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In a Spring Boot application, the @OneToMany annotation in JPA is used to define a one-to-many relationship between two entities. Let’s walk through an example where an Author can have multiple Books.

📦 Project Setup

Add the following dependencies to your pom.xml:

xml

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-data-jpa</artifactId>
</dependency>
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
</dependency>
<dependency>
    <groupId>com.h2database</groupId>
    <artifactId>h2</artifactId>
    <scope>runtime</scope>
</dependency>

🧱 Entities: Author and Book

Author Entity

java

import javax.persistence.*;
import java.util.ArrayList;
import java.util.List;

@Entity
public class Author {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String name;

    @OneToMany(mappedBy = "author", cascade = CascadeType.ALL, orphanRemoval = true)
    private List<Book> books = new ArrayList<>();

    // Getters and setters

    public void addBook(Book book) {
        books.add(book);
        book.setAuthor(this);
    }

    public void removeBook(Book book) {
        books.remove(book);
        book.setAuthor(null);
    }

    // standard getters and setters
}

Book Entity

java

import javax.persistence.*;

@Entity
public class Book {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String title;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "author_id")
    private Author author;

    // standard getters and setters
}

🗃️ Repositories

AuthorRepository

java

import org.springframework.data.jpa.repository.JpaRepository;

public interface AuthorRepository extends JpaRepository<Author, Long> {}

BookRepository

java

import org.springframework.data.jpa.repository.JpaRepository;

public interface BookRepository extends JpaRepository<Book, Long> {}

💼 Service Layer (Optional)

java

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

import java.util.List;

@Service
public class AuthorService {

    @Autowired
    private AuthorRepository authorRepository;

    public Author saveAuthorWithBooks(Author author) {
        return authorRepository.save(author);
    }

    public List<Author> getAllAuthors() {
        return authorRepository.findAll();
    }
}

🧑‍💻 Controller

java

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.*;

import java.util.List;

@RestController
@RequestMapping("/api/authors")
public class AuthorController {

    @Autowired
    private AuthorService authorService;

    @PostMapping
    public Author createAuthor(@RequestBody Author author) {
        return authorService.saveAuthorWithBooks(author);
    }

    @GetMapping
    public List<Author> getAuthors() {
        return authorService.getAllAuthors();
    }
}

⚙️ application.properties

properties

spring.datasource.url=jdbc:h2:mem:testdb
spring.datasource.driverClassName=org.h2.Driver
spring.datasource.username=sa
spring.datasource.password=
spring.jpa.database-platform=org.hibernate.dialect.H2Dialect
spring.jpa.hibernate.ddl-auto=update
spring.h2.console.enabled=true

🧪 Sample JSON Request

To test your endpoint (POST /api/authors), try this payload:

json

{
  "name": "George R.R. Martin",
  "books": [
    {
      "title": "A Game of Thrones"
    },
    {
      "title": "A Clash of Kings"
    }
  ]
}

✅ Summary

• @OneToMany is used on the parent entity (Author) referencing the child (Book)
• mappedBy tells JPA which side owns the relationship
• CascadeType.ALL and orphanRemoval = true allow full lifecycle management of children
• Use addBook() and removeBook() methods to keep both sides of the association in sync