect(*selections.toTypedArray())

    val query = entityManager.createQuery(criteriaQuery)
    val resultList = query.resultList