Spring Boot Hibernate Many to Many Example

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In this tutorial, we will learn how to implement step by step many-to-many entity mapping using JPA/Hibernate with Spring Boot, Spring Data JPA, and MySQL database.

In many-to-many association, the source entity has a field that stores a collection of target entities. The @ManyToMany JPA annotation is used to link the source entity with the target entity.

A many-to-many association always uses an intermediate join table to store the association that joins two entities. The join table is defined using the @JoinTable JPA annotation.

Video

Overview

Consider the following tables where employees and projects exhibit a many-to-many relationship between each other -

The many-to-many relationship is implemented using a third table called employees_projects which contains the details of the employees and their associated projects. Note that here Employee is a primary entity.

Let’s now create a Spring boot project from scratch and learn how to go about implementing such a many-to-many relationship using JPA and Hibernate.

Tools and Technologies used

• Spring boot 2+
• Hibernate 5+
• JDK 1.8+
• Maven 3+
• IDE - STS or Eclipse
• Spring Data JPA
• MySQL 5+

Development Steps

1. Create a Spring Boot Application
2. Maven pom Dependencies
3. Project Structure
4. Configuring the Database and Hibernate Log levels
5. Defining the Domain Models
6. Defining the Repositories
7. Run and Test the Application

1. Create a Spring Boot Application

There are many ways to create a Spring Boot application. You can refer below articles to create a Spring Boot application.

>> Create Spring Boot Project With Spring Initializer
>> Create Spring Boot Project in Spring Tool Suite [STS]

Refer to project structure or packaging structure from step 3.

2. Maven Dependencies

Let's add required maven dependencies to pom.xml:

<?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 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>net.guides.springboot2</groupId>
    <artifactId>springboot-jpa-many-to-many-example</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <packaging>jar</packaging>
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.0.5.RELEASE</version>
        <relativePath />
        <!-- lookup parent from repository -->
    </parent>
    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
        <java.version>1.8</java.version>
    </properties>
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-data-jpa</artifactId>
        </dependency>
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
        </dependency>
    </dependencies>
    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>
</project>

3. Project Structure

Let's refer below screenshot to create our Project packaging structure - 

4. Configuring the Database and Hibernate Log levels

We’ll need to configure MySQL database URL, username, and password so that Spring can establish a connection with the database on startup.

Open src/main/resources/application.properties and add the following properties to it -

logging.pattern.console=%clr(%d{yy-MM-dd E HH:mm:ss.SSS}){blue} %clr(%-5p) %clr(%logger{0}){blue} %clr(%m){faint}%n

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

spring.jpa.hibernate.ddl-auto=create
spring.jpa.database-platform=org.hibernate.dialect.MySQL57Dialect
spring.jpa.generate-ddl=true
spring.jpa.show-sql=true

logging.level.org.hibernate.SQL=DEBUG
logging.level.org.hibernate.type=TRACE

5. Defining the Domain Models

We’ll abstract out these common fields in a separate class called AuditModel and extend this class in the Employee and Project entities.

AuditModel

package net.guides.springboot.jparepository.model;

import java.io.Serializable;
import java.util.Date;

import javax.persistence.Column;
import javax.persistence.EntityListeners;
import javax.persistence.MappedSuperclass;
import javax.persistence.Temporal;
import javax.persistence.TemporalType;

import org.springframework.data.annotation.CreatedDate;
import org.springframework.data.annotation.LastModifiedDate;
import org.springframework.data.jpa.domain.support.AuditingEntityListener;


@MappedSuperclass
@EntityListeners(AuditingEntityListener.class)

public abstract class AuditModel implements Serializable {

    private static final long serialVersionUID = 1 L;

    @Temporal(TemporalType.TIMESTAMP)
    @Column(name = "created_at", nullable = false, updatable = false)
    @CreatedDate
    private Date createdAt;

    @Temporal(TemporalType.TIMESTAMP)
    @Column(name = "updated_at", nullable = false)
    @LastModifiedDate
    private Date updatedAt;

    public Date getCreatedAt() {
        return createdAt;
    }

    public void setCreatedAt(Date createdAt) {
        this.createdAt = createdAt;
    }

    public Date getUpdatedAt() {
        return updatedAt;
    }

    public void setUpdatedAt(Date updatedAt) {
        this.updatedAt = updatedAt;
    }
}

In the above class, we’re using Spring Boot’s AuditingEntityListener to automatically populate the createdAt and updatedAt fields.

