JPA/Hibernate One to Many Bidirectional Mapping Example

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In this tutorial, we will learn how to implement step-by-step one-to-many bidirectional entity mapping using JPA and Hibernate, and the MySQL database.

The @OneToMany and @ManyToOne JPA annotations are used to link one-to-many bidirectional entity mapping.

There are two types of one-to-many associations -
  • Unidirectional → In this type of association, only the source entity has a relationship field that refers to the target entity. We can navigate this type of association from one side.
  • Bidirectional → In this type of association, each entity (i.e. source and target) has a relationship field that refers to each other. We can navigate this type of association from both sides.
In this example, we implement step-by-step one-to-many bidirectional entity mapping using JPA and Hibernate, and the MySQL database.

Overview

Simply put, one-to-many mapping means that one row in a table is mapped to multiple rows in another table.

The below ER diagram shows one Instructor can have many Courses and many Courses belong to one Instructor only.
We will implement CRUD (create/save, read, update, delete) operations for Instructor and Course entities. You can download the source code of this example at my GitHub repository (link given at end of this article).

For Instructor DAO CRUD operations:
void saveInstructor(Instructor instructor);

void updateInstructor(Instructor instructor);

void deleteInstructor(int id);

Instructor getInstructor(int id);
For Course DAO CRUD operations:
void saveCourse(Course course);

void updateCourse(Course course);

void deleteCourse(int id);

Course getCourse(int id);

Development Steps

  1. Create a Simple Maven Project
  2. Project Directory Structure
  3. Add jar Dependencies to pom.xml
  4. Creating the JPA Entities(Persistent classes)
  5. Create Hibernate DAO Classes
  6. Create a Hibernate configuration file - HibernateUtil.java (Java Configuration)
  7. Create the Main class and Run an Application

1. Create a Simple Maven Project

Use the How to Create a Simple Maven Project in Eclipse article to create a simple Maven project in Eclipse IDE.

2. Project Directory Structure

Let's create a packaging structure for the above-created simple maven project:

2. POM Dependencies

Let's add Hibernate and MySQL dependencies to pom.xml:
<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>
    <parent>
        <groupId>net.javaguides.hibernate</groupId>
        <artifactId>hibernate-tutorial</artifactId>
        <version>0.0.1-SNAPSHOT</version>
    </parent>
    <artifactId>hibernate-one-to-many-bi-example</artifactId>
    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    </properties>
    <dependencies>
        <!-- https://mvnrepository.com/artifact/mysql/mysql-connector-java -->
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>8.0.32</version>
        </dependency>
        <!-- https://mvnrepository.com/artifact/org.hibernate/hibernate-core -->
        <dependency>
            <groupId>org.hibernate</groupId>
            <artifactId>hibernate-core</artifactId>
            <version>6.1.7.Final</version>
        </dependency>
    </dependencies>
    <build>
        <sourceDirectory>src/main/java</sourceDirectory>
        <plugins>
            <plugin>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.5.1</version>
                <configuration>
                    <source>17</source>
                    <target>17</target>
                </configuration>
            </plugin>
        </plugins>
    </build>
</project>

3. Creating the JPA Entities(Persistent classes)

Let's create JPA entities that we map with database tables.

The @OneToMany and @ManyToOne JPA annotations are used to link one-to-many bidirectional entity mapping.

Instructor JPA Entity

package net.javaguides.hibernate.entity;

import java.util.List;

import jakarta.persistence.*;

@Entity
@Table(name = "instructor")
public class Instructor {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id")
    private int id;

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

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

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

    @OneToMany(mappedBy = "instructor", cascade = { CascadeType.PERSIST, CascadeType.MERGE, CascadeType.DETACH,
      CascadeType.REFRESH })
    private List<Course> courses;

    public Instructor() {

    }

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

    public int getId() {
         return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    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 String getEmail() {
         return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public List<Course> getCourses() {
        return courses;
    }

    public void setCourses(List<Course> courses) {
        this.courses = courses;
    }
}

Course JPA Entity

package net.javaguides.hibernate.entity;

import jakarta.persistence.*;

@Entity
@Table(name = "course")
public class Course {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id")
    private int id;

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

    @ManyToOne(cascade = CascadeType.ALL)
    @JoinColumn(name = "instructor_id")
    private Instructor instructor;

    public Course() {

    }

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

    public int getId() {
         return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getTitle() {
        return title;
    }

