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In the previous tutorial, we have implemented many-to-many domain model Unidirectional mapping using Spring Data JPA.

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In this tutorial, we will learn how to implement many-to-many domain model Bidirectional mapping using Spring Data JPA (Hibernate as JPA provider).

We use User and the Role entities to perform Many to Many bidirectional mapping.

The bidirectional relationship mapping allows you to navigate the association in both directions.

In bidirectional mapping, we use @ManyToMany annotation on both entities (User and Role).

Many to Many Bidirectional Mapping - ER Diagram

The many-to-many relationship is implemented using a third table called users_roles which contains foreign keys for users and roles table.

1. Creating Spring Boot Project

Spring Boot provides a web tool called https://start.spring.io to bootstrap an application quickly. Just go to https://start.spring.io and generate a new spring boot project.
Use the below details in the Spring boot creation:
Project Name: spring-data-jpa-course
Project Type: Maven
Choose dependencies: Spring Data JPA, MySQL Driver, Lombok
Package name: net.javaguides.springboot

2. Maven Dependencies

Here is the complete pom.xml for your reference:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
	<modelVersion>4.0.0</modelVersion>
	<parent>
		<groupId>org.springframework.boot</groupId>
		<artifactId>spring-boot-starter-parent</artifactId>
		<version>3.0.4</version>
		<relativePath/> <!-- lookup parent from repository -->
	</parent>
	<groupId>net.javaguides</groupId>
	<artifactId>spring-data-jpa-course</artifactId>
	<version>0.0.1-SNAPSHOT</version>
	<name>spring-data-jpa-course</name>
	<description>Demo project for Spring Boot</description>
	<properties>
		<java.version>17</java.version>
	</properties>
	<dependencies>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-data-jpa</artifactId>
		</dependency>

		<dependency>
			<groupId>com.mysql</groupId>
			<artifactId>mysql-connector-j</artifactId>
			<scope>runtime</scope>
		</dependency>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-test</artifactId>
			<scope>test</scope>
		</dependency>
	</dependencies>

	<build>
		<plugins>
			<plugin>
				<groupId>org.springframework.boot</groupId>
				<artifactId>spring-boot-maven-plugin</artifactId>
			</plugin>
		</plugins>
	</build>

</project>

3. Configure MySQL database

Let's use the MySQL database to store and retrieve the data in this example and we gonna use Hibernate properties to create and drop tables.

Open the application.properties file and add the following configuration to it:

spring.datasource.url=jdbc:mysql://localhost:3306/demo?useSSL=false
spring.datasource.username=root
spring.datasource.password=Mysql@123

spring.jpa.properties.hibernate.dialect = org.hibernate.dialect.MySQLDialect

spring.jpa.hibernate.ddl-auto = create-drop

Make sure that you will create a demo database before running the Spring boot application.

Also, change the MySQL username and password as per your MySQL installation on your machine.

4. Create JPA Entities

Let's create an entity package inside a base package "net.javaguides.springboot".

Within the entity package, create User and Role classes with the following content:

User.java

package com.springdatajpa.springboot.entity;

import lombok.Getter;
import lombok.Setter;

import jakarta.persistence.*;
import java.util.HashSet;
import java.util.Set;

@Getter
@Setter
@Entity
@Table(
        name = "users",
        uniqueConstraints = @UniqueConstraint(
                name = "unique_email",
                columnNames = "email"
        )
)
public class User {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private long id;
    private String firstName;
    private String lastName;
    private String email;
    private String password;

    @ManyToMany(fetch = FetchType.EAGER, cascade = CascadeType.ALL)
    @JoinTable(
            name = "users_roles",
            joinColumns = @JoinColumn(
                    name = "user_id", referencedColumnName = "id"
            ),
            inverseJoinColumns = @JoinColumn(
                    name = "role_id", referencedColumnName = "id"
            )
    )
    private Set<Role> roles = new HashSet<>();
}

