lazyinitializationexception with Hibernate

Question

I use lazy initialization of entities from the database in the project. As a JPA implementation, I use Hibernate. But when I need to get a collection of entities on a custom JSP, I get a lazyinitializationexception, because Hibernate session is already closed. Please tell me whether it is possible to prevent this exception without resorting to using a special request, as described in https://thorben-janssen.com/hibernate-tips-initialize-lazy-relationships-within-query/ . Here is some of my code:

@Entity
public class Statistic extends EntityAbstract {

    private static final long serialVersionUID = -6372957177860305322L;
    @Temporal(TemporalType.TIMESTAMP)
    private Date date;
    private boolean correct;

    @ManyToOne(fetch = FetchType.LAZY, cascade = {MERGE, PERSIST, REFRESH, DETACH})
    @JoinColumn(name = "questionId", nullable = false)
    private Question question;

    @ManyToOne(fetch = FetchType.LAZY, cascade = {MERGE, PERSIST, REFRESH, DETACH})
    @JoinColumn(name = "userId", nullable = false)
    private User user;

    public Statistic() {
    }

    public Statistic(Date date, boolean correct, Question question, User user) {
        this.date = date;
        this.correct = correct;
        this.question = question;
        this.user = user;
    }

    getter... setter...
}

@Entity
public class Question extends EntityAbstract {

    private static final long serialVersionUID = -2751224412459986012L;
    private String description;

    @ManyToOne(fetch = FetchType.LAZY, cascade = {MERGE, PERSIST, REFRESH, DETACH})
    @JoinColumn(name = "testId", nullable = false)
    private Test test;

    @OneToMany(fetch = FetchType.LAZY, mappedBy = "question")
    @OnDelete(action = OnDeleteAction.CASCADE)
    private Set<Answer> answers;

    @OneToMany(fetch = FetchType.LAZY, mappedBy = "question")
    @OnDelete(action = OnDeleteAction.CASCADE)
    private Set<Literature> literature;

    @OneToMany(fetch = FetchType.LAZY, mappedBy = "question")
    @OnDelete(action = OnDeleteAction.CASCADE)
    private Set<Statistic> statistics;

    public Question() {
    }

    public Question(String description, Test test) {
        this.description = description;
        this.test = test;
    }

    getter... setter...
}

@Entity
public class Literature extends EntityAbstract {

    private static final long serialVersionUID = 218407080623072886L;
    private String description;

    @ManyToOne(fetch = FetchType.LAZY, cascade = {MERGE, PERSIST, REFRESH, DETACH})
    @JoinColumn(name = "questionId", nullable = false)
    private Question question;

    @OneToMany(fetch = FetchType.LAZY, mappedBy = "literature")
    @OnDelete(action = OnDeleteAction.CASCADE)
    private Set<Link> links;

    public Literature() {
    }

    public Literature(String description, Question question) {
        this.description = description;
        this.question = question;
    }

    getter... setter...
}

@Entity
public class Link extends EntityAbstract {

    private static final long serialVersionUID = 6494218299043499655L;
    private String link;

    @ManyToOne(fetch = FetchType.LAZY, cascade = {MERGE, PERSIST, REFRESH, DETACH})
    @JoinColumn(name = "literatureId", nullable = false)
    private Literature literature;

    public Link() {
    }

    public Link(String link, Literature literature) {
        this.link = link;
        this.literature = literature;
    }

    getter... setter...
}

And I need to get a collection of questions from statistics and further along the chain. I use this service:

@Repository
@Transactional
public interface CrudRepository<T extends EntityAbstract> {

    SessionFactory getBeanToBeAutowired();

    // create
    @SuppressWarnings("unchecked")
    default T add(T entity){
        return (T) getBeanToBeAutowired().getCurrentSession().merge(entity);
    }

    // read
    @SuppressWarnings("unchecked")
    default T getById(Class<T> entityClass, long id){
        return (T)getBeanToBeAutowired().getCurrentSession().find(entityClass, id);
    }