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

    public Instructor getInstructor() {
        return instructor;
    }

    public void setInstructor(Instructor instructor) {
        this.instructor = instructor;
    }

    @Override
    public String toString() {
        return "Course [id=" + id + ", title=" + title + "]";
    }
}
  • @Table maps the entity with the table. If no @Table is defined, the default value is used: the class name of the entity.
  • @Id declares the identifier property of the entity.
  • @Column maps the entity's field with the table's column. If @Column is omitted, the default value is used: the field name of the entity.
  • @OneToMany and @ManyToOne define a one-to-many and many-to-one relationship between 2 entities. 
  • @JoinColumn indicates the entity is the owner of the relationship: the corresponding table has a column with a foreign key to the referenced table. mappedBy indicates the entity is the inverse of the relationship.

4. Create Hibernate DAO Classes

InstructorDao.java

The InstructorDao class provides the implementation of DAO CRUD methods for the Instructor entity:
import org.hibernate.Session;
import org.hibernate.Transaction;

import net.javaguides.hibernate.entity.Instructor;
import net.javaguides.hibernate.util.HibernateUtil;

public class InstructorDao {
	public void saveInstructor(Instructor instructor) {
		Transaction transaction = null;
		try (Session session = HibernateUtil.getSessionFactory().openSession()) {
			// start a transaction
			transaction = session.beginTransaction();
			// save the student object
			session.save(instructor);
			// commit transaction
			transaction.commit();
		} catch (Exception e) {
			if (transaction != null) {
				transaction.rollback();
			}
			e.printStackTrace();
		}
	}

	public void updateInstructor(Instructor instructor) {
		Transaction transaction = null;
		try (Session session = HibernateUtil.getSessionFactory().openSession()) {
			// start a transaction
			transaction = session.beginTransaction();
			// save the student object
			session.update(instructor);
			// commit transaction
			transaction.commit();
		} catch (Exception e) {
			if (transaction != null) {
				transaction.rollback();
			}
			e.printStackTrace();
		}
	}

	public void deleteInstructor(int id) {

		Transaction transaction = null;
		try (Session session = HibernateUtil.getSessionFactory().openSession()) {
			// start a transaction
			transaction = session.beginTransaction();

			// Delete a instructor object
			Instructor instructor = session.get(Instructor.class, id);
			if (instructor != null) {
				session.delete(instructor);
				System.out.println("instructor is deleted");
			}

			// commit transaction
			transaction.commit();
		} catch (Exception e) {
			if (transaction != null) {
				transaction.rollback();
			}
			e.printStackTrace();
		}
	}

	public Instructor getInstructor(int id) {

		Transaction transaction = null;
		Instructor instructor = null;
		try (Session session = HibernateUtil.getSessionFactory().openSession()) {
			// start a transaction
			transaction = session.beginTransaction();
			// get an instructor object
			instructor = session.get(Instructor.class, id);
			// commit transaction
			transaction.commit();
		} catch (Exception e) {
			if (transaction != null) {
				transaction.rollback();
			}
			e.printStackTrace();
		}
		return instructor;
	}
}

CourseDao.java

The CourseDao class provides the following implementation DAO CRUD methods for the Course entity:
import org.hibernate.Session;
import org.hibernate.Transaction;

import net.javaguides.hibernate.entity.Course;
import net.javaguides.hibernate.util.HibernateUtil;

public class CourseDao {
	public void saveCourse(Course course) {
		Transaction transaction = null;
		try (Session session = HibernateUtil.getSessionFactory().openSession()) {
			// start a transaction
			transaction = session.beginTransaction();
			// save the student object
			session.save(course);
			// commit transaction
			transaction.commit();
		} catch (Exception e) {
			if (transaction != null) {
				transaction.rollback();
			}
			e.printStackTrace();
		}
	}

	public void updateCourse(Course course) {
		Transaction transaction = null;
		try (Session session = HibernateUtil.getSessionFactory().openSession()) {
			// start a transaction
			transaction = session.beginTransaction();
			// save the student object
			session.update(course);
			// commit transaction
			transaction.commit();
		} catch (Exception e) {
			if (transaction != null) {
				transaction.rollback();
			}
			e.printStackTrace();
		}
	}

	public void deleteCourse(int id) {

		Transaction transaction = null;
		try (Session session = HibernateUtil.getSessionFactory().openSession()) {
			// start a transaction
			transaction = session.beginTransaction();