Role.java

package com.springdatajpa.springboot.entity;

import lombok.Getter;
import lombok.Setter;

import jakarta.persistence.*;
import java.util.HashSet;
import java.util.Set;

@Getter
@Setter
@Entity
@Table(name = "roles")
public class Role {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private long id;
    private String name;

    @ManyToMany(cascade = {
            CascadeType.PERSIST,
            CascadeType.MERGE
    }, fetch = FetchType.EAGER,
    mappedBy = "roles")
    private Set<User> users = new HashSet<>();
}

5. Create Spring Data JPA Repositories

Next, let's create Spring Data JPA repositories to access the User and Role entities from the database.
The JpaRepository interface defines methods for all the CRUD operations on the entity, and a default implementation of the JpaRepository called SimpleJpaRepository.
Let's create a repository package inside a base package "net.javaguides.springdatarest".

Within the repository package, create UserRepository and RoleRepository interfaces with the following content:

UserRepository.java

package com.springdatajpa.springboot.repository;

import com.springdatajpa.springboot.entity.User;
import org.springframework.data.jpa.repository.JpaRepository;

public interface UserRepository extends JpaRepository<User, Long> {
}

RoleRepository.java

package com.springdatajpa.springboot.repository;

import com.springdatajpa.springboot.entity.Role;
import org.springframework.data.jpa.repository.JpaRepository;

public interface RoleRepository extends JpaRepository<Role, Long> {
}

6. Testing Many to Many Bidirectional Mapping

Let's write the JUnit tests to test many-to-many Bidirectional mapping using Spring Data JPA:

package com.springdatajpa.springboot.repository;

import com.springdatajpa.springboot.entity.Role;
import com.springdatajpa.springboot.entity.User;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;

import java.util.List;

@SpringBootTest
public class ManyToManyBidirectionalTest {

    @Autowired
    private RoleRepository roleRepository;

    @Test
    void saveRole(){
        User user = new User();
        user.setFirstName("ramesh");
        user.setLastName("fadatare");
        user.setEmail("ramesh@gmail.com");
        user.setPassword("secrete");

        User admin = new User();
        admin.setFirstName("admin");
        admin.setLastName("admin");
        admin.setEmail("admin@gmail.com");
        admin.setPassword("admin");

        Role roleAdmin = new Role();
        roleAdmin.setName("ROLE_ADMIN");

        roleAdmin.getUsers().add(user);
        roleAdmin.getUsers().add(admin);

        user.getRoles().add(roleAdmin);
        admin.getRoles().add(roleAdmin);

        roleRepository.save(roleAdmin);
    }

    @Test
    void fetchRole(){
        List<Role> roles = roleRepository.findAll();
        roles.forEach((r) ->{
            System.out.println(r.getName());
            r.getUsers().forEach((u) ->{
                System.out.println(u.getFirstName());
            });
        });
    }
}

Save Role will also save associated users (Cascade. PERSIST):

    @Test
    void saveRole(){
        User user = new User();
        user.setFirstName("ramesh");
        user.setLastName("fadatare");
        user.setEmail("ramesh@gmail.com");
        user.setPassword("secrete");

        User admin = new User();
        admin.setFirstName("admin");
        admin.setLastName("admin");
        admin.setEmail("admin@gmail.com");
        admin.setPassword("admin");

        Role roleAdmin = new Role();
        roleAdmin.setName("ROLE_ADMIN");

        roleAdmin.getUsers().add(user);
        roleAdmin.getUsers().add(admin);

        user.getRoles().add(roleAdmin);
        admin.getRoles().add(roleAdmin);

        roleRepository.save(roleAdmin);
    }

Fetch Role will also fetch its associated user (Fetch type EAGER):

    @Test
    void fetchRole(){
        List<Role> roles = roleRepository.findAll();
        roles.forEach((r) ->{
            System.out.println(r.getName());
            r.getUsers().forEach((u) ->{
                System.out.println(u.getFirstName());
            });
        });
    }

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Spring Data JPA Many to Many Bidirectional Mapping