Hibernate

One to Many Mapping

One To Many mapping to represent one to many relationships between tables. A one to many relationships occurs between tables when one record from a table can correspond to many records from another table.

For example, In a department, many employees work. So one record in the Department table will correspond to many records in the Employee table.

Tools and Technologies Used:

1. Java 8

2. MySQL 8

3. Eclipse IDE

4. mysql-connector-java-8.0.23.jar

5. Hibernate 5.5.7.Final

6. Maven 3.6.1

Maven Project for One to Many Mapping

We need to follow below steps:

Step 1: Click on File --> New --> Maven Project.

Step 2: Check Checkbox Create a simple project (skip archetype selection). Click on Next button.

Step 3: Enter Group Id, Artifact Id, Version, Packaging, Name, Description details and click on Finish button.

Update pom.xml file.

Open pom.xml file and paste below pom.xml file code.

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>
		<groupId>com.hareeshsoni.blog.hibernate.mapping</groupId>
		<artifactId>HibernateOneToManyExample</artifactId>
		<version>0.0.1-SNAPSHOT</version>
		<name>HibernateOneToManyExample</name>
		<description>Hibernate One To Many Example</description>
		<dependencies>
			<!-- https://mvnrepository.com/artifact/org.hibernate/hibernate-core -->
			<dependency>
				<groupId>org.hibernate</groupId>
				<artifactId>hibernate-core</artifactId>
				<version>5.5.7.Final</version>
			</dependency>
	
			<!-- https://mvnrepository.com/artifact/mysql/mysql-connector-java -->
			<dependency>
				<groupId>mysql</groupId>
				<artifactId>mysql-connector-java</artifactId>
				<version>8.0.23</version>
			</dependency>
	
		</dependencies>
		<build>
			<finalName></finalName>
			<plugins>
				<plugin>
					<groupId>org.apache.maven.plugins</groupId>
					<artifactId>maven-compiler-plugin</artifactId>
					<version>3.6.1</version>
					<configuration>
						<source>1.8</source>
						<target>1.8</target>
					</configuration>
				</plugin>
			</plugins>
		</build>
	</project>
		

Update Maven Project

Step 1: Right-click on project HibernateOneToManyExample --> Maven --> Update Project…

Step 2: Check Checkbox Force Update of Snapshots/Releases. Click on OK Button.

Project Structure

We have used the below JPA Annotations:

@Entity: @Entity annotation specifies that the class is an entity.

@Table: @Table annotation specifies the primary table for the annotated entity.

@Id: @Id annotation marks the particular field as the primary key of the Entity.

@GeneratedValue: This annotation is used to specify how the primary key should be generated.

@Column: This annotation maps the corresponding fields to their respective columns in the database table.

@JoinColumn: This annotation defines the foreign key. It is the column that associates the two tables.

@OneToMany: This annotation is used to create one to many relationship between Department and Employee entities.

@ManyToOne: This annotation is used to create many to one relationship between Employee and Department entities.

Department.java

	package com.hareeshsoni.blog.hibernate.mapping.model;
	
	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.OneToMany;
	import javax.persistence.Table;
	
	@Entity
	@Table(name = "department")
	public class Department {
		@Id
		@GeneratedValue(strategy = GenerationType.IDENTITY)
		@Column(name = "department_id")
		private int departmentId;
	
		@Column(name = "department_name")
		private String departmentName;
	
		@OneToMany(mappedBy = "department", cascade = CascadeType.ALL)
		private Set employee;
	
		public Department() {
		}
	
		public Department(String departmentName) {
			this.departmentName = departmentName;
		}
	
		public int getDepartmentId() {
			return departmentId;
		}
	
		public void setDepartmentId(int departmentId) {
			this.departmentId = departmentId;
		}
	
		public String getDepartmentName() {
			return departmentName;
		}
	
		public void setDepartmentName(String departmentName) {
			this.departmentName = departmentName;
		}
	
		public Set getEmployee() {
			return employee;
		}
	
		public void setEmployee(Set employee) {
			this.employee = employee;
		}
	
	}
	

Employee.java

	package com.hareeshsoni.blog.hibernate.mapping.model;
	
	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.ManyToOne;
	import javax.persistence.Table;
	
	@Entity
	@Table(name = "employee")
	public class Employee {
		@Id
		@GeneratedValue(strategy = GenerationType.IDENTITY)
		@Column(name = "employee_id")
		private int employeeId;
	
		@Column(name = "employee_name")
		private String employeeName;
	
		@ManyToOne(cascade = CascadeType.ALL)
		@JoinColumn(name = "department_id")
		private Department department;
	
		public Employee() {
		}
	
		public Employee(String employeeName) {
			this.employeeName = employeeName;
		}
	
		public int getEmployeeId() {
			return employeeId;
		}
	
		public void setEmployeeId(int employeeId) {
			this.employeeId = employeeId;
		}
	
		public String getEmployeeName() {
			return employeeName;
		}
	
		public void setEmployeeName(String employeeName) {
			this.employeeName = