使用带有fetch = FetchType.LAZY的休眠状态从表中获取数据

Question

I'm trying to get all user's emails from table. Entity user:

     @Entity
     @Table(name = "tbl_User")
     public class User {
          @Expose
          @Id
          @GeneratedValue
          @Column(name = "id")
          private Long id;
          .....
          @OneToMany(mappedBy = "user", fetch = FetchType.LAZY)
          List<CommunicationAddress> communicationAddresses = new ArrayList<CommunicationAddress>();
          .....
     } 

In the service I'm getting user and trying to look emails:

User user = userDAO.getUserById(id);
        if (user == null) {
            throw new Exception("User not found");
        } else {
            List<Email> addresses = user.getCommunicationAddresses();
        }

But I received the next exception:

 org.hibernate.LazyInitializationException: could not initialize proxy - no Session
        at org.hibernate.collection.internal.AbstractPersistentCollection.withTemporarySessionIfNeeded(AbstractPersistentCollection.java:186)
        at org.hibernate.collection.internal.AbstractPersistentCollection.readSize(AbstractPersistentCollection.java:137)
        at org.hibernate.collection.internal.PersistentBag.isEmpty(PersistentBag.java:249)

The method for getting user:

@Transactional
@Override
public User getUserById(Long userId) {
    Criteria criteria = sessionFactory.getCurrentSession().createCriteria(User.class);
    criteria.add(Restrictions.eq("id", userId));
    return (User) criteria.uniqueResult();
}

I understand that I must to get communicationAddresses when I get User using Criteria... How to do it? Thank's all.

Answer 1

It seems your service method is not annotated with @Transactional . Thus, after calling userDAO.getUserById(id); , there is no longer a transaction. That means that you cannot access any lazy-loaded properties of the loaded entity that hasn't been accessed/pre-fetched inside the transaction without running into a LazyInitializationException.
So you can either think about replacing LAZY with EAGER fetching (this mostly depends on the use cases you are facing) or you should annotate your Service method with @Transactional .

I'd highly suggest to annotate your service methods (instead of the DAO methods), as only there you can establish meaningful transactional boundaries when interacting with multiple entities.
Also, if you make usage of lazy loading, you must be aware of the possibility to run into that kind of exception after leaving the service layer, eg when rendering your view (assuming you somehow present the data).

"Prefetching' lazy associations

To trigger instant loading of lazy associations (called " dynamic association fetching "), add this line in getUserById :

criteria.setFetchMode("communicationAddresses", FetchMoode.EAGER);

However, if you do that in this specific method, I wonder why you stick to lazy loading at all?