			// Delete a course object
			Course course = session.get(Course.class, id);
			if (course != null) {
				session.delete(course);
				System.out.println("course is deleted");
			}

			// commit transaction
			transaction.commit();
		} catch (Exception e) {
			if (transaction != null) {
				transaction.rollback();
			}
			e.printStackTrace();
		}
	}

	public Course getCourse(int id) {

		Transaction transaction = null;
		Course course = null;
		try (Session session = HibernateUtil.getSessionFactory().openSession()) {
			// start a transaction
			transaction = session.beginTransaction();
			// get an course object
			course = session.get(Course.class, id);
			// commit transaction
			transaction.commit();
		} catch (Exception e) {
			if (transaction != null) {
				transaction.rollback();
			}
			e.printStackTrace();
		}
		return course;
	}
}

5. Create a Hibernate configuration file - HibernateUtil.java (Java Configuration)

The HibernateUtil Java configuration file contains information about the database and mapping file. 
Let's create a HibernateUtil file and write the following code in it.
package net.javaguides.hibernate.util;

import java.util.Properties;

import org.hibernate.SessionFactory;
import org.hibernate.boot.registry.StandardServiceRegistryBuilder;
import org.hibernate.cfg.Configuration;
import org.hibernate.cfg.Environment;
import org.hibernate.service.ServiceRegistry;

import net.javaguides.hibernate.entity.Course;
import net.javaguides.hibernate.entity.Instructor;

/**
 * Java based configuration
 * @author ramesh Fadatare
 *
 */
public class HibernateUtil {
 private static SessionFactory sessionFactory;

 public static SessionFactory getSessionFactory() {
  if (sessionFactory == null) {
   try {
    Configuration configuration = new Configuration();

    // Hibernate settings equivalent to hibernate.cfg.xml's properties
    Properties settings = new Properties();
    settings.put(Environment.DRIVER, "com.mysql.cj.jdbc.Driver");
    settings.put(Environment.URL, "jdbc:mysql://localhost:3306/hibernate_db?useSSL=false");
    settings.put(Environment.USER, "root");
    settings.put(Environment.PASS, "root");
    settings.put(Environment.DIALECT, "org.hibernate.dialect.MySQL5InnoDBDialect");

    settings.put(Environment.SHOW_SQL, "true");

    settings.put(Environment.CURRENT_SESSION_CONTEXT_CLASS, "thread");

    settings.put(Environment.HBM2DDL_AUTO, "create-drop");

    configuration.setProperties(settings);
    configuration.addAnnotatedClass(Instructor.class);
    configuration.addAnnotatedClass(Course.class);

    ServiceRegistry serviceRegistry = new StandardServiceRegistryBuilder()
      .applySettings(configuration.getProperties()).build();
    System.out.println("Hibernate Java Config serviceRegistry created");
    sessionFactory = configuration.buildSessionFactory(serviceRegistry);
    return sessionFactory;

   } catch (Exception e) {
    e.printStackTrace();
   }
  }
  return sessionFactory;
 }
}

6. Create the Main class and Run an Application

Let's test a Hibernate application to connect to the MySQL database.
package net.javaguides.hibernate;

import net.javaguides.hibernate.dao.CourseDao;
import net.javaguides.hibernate.dao.InstructorDao;
import net.javaguides.hibernate.entity.Course;
import net.javaguides.hibernate.entity.Instructor;

public class ManApp {
 public static void main(String[] args) {

  InstructorDao instructorDao = new InstructorDao();
  CourseDao courseDao = new CourseDao();
  
  Instructor instructor = new Instructor("Ramesh", "Fadatare", "ramesh@javaguides.com");
  instructorDao.saveInstructor(instructor);
  
  // create some courses
  Course tempCourse1 = new Course("Air Guitar - The Ultimate Guide");
  tempCourse1.setInstructor(instructor);
  courseDao.saveCourse(tempCourse1);  
  
  Course tempCourse2 = new Course("The Pinball Masterclass");
  tempCourse2.setInstructor(instructor);
  courseDao.saveCourse(tempCourse2);
 }
}

7. Output

Conclusion


In this tutorial, we successfully built a project from scratch and learned how to map a one-to-many bidirectional database relationship using Hibernate/JPA.

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

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

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