How to load lazy fetched items from Hibernate/JPA

Question

I have a Person class:

@Entity
public class Person {

    @Id
    @GeneratedValue
    private Long id;

    @ManyToMany(fetch = FetchType.LAZY)
    private List<Role> roles;
    // etc
}

With a many-to-many relation that is lazy.

In my controller I have

@Controller
@RequestMapping("/person")
public class PersonController {
    @Autowired
    PersonRepository personRepository;

    @RequestMapping("/get")
    public @ResponseBody Person getPerson() {
        Person person = personRepository.findOne(1L);
        return person;
    }
}

And the PersonRepository is just this code, written according to this guide

public interface PersonRepository extends JpaRepository<Person, Long> {
}

However, in this controller I actually need the lazy-data. How can I trigger its loading?

Trying to access it will fail with

failed to lazily initialize a collection of role: no.dusken.momus.model.Person.roles, could not initialize proxy - no Session

or other exceptions depending on what I try.

My xml-description, in case needed.

Thanks.

Answer 1

Though this is an old post, please consider using @NamedEntityGraph (Javax Persistence) and @EntityGraph (Spring Data JPA). The combination works.

Example

@Entity
@Table(name = "Employee", schema = "dbo", catalog = "ARCHO")
@NamedEntityGraph(name = "employeeAuthorities",
            attributeNodes = @NamedAttributeNode("employeeGroups"))
public class EmployeeEntity implements Serializable, UserDetails {
// your props
}

and then the spring repo as below

@RepositoryRestResource(collectionResourceRel = "Employee", path = "Employee")
public interface IEmployeeRepository extends PagingAndSortingRepository<EmployeeEntity, String>           {

    @EntityGraph(value = "employeeAuthorities", type = EntityGraphType.LOAD)
    EmployeeEntity getByUsername(String userName);

}

Answer 2

You can do the same like this:

@Override
public FaqQuestions getFaqQuestionById(Long questionId) {
    session = sessionFactory.openSession();
    tx = session.beginTransaction();
    FaqQuestions faqQuestions = null;
    try {
        faqQuestions = (FaqQuestions) session.get(FaqQuestions.class,
                questionId);
        Hibernate.initialize(faqQuestions.getFaqAnswers());

        tx.commit();
        faqQuestions.getFaqAnswers().size();
    } finally {
        session.close();
    }
    return faqQuestions;
}

Just use faqQuestions.getFaqAnswers().size()nin your controller and you will get the size if lazily intialised list, without fetching the list itself.

Answer 3

You have some options

• Write a method on repository that return a initialized entity as R.J suggested.

More work, best performance.

• Use OpenEntityManagerInViewFilter to keep session open for the entire request.

Less work, usually acceptable in web enviroments.

• Use a helper class to initialize entities when required.

Less work, useful when OEMIV is not at option, for example in a Swing application, but may be useful too on repository implementations to initialize any entity in one shot.

For the last option, I wrote a utility class, JpaUtils to initilize entities at some deph.

For example:

@Transactional
public class RepositoryHelper {

    @PersistenceContext
    private EntityManager em;

    public void intialize(Object entity, int depth) {
        JpaUtils.initialize(em, entity, depth);
    }
}

Answer 4

I think you need OpenSessionInViewFilter to keep your session open during view rendering (but it is not too good practice).

Answer 5

it can only be lazily loaded whilst within a transaction. So you could access the collection in your repository, which has a transaction - or what I normally do is a get with association, or set fetchmode to eager.

Answer 6

You will have to make an explicit call on the lazy collection in order to initialize it (common practice is to call .size() for this purpose). In Hibernate there is a dedicated method for this (Hibernate.initialize()), but JPA has no equivalent of that. Of course you will have to make sure that the invocation is done, when the session is still available, so annotate your controller method with @Transactional. An alternative is to create an intermediate Service layer between the Controller and the Repository that could expose methods which initialize lazy collections.

Update:

Please note that the above solution is easy, but results in two distinct queries to the database (one for the user, another one for its roles). If you want to achieve better performace add the following method to your Spring Data JPA repository interface:

public interface PersonRepository extends JpaRepository<Person, Long> {

    @Query("SELECT p FROM Person p JOIN FETCH p.roles WHERE p.id = (:id)")
    public Person findByIdAndFetchRolesEagerly(@Param("id") Long id);

}

This method will use JPQL's fetch join clause to eagerly load the roles association in a single round-trip to the database, and will therefore mitigate the performance penalty incurred by the two distinct queries in the above solution.