How does the FetchMode work in Spring Data JPA

Spring-jpa creates the query using the entity manager, and Hibernate will ignore the fetch mode if the query was built by the entity manager.

The following is the work around that I used:

1. Implement a custom repository which inherits from SimpleJpaRepository

2. Override the method getQuery(Specification<T> spec, Sort sort):

   @Override
   protected TypedQuery<T> getQuery(Specification<T> spec, Sort sort) { 
       CriteriaBuilder builder = entityManager.getCriteriaBuilder();
       CriteriaQuery<T> query = builder.createQuery(getDomainClass());
   
       Root<T> root = applySpecificationToCriteria(spec, query);
       query.select(root);
   
       applyFetchMode(root);
   
       if (sort != null) {
           query.orderBy(toOrders(sort, root, builder));
       }
   
       return applyRepositoryMethodMetadata(entityManager.createQuery(query));
   }
   

   In the middle of the method, add applyFetchMode(root); to apply the fetch mode, to make Hibernate create the query with the correct join.

   (Unfortunately we need to copy the whole method and related private methods from the base class because there was no other extension point.)

3. Implement applyFetchMode:

   private void applyFetchMode(Root<T> root) {
       for (Field field : getDomainClass().getDeclaredFields()) {
   
           Fetch fetch = field.getAnnotation(Fetch.class);
   
           if (fetch != null && fetch.value() == FetchMode.JOIN) {
               root.fetch(field.getName(), JoinType.LEFT);
           }
       }
   }
   

I think that Spring Data ignores the FetchMode. I always use the @NamedEntityGraph and @EntityGraph annotations when working with Spring Data

@Entity
@NamedEntityGraph(name = "GroupInfo.detail",
  attributeNodes = @NamedAttributeNode("members"))
public class GroupInfo {

  // default fetch mode is lazy.
  @ManyToMany
  List<GroupMember> members = new ArrayList<GroupMember>();

  …
}

@Repository
public interface GroupRepository extends CrudRepository<GroupInfo, String> {

  @EntityGraph(value = "GroupInfo.detail", type = EntityGraphType.LOAD)
  GroupInfo getByGroupName(String name);

}

Check the documentation here

First of all, @Fetch(FetchMode.JOIN) and @ManyToOne(fetch = FetchType.LAZY) are antagonistic because @Fetch(FetchMode.JOIN) is equivalent to the JPA FetchType.EAGER.

Eager fetching is rarely a good choice, and for predictable behavior, you are better off using the query-time JOIN FETCH directive:

public interface PlaceRepository extends JpaRepository<Place, Long>, PlaceRepositoryCustom {

    @Query(value = "SELECT p FROM Place p LEFT JOIN FETCH p.author LEFT JOIN FETCH p.city c LEFT JOIN FETCH c.state where p.id = :id")
    Place findById(@Param("id") int id);
}

public interface CityRepository extends JpaRepository<City, Long>, CityRepositoryCustom { 
    @Query(value = "SELECT c FROM City c LEFT JOIN FETCH c.state where c.id = :id")   
    City findById(@Param("id") int id);
}