    // update
    @SuppressWarnings("unchecked")
    default T update(T entity){
        return (T) getBeanToBeAutowired().getCurrentSession().merge(entity);
    }

    // delete
    default void delete(T entity){
        getBeanToBeAutowired().getCurrentSession().remove(entity);
    }

    @SuppressWarnings("unchecked")
    default List<T> getAll(Class<T> t){
        return (List<T>)getBeanToBeAutowired()
                .getCurrentSession()
                .createQuery("FROM " + t.getSimpleName())
                .list();
    }
}

public interface StatisticRepository extends CrudRepository<Statistic> {
}

public interface StatisticService extends StatisticRepository {

    Statistic getById(long id);
    List<Statistic> getAll();
}

@Service("statisticService")
public class StatisticServiceImpl implements StatisticService {

    private SessionFactory sessionFactory;

    @Autowired
    public void setSessionFactory(SessionFactory sessionFactory) {
        this.sessionFactory = sessionFactory;
    }

    @Override
    public SessionFactory getBeanToBeAutowired() {
        return sessionFactory;
    }

    @Override
    public Statistic getById(long id) {
        return getById(Statistic.class, id);
    }

    @Override
    public List<Statistic> getAll() {
        return getAll(Statistic.class);
    }
}

Answer 1

I solved my question as follows (using this article herehttps://thorben-janssen.com/hibernate-tip-entitygraph-multiple- ):

@NamedEntityGraph(
        name = "graph.statistic",
        attributeNodes = @NamedAttributeNode(value = "question", subgraph = "questionGraph"),
        subgraphs = {
                @NamedSubgraph(name = "questionGraph",
                        attributeNodes = @NamedAttributeNode(value = "literature", subgraph = "literatureGraph")),
                @NamedSubgraph(name = "literatureGraph",
                        attributeNodes = @NamedAttributeNode(value = "links"))}
)
@Entity
public class Statistic extends EntityAbstract {

    private static final long serialVersionUID = -6372957177860305322L;
    @Temporal(TemporalType.TIMESTAMP)
    private Date date;
    private boolean correct;

    @ManyToOne(fetch = FetchType.LAZY, cascade = {MERGE, PERSIST, REFRESH, DETACH})
    @JoinColumn(name = "questionId", nullable = false)
    private Question question;

    @ManyToOne(fetch = FetchType.LAZY, cascade = {MERGE, PERSIST, REFRESH, DETACH})
    @JoinColumn(name = "userId", nullable = false)
    private User user;

    public Statistic() {
    }

    public Statistic(Date date, boolean correct, Question question, User user) {
        this.date = date;
        this.correct = correct;
        this.question = question;
        this.user = user;
    }

    getters... and setters..... 
}

@Service("statisticService")
public class StatisticServiceImpl implements StatisticService {

 ...

    @Override
    @Transactional
    public List<Statistic> getUserStatisticByUserIdAndDate(long id, Date startDate, Date endDate) {
        Session session = sessionFactory.openSession();
        session.beginTransaction();
        RootGraph<?> graph = session.getEntityGraph("graph.statistic");
        List<Statistic> statistics = session
                .createQuery("FROM Statistic WHERE user.id = :id AND date between :startDate AND :endDate", Statistic.class)
                .setParameter("id", id)
                .setParameter("startDate", startDate)
                .setParameter("endDate", endDate)
                .setHint("javax.persistence.fetchgraph", graph)
                .getResultList();
        session.getTransaction().commit();
        session.close();
        return statistics;
}

Answer 2

There are a few problems with entity graphs. IMO the most pressing issue is, that you use the same java model that exposes all state although not everything is available. You always have to keep track about where an object came from which is a pain as well.

I can recommend you take a look atBlaze-Persistence Entity-Views which is a more appropriate solution: https://blazebit.com/blog/2016/getting-started-with-blaze-persistence-entity-views.html