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Consider the following two tables - posts and comments of a Blog database schema where the posts table has a one-to-many relationship with the comments table

I'm using Bidirectional One to Many Mapping, because my child entities are limited only.

In Bidirectional One to Many Mapping, I wrote one rest call to get all the post with appropriate comments for the post Using default find all method in JPA Repository.

But the following exception occurred:

"message": "Could not write JSON: failed to lazily initialize a collection of role: com.insights.apartmento.modal.Post.comments, could not initialize proxy - no Session; nested exception is com.fasterxml.jackson.databind.JsonMappingException: failed to lazily initialize a collection of role: com.insights.apartmento.modal.Post.comments, could not initialize proxy - no Session (through reference chain: java.util.ArrayList[0]->com.insights.apartmento.modal.Post["comments"])",

My Pojo class as follows

In POST POJO

@Entity
@Table(name = "posts")
public class Post 
 @Id
 @GeneratedValue(strategy = GenerationType.IDENTITY)
 private Long id;

 @Size(max = 100)
 @Column(unique = true)
 private String title;

 @Size(max = 250)
 private String description;

 @Lob
 private String content;

 @OneToMany(cascade = CascadeType.ALL,
 fetch = FetchType.LAZY,
 mappedBy = "post")
 private Set<Comment> comments = new HashSet<>();

 // Getters and Setters (Omitted for brevity)

In Comments POJO

@Entity
@Table(name = "comments")
public class Comment
 @Id
 @GeneratedValue(strategy = GenerationType.IDENTITY)
 private Long id;

 @NotNull
 @Lob
 private String text;

 @ManyToOne(fetch = FetchType.LAZY)
 @JoinColumn(name = "post_id", nullable = false)
 private Post post;

 // Getters and Setters (Omitted for brevity)

You are trying to serialize directly the Post Object. What happens is that Jackson marshaller tries to build a JSON from the full class, included the private Set<Comment> comments

Since you are using a Lazy FetchType (correctly according to me) Hibernate will throw a LazyInitializationException
You can avoid this in several way:

1. You can ignore the comments Set in the generated JSON by using the @JsonIgnore annotation on this property


2. You can fully load the Post object but using a different FetchType (e.g EAGER or JOIN)


3. You can use the Hibernate.initialize method

Which solution to use? Well this depends on your use case. In any case I strongly suggest you to avoid the solution number 2 (fully load the Post object but using a different FetchType (e.g. EAGER or JOIN) because this means you'll always load Posts with all comments and this is very impactful on performance

Your @OneToMany association gets fetched lazily, which should always be your preferred FetchType. It performs much better than EAGER fetching.

But then you need to make sure that you initialize all required associations before you close the Session or send the entity object to any client. In your case, the JSON marshaling happens after your Session got closed, which caused the exception.

There are multiple ways to initialize a lazily fetched association. The easiest one is a JOIN FETCH clause in a JPQL query, e.g.:

SELECT p FROM Post p LEFT JOIN FETCH p.comments c WHERE p.id = :id

If you're using em.find to load the Post entity, you either need to replace it with a JPQL query or use an EntityGraph.

In one to many mapping, define fetch type as EAGER.

@OneToMany(cascade = CascadeType.ALL,
 fetch = FetchType.EAGER,
 mappedBy = "post")