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.
ベストアンサー1
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句を使用すると、データベースへの 1 回のラウンドトリップでロールの関連付けを積極的にロードできるため、上記のソリューションの 2 つの異なるクエリによって発生するパフォーマンスの低下が軽減されます。