1. Introduction
Handling bidirectional relationships in Java can be tricky when converting objects to JSON, as they can lead to infinite loops during serialization. Jackson provides a solution with the @JsonIdentityInfo and @JsonIdentityReference annotations. In this guide, we will focus on the @JsonIdentityReference annotation, which allows representing a referenced entity with its ID rather than the full serialization.
2. Example Steps
1. Add the necessary Jackson library dependencies to your project.
2. Create Java classes representing a bidirectional relationship.
3. Annotate one side of the relationship with @JsonIdentityInfo and @JsonIdentityReference.
4. Use Jackson's ObjectMapper class to serialize the Java object.
5. Display the results.
3. Java Jackson @JsonIdentityReference Example
import com.fasterxml.jackson.annotation.JsonIdentityInfo;
import com.fasterxml.jackson.annotation.JsonIdentityReference;
import com.fasterxml.jackson.annotation.ObjectIdGenerators;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;
public class JacksonIdentityReferenceExample {
public static void main(String[] args) {
// Initialize ObjectMapper instance.
ObjectMapper objectMapper = new ObjectMapper();
// Create objects with a bidirectional relationship.
Book book = new Book("A Tale of Two Cities");
Author author = new Author("Charles Dickens");
book.setAuthor(author);
author.setBook(book);
try {
// Serialize the Java object to JSON string.
String jsonString = objectMapper.writeValueAsString(book);
System.out.println("Serialized JSON: " + jsonString);
} catch (JsonProcessingException e) {
e.printStackTrace();
}
}
static class Author {
private String name;
@JsonIdentityReference(alwaysAsId = true)
private Book book;
// Constructor, getters, setters, and other methods...
}
@JsonIdentityInfo(generator = ObjectIdGenerators.PropertyGenerator.class, property = "title")
static class Book {
private String title;
private Author author;
// Constructor, getters, setters, and other methods...
}
}
Output:
Serialized JSON: {"title":"A Tale of Two Cities","author":{"name":"Charles Dickens","book":"A Tale of Two Cities"}}
4. Step By Step Explanation
1. We have two classes, Book and Author, representing a bidirectional relationship.
2. The @JsonIdentityInfo annotation on the Book class instructs Jackson to use the title property of the book as its identifier.
3. In the Author class, the @JsonIdentityReference annotation with the attribute alwaysAsId = true tells Jackson to serialize the book reference using its ID (in this case, the title) rather than the full object.
4. After setting up the bidirectional relationship between a book and its author, we serialize the book object. In the JSON output, the book property in the Author class is represented by its title, as instructed by our annotations.