Enabling JPA Auditing

To enable JPA Auditing, we need to add @EnableJpaAuditing annotation to one of our configuration classes. Let's open the main class Application.java and add the @EnableJpaAuditing to the main class like so -

@SpringBootApplication
@EnableJpaAuditing // Enabling JPA Auditing
public class Application implements CommandLineRunner {
   // code goes here
}

Employee

package net.guides.springboot.jparepository.model;

import java.util.HashSet;
import java.util.Set;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.JoinTable;
import javax.persistence.ManyToMany;
import javax.persistence.Table;

@Entity
@Table(name = "employees")
public class Employee extends AuditModel {

    private static final long serialVersionUID = 1 L;

    @Id
    @Column(name = "employee_id")
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long employeeId;

    @Column(name = "first_name")
    private String firstName;

    @Column(name = "last_name")
    private String lastName;

    @ManyToMany(cascade = {
        CascadeType.ALL
    })
    @JoinTable(
        name = "employees_projects",
        joinColumns = {
            @JoinColumn(name = "employee_id")
        },
        inverseJoinColumns = {
            @JoinColumn(name = "project_id")
        }
    )
    Set < Project > projects = new HashSet < Project > ();


    public Employee() {
        super();
    }

    public Employee(String firstName, String lastName) {
        this.firstName = firstName;
        this.lastName = lastName;
    }

    public Employee(String firstName, String lastName, Set < Project > projects) {
        this.firstName = firstName;
        this.lastName = lastName;
        this.projects = projects;
    }


    public Long getEmployeeId() {
        return employeeId;
    }

    public void setEmployeeId(Long employeeId) {
        this.employeeId = employeeId;
    }

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    public Set < Project > getProjects() {
        return projects;
    }

    public void setProjects(Set < Project > projects) {
        this.projects = projects;
    }
}

Project 

package net.guides.springboot.jparepository.model;

import java.util.HashSet;
import java.util.Set;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.ManyToMany;
import javax.persistence.Table;

@Entity
@Table(name = "projects")
public class Project extends AuditModel {

 private static final long serialVersionUID = 1L;

 @Id
    @Column(name = "project_id")
    @GeneratedValue
    private Long projectId;

    @Column(name = "title")
    private String title;

    @ManyToMany(mappedBy = "projects", cascade = { CascadeType.ALL })
    private Set<Employee> employees = new HashSet<Employee>();
    
    public Project() {
        super();
    }

    public Project(String title) {
        this.title = title;
    }

    public Long getProjectId() {
        return projectId;
    }

    public void setProjectId(Long projectId) {
        this.projectId = projectId;
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public Set<Employee> getEmployees() {
        return employees;
    }

    public void setEmployees(Set<Employee> employees) {
        this.employees = employees;
    }
}

We use @ManyToMany annotation to create a many-to-many relationship between two entities. In a bi-directional association, the @ManyToMany annotation is used on both the entities but only one entity can be the owner of the relationship.

The entity that specifies the @JoinTable is the owning side of the relationship and the entity that specifies the mappedBy attribute is the inverse side.

6. Defining the Repositories

Let’s define the repositories for accessing the Employee and Project data from the database.

EmployeeRepository

package net.guides.springboot.jparepository.repository;

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

import net.guides.springboot.jparepository.model.Employee;

@Repository
public interface EmployeeRepository extends JpaRepository<Employee, Long>{

}

ProjectRepository

package net.guides.springboot.jparepository.repository;

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

import net.guides.springboot.jparepository.model.Project;

@Repository
public interface ProjectRepository extends JpaRepository<Project, Long>{

}

7. Run and Test Many-to-Many relationship

Time to write some code to test our many-to-many association setup. Open the main class Application.java and replace it with the following code. Let's run this Application.main() method:

package net.guides.springboot.jparepository;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.data.jpa.repository.config.EnableJpaAuditing;

import net.guides.springboot.jparepository.model.Employee;
import net.guides.springboot.jparepository.model.Project;
import net.guides.springboot.jparepository.repository.EmployeeRepository;

@SpringBootApplication
@EnableJpaAuditing // Enabling JPA Auditing
public class Application implements CommandLineRunner {

    @Autowired
    private EmployeeRepository employeeRepository;

    @Override
    public void run(String...args) throws Exception {

        // Create an employee
        Employee employee = new Employee();
        employee.setFirstName("Ramesh");
        employee.setLastName("Fadatare");

        // Create project1
        Project project = new Project();
        project.setTitle("Employee Management System");

        // Create project2
        Project project1 = new Project();
        project1.setTitle("Content Management System");

        // employee can work on two projects,  Add project references in the employee
        employee.getProjects().add(project);
        employee.getProjects().add(project1);

        // Add employee reference in the projects
        project.getEmployees().add(employee);
        project1.getEmployees().add(employee);

        employeeRepository.save(employee);
    }

    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }
}

Output

Conclusion

In this tutorial, we successfully built a spring boot project from scratch and learned how to map a many-to-many database relationship using hibernate.

You might also be interested in checking out the following articles on JPA and Hibernate -

• JPA/Hibernate One to One Mapping Annotation Example

• JPA/Hibernate One to One Bidirectional Mapping Annotation Example

• JPA Hibernate One to Many Unidirectional Mapping Example

• JPA/Hibernate One to Many Bidirectional Mapping Example

Get the source code of this tutorial on my GitHub Repository.

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