JPA / Hibernate One to Many Unidirectional Mapping Example

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

Overview

There are two types of one-to-many association - 

• 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 tutorial, we will implement unidirectional entity mapping using @OneToMany JPA annotation.

Consider the following domain model and relational model diagrams of one-to-many unidirectional association.

Development Steps

1. Create a Simple Maven Project
2. Project Directory Structure
3. POM Dependencies
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 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 a simple maven project. Refer below screenshot for your reference.

3. POM Dependencies

We are using MySQL database so add MySQL dependency 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-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>8.0.13</version>
        </dependency>
        <!-- https://mvnrepository.com/artifact/org.hibernate/hibernate-core -->
        <dependency>
            <groupId>org.hibernate</groupId>
            <artifactId>hibernate-core</artifactId>
            <version>5.3.7.Final</version>
        </dependency>
    </dependencies>
    <build>
        <sourceDirectory>src/main/java</sourceDirectory>
        <plugins>
            <plugin>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.5.1</version>
                <configuration>
                    <source>1.8</source>
                    <target>1.8</target>
                </configuration>
            </plugin>
        </plugins>
    </build>
</project>

4. Creating the JPA Entities(Persistent classes)

Let's create JPA entities which we map with database tables. The @OneToMany JPA annotation is used to link one-to-many unidirectional entity mapping.

Instructor JPA Entity - Instructor.java

package net.javaguides.hibernate.entity;

import java.util.ArrayList;
import java.util.List;

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.OneToMany;
import javax.persistence.Table;

@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(cascade = CascadeType.ALL)
    private List < Course > courses = new ArrayList < Course > ();

    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 - Course.java

package net.javaguides.hibernate.entity;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.Table;

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

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

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

 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;
 }

 @Override
 public String toString() {
  return "Course [id=" + id + ", title=" + title + "]";
 }
}

5. Create Hibernate DAO Classes

InstructorDao.java

Let's create InstructorDao class add following hibernate DAO operations for Instructor entity:

package net.javaguides.hibernate.dao;

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

Let's create CourseDao class add following hibernate DAO operations for Course entity:

package net.javaguides.hibernate.dao;

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;
    }
}

6. Hibernate Utility File(Java Configuration)

Create a helper class HibernateUtil to bootstrap hibernate.

Map the Instructor and Course entities using the MetadataSources.addAnnotatedClass() method.

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;
    }
}

7. Create the Main class and Run an Application

package net.javaguides.hibernate;

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();

        Instructor instructor = new Instructor("Ramesh", "Fadatare", "ramesh@javaguides.com");
        instructorDao.saveInstructor(instructor);

        // create some courses
        Course tempCourse1 = new Course("Learn Spring Boot");
        instructor.getCourses().add(tempCourse1);

        Course tempCourse2 = new Course("Learn hibernate");
        instructor.getCourses().add(tempCourse2);

        instructorDao.saveInstructor(instructor);
    }
}

Output

Conclusion

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

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

• Spring Boot Hibernate Many to Many Example

• Spring Boot + JPA/Hibernate One to One Mapping Example

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