Vaadin Spring 和 Hibernate：无法初始化代理 - 否 Session

Question

The Model:

    @Entity
    ...
    public class ScolProgressionPopulation {    
       ...        
        @OneToMany(mappedBy = "population")
        private List<ScolProgressionSemestre> progressions;        
       ...
    }

The repo:

    public interface ScolProgressionPopulationRepository extends JpaRepository<ScolProgressionPopulation, Long>

The View:

    @Route(value = "progression", layout = ApplicationLayout.class)
    @RouteAlias(value = "", layout = ApplicationLayout.class)
    public class ProgressionView extends Div {
           public ProgressionView(@Autowired ScolProgressionPopulationRepository scolProgressionPopulationRepository) {
               var items = scolProgressionPopulationRepository.findAll();

               // I try this..
               Hibernate.initialize(items);
               // and this ...
               Hibernate.initialize(items.get(0));

               // this works:
               System.out.println("this is ok:" +items.size());

               // but not this:
               System.out.println("err, could not initialize proxy - no Session :" +items.get(0).getProgressions().size());

I try many thing to make lazy loading work, (adding @Transactional, Hibernate.initialize...)

But I always end up with this error:

org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'nc.unc.scolarite.lmd.progression.gui.ProgressionView': Bean instantiation via constructor failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [nc.unc.scolarite.lmd.progression.gui.ProgressionView]: Constructor threw exception; nested exception is org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: nc.unc.scolarite.lmd.progression.entities.ScolProgressionPopulation.progressions, could not initialize proxy - no Session

PS: I don't want to use EAGER fetch. I want to lazy load the data.

Answer 1

You could create a new method in your repository that uses join fetch to fetch the related entities, something like

@Query("SELECT spp FROM ScolProgressionPopulation spp JOIN FETCH spp.progressions")
public List<ScolProgressionPopulation> findAllWithProgressions();

Another way is to add the @Transactional annotation to a method in your repository (then it can't be an interface), it might also work on a method in your view. You probably need to call initialize for all progression collections, which is not very efficient as it may create many DB queries.

@Transactional
public List<ScrolProgressionPopulation> findAllWithProgressions() {
    val spps = findAll();
    for(ScrolProgressionPopulation spp: spps) {
        Hibernate.initialize(spp.getProgressions();
    }
    return spps;
}

Edit: If you don't want to initialize all items, you can do something like this

@Transactional
public void initializeProgressions(ScrolProgressionPopulation spp) {
    Hibernate.initialize(spp.getProgressions());
}

and call this on demand.