Spring Data JPA. How to get only a list of IDs from findAll() method

I have a very complicated model. Entity has a lot relationship and so on.

I try to use Spring Data JPA and I prepared a repository.

but when I invoke a method findAll() with specification for the object a have a performance issue because objects are very big. I know that because when I invoke a method like this:

@Query(value = "select id, name from Customer ")
List<Object[]> myFindCustomerIds();

I didn't have any problems with performance.

But when I invoke

List<Customer> findAll(); 

I had a big problem with performance.

The problem is that I need to invoke findAll method with Specifications for Customer that is why I cannot use method which returns a list of arrays of objects.

How do I write a method to find all customers with specifications for the Customer entity but which returns only IDs.

like this:

List<Long> findAll(Specification<Customer> spec);

• I cannot use in this case pagination.

Please help.

I solved the problem.

(As a result we will have a sparse Customer object only with id and name)

Define their own repository:

public interface SparseCustomerRepository {
    List<Customer> findAllWithNameOnly(Specification<Customer> spec);
}

And an implementation (remember about suffix - Impl as default)

@Service
public class SparseCustomerRepositoryImpl implements SparseCustomerRepository {
    private final EntityManager entityManager;

    @Autowired
    public SparseCustomerRepositoryImpl(EntityManager entityManager) {
        this.entityManager = entityManager;
    }

    @Override
    public List<Customer> findAllWithNameOnly(Specification<Customer> spec) {
        CriteriaBuilder criteriaBuilder = entityManager.getCriteriaBuilder();
        CriteriaQuery<Tuple> tupleQuery = criteriaBuilder.createTupleQuery();
        Root<Customer> root = tupleQuery.from(Customer.class);
        tupleQuery.multiselect(getSelection(root, Customer_.id),
                getSelection(root, Customer_.name));
        if (spec != null) {
            tupleQuery.where(spec.toPredicate(root, tupleQuery, criteriaBuilder));
        }

        List<Tuple> CustomerNames = entityManager.createQuery(tupleQuery).getResultList();
        return createEntitiesFromTuples(CustomerNames);
    }

    private Selection<?> getSelection(Root<Customer> root,
            SingularAttribute<Customer, ?> attribute) {
        return root.get(attribute).alias(attribute.getName());
    }

    private List<Customer> createEntitiesFromTuples(List<Tuple> CustomerNames) {
        List<Customer> customers = new ArrayList<>();
        for (Tuple customer : CustomerNames) {
            Customer c = new Customer();
            c.setId(customer.get(Customer_.id.getName(), Long.class));
            c.setName(customer.get(Customer_.name.getName(), String.class));
            c.add(customer);
        }
        return customers;
    }
}