LazyInitializationException 即使使用了 @Transactional

Question

Currently I am getting the following exception:

org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: com.example.model.Link.subLinks, could not initialize proxy - no Session

I googled and I found another solution for this exception but I would like to know why @Transactional in my case is not working. I am sure that I am doing something wrong. What am I doing wrong?

The weird part is that I used @Transactional already somewhere else in this project and there it works.

Thank you in advance for any tips.

My Link class: The entity Link contains a collection of SubLink.

@Data
@Builder
@AllArgsConstructor
@NoArgsConstructor
@Entity
public class Link {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long id;
    
    @NotEmpty
    private String title;
    
    @NotEmpty
    private String description;
    
    @OneToMany(mappedBy = "link")
    private Collection<SubLink> subLinks;
}

My SubLink class:

@Data
@NoArgsConstructor
@AllArgsConstructor
@Builder
@Entity
public class SubLink {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long id;
    
    @NotEmpty
    private String type;
    
    @NotEmpty
    @URL
    private String url;
    
    @ManyToOne
    @JoinColumn(name = "link_id", referencedColumnName = "id")
    private Link link;
}

My LinkServiceImpl class:

    @Service
    public class LinkServiceImpl implements LinkService {
        @Autowired
        public LinkRepository linkRepository;
        
        @Override
        @Transactional
        public Link findById(long id) {
            return linkRepository.findById(id);
        }
    }

In my controller class there is the method showLink():

    @GetMapping("/link/{linkId}")
    public String showLink(@PathVariable(value = "linkId") long linkId, Model model) {
        Link link = linkService.findById(linkId);

        
        Collection<SubLink> subLinkCollection = link.getSubLinks(); //Error
        model.addAttribute("subLinkCollection", subLinkCollection);
        return "link";
    }

Answer 1

LazyInitializationException indicates access to unfetched data outside of a session context. To fix this, you have to fetch all required associations within your service layer.

I think in your case you are fetching Link without its SubLink , but trying to access sublinks. So, using JOIN FETCH would be the best strategy.

Note: You can use FetchType.EAGER too, but it might affect performance. Because, it will always fetch fetch the association, even if you don't use them.

= UPDATE =

Thanks to crizzi for mentioning an interesting feature called Open Session In View(OSIV) (enabled by default in spring-boot applications). Though it depends on who you may ask to label it as a pattern or an antipattern .

The main purpose is to to facilitate working with lazy associations(avoids LazyInitializationException ). Very detailed explanation can be found on

• https://www.baeldung.com/spring-open-session-in-view and
• https://vladmihalcea.com/the-open-session-in-view-anti-pattern/

Answer 2

Aman explained well the cause of the LazyInitializationException in your case and crizzis added more information about why @Transactional is not worked as you expected and how it should be used.

So, to summarize the options you have to solve your problem:

---

Fetch Eager

Commented by Aman and crizzis , this is the easier but not the best from the point of view of the performance.

To do it, include fetch = FetchType.EAGER in subLinks property definition:

@OneToMany(mappedBy = "link", fetch = FetchType.EAGER)
private Collection<SubLink> subLinks;

However, every time you get from database instances of Link => its collection of related SubLink will be included.

---

Custom Query

Commented by Aman too (although is not necessary on (link.id = sublink.link.id) ). In this case, you can create a new method in your repository to get the SubLink collection of an specific Link .

In this way, you will be able to deal with situations on which you want to receive such "extra information" ( findWithSubLinkById ) or not (provided findById ).

With a bidirectional OneToMany , it is enough with something like:

@Query("select l from Link l left join fetch l.subLinks where l.id = :id")
Link findWithSubLinkById(Long id);

With a unidirectional OneToMany , besides the above query you have to include how "join" both tables:

@OneToMany(cascade = CascadeType.ALL, orphanRemoval = true)
@JoinColumn(name = "link_id")
private Collection<SubLink> subLinks;

Removing link property from SubLink class.

More information in Hibernate associations and unidirectional one-to-many tips

---

EntityGraph

This is a variation of the previous one. So, instead of create the new method using @Query , an easier choice will be configured with @EntityGraph :

@EntityGraph(attributePaths = "subLinks")
Link findWithSubLinkById(Long id);

More information here ( Example 77 )

---

Work inside Transactional method

As crizzis commented, inside the method configured with @Transactional you won't receive such exception. So, another option is move the code that uses Sublink from your controller to your service.