Spring boot SUM(column) with specification

I have an entity, says:

class MyEntity {
    Long id;
    String attr1;
    String attr2;
    String attr3;
    String attr4;
    Double attr5;
}

I use Specification to query the result filtered by attributes like:

class MySpecification implements Specification<MyEntity> {
    private String attr1;
    private String attr2;
    private String attr3;
    private String attr4;

@Override
public Predicate toPredicate(Root<MyEntity> root, CriteriaQuery<?> query, CriteriaBuilder criteriaBuilder) {
    List<Predicate> restriction = new ArrayList<>();
    if (!StringUtils.isEmpty(attr1)) {
        restriction.add(criteriaBuilder.equal(root.get("attr1"), attr1));
    }
    if (!StringUtils.isEmpty(attr2)) {
        restriction.add(criteriaBuilder.equal(root.get("attr2"), attr2));
    }
    // And so on
    Predicate predicate = criteriaBuilder.disjunction();
    predicate.getExpressions().add(criteriaBuilder.and(restriction.toArray(new Predicate[restriction.size()])));
    return predicate;
    }
}

Now I want to get sum of attr5 by the Specification, how can I do that ?

Thank you in advanced.

After some research, here is solution:

Create an interface:

interface MyRepositoryCustom {
    <S extends Number> S sum(Specification<MyEntity> spec, Class<S> resultType, String fieldName);
}

Implementation:

@Repository
class MyRepositoryCustomImpl implements MyRepositoryCustom {

    @Autowired
    private EntityManager entityManager;

    @Override
    public <S extends Number> S sum(Specification<MyEntity> spec, Class<S> resultType, String fieldName) {
	    CriteriaBuilder builder = entityManager.getCriteriaBuilder();
	    CriteriaQuery<S> query = builder.createQuery(resultType);
	    Root<MyEntity> root = applySpecificationToCriteria(spec, query);
	    query.select(builder.sum(root.get(fieldName).as(resultType)));
	    TypedQuery<S> typedQuery = entityManager.createQuery(query);
	    return typedQuery.getSingleResult();
    }

    protected <S> Root<MyEntity> applySpecificationToCriteria(Specification<MyEntity> spec, CriteriaQuery<S> query) {
	    Root<MyEntity> root = query.from(MyEntity.class);
	    if (spec == null) {
		    return root;
	    }
	    CriteriaBuilder builder = entityManager.getCriteriaBuilder();
	    Predicate predicate = spec.toPredicate(root, query, builder);
	    if (predicate != null) {
		    query.where(predicate);
	    }
	    return root;
    }
}

Main repository should extend both JpaRepository and MyRepositoryCustom:

@Repository
interface MyEntityRepository extends JpaRepository<MyEntity, Long>, MyRepositoryCustom {

}