1. Introduction
In scenarios involving bidirectional relationships between entities, serializing such entities can lead to infinite recursion and consequently a StackOverflowError. Jackson provides two annotations, @JsonManagedReference and @JsonBackReference, to handle such situations. In this post, we'll focus on the @JsonManagedReference.
2. Example Steps
1. Create two classes, User and Post, representing a user and their posts respectively. There's a bidirectional relationship: a user can have multiple posts, and each post has a reference to its user.
2. Annotate the 'posts' field in the User class with @JsonManagedReference to indicate it's the forward part of the relationship.
3. Serialize a sample User object with associated Post objects to JSON.
4. Print the serialized JSON string.
3. Java Jackson @JsonManagedReference Example
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.annotation.JsonManagedReference;
import java.util.List;
public class JsonManagedReferenceExample {
public static class User {
public int id;
public String name;
// Annotating the forward part of the relationship
@JsonManagedReference
public List<Post> posts;
public User(int id, String name, List<Post> posts) {
this.id = id;
this.name = name;
this.posts = posts;
}
}
public static class Post {
public int postId;
public String content;
public User user;
public Post(int postId, String content, User user) {
this.postId = postId;
this.content = content;
this.user = user;
}
}
public static void main(String[] args) throws JsonProcessingException {
User user = new User(1, "John", List.of(new Post(101, "Hello World", null)));
user.posts.get(0).user = user;
ObjectMapper mapper = new ObjectMapper();
String result = mapper.writeValueAsString(user);
System.out.println(result);
}
}
Output:
{"id":1,"name":"John","posts":[{"postId":101,"content":"Hello World"}]}
4. Step By Step Explanation
@JsonManagedReference and its counterpart @JsonBackReference help in handling and resolving infinite recursion problems that can arise due to bidirectional relationships in entities.
In our example, a User can have multiple Post entities and each Post references back to its User. Without any special handling, serializing a User object would involve serializing its posts, and each post would then serialize its user, leading to an endless cycle.
By marking the 'posts' field in the User class with @JsonManagedReference, we indicate that this side of the relationship should be serialized, while the back-reference in the Post class (the 'user' field) won't be serialized, thus preventing the infinite loop.
The output JSON string correctly represents the User and their Post without redundant back-references.