How to use declare Stream as return type when dealing with JPA Specification and spring-data-jpa

Question

I'm wondering if I can use JPA specification predicates in custom queries?

I've tried but with no success.

Let's say I have an Entity Customer and a repository:

@Repository
public interface CustomerRepository 
    extends JpaRepository<Customer, Long>,
   JpaSpecificationExecutor<Customer> {
}

Querying like this is OK

@Query("select c from Customer c")
Stream<Customer> streamAllCustomers();

This is Not OK

Stream<Customer> streamAllCustomersWithFilter(Specification<Customer> filter);

Is there a way to achieve this ?

NB I know I can put params in the @Query but I would like to stay in the design of the current app and use Specifications all the way.

Answer 1

There is the way to stream data from Spring Data JPA that I use. This approach is useful to process huge amount of data while avoiding high memory consumption, because whole query result is not loaded to memory.

Create custom individual repository with following implementation

public class YourCustomRepositoryImpl implements YourCustomRepository {

    @PersistenceContext(unitName = "yorEntityManagerFactory")
    private EntityManager em;

    @Override
    public Stream<SomeEntity> streamAll(Specification<SomeEntity> spec) {

            CriteriaBuilder cb = em.getCriteriaBuilder();
            CriteriaQuery<SomeEntity> query = cb.createQuery(SomeEntity.class);
            Root<SomeEntity> root = query.from(SomeEntity.class);
            query.where(spec.toPredicate(root, query, cb));

            return em.createQuery(query).getResultStream();
    }

}

Sure, this requires JPA 2.2 supporting ORM framework (I used Hibernate 5.3).

Also, you should care to provide a connection to be alive while stream is being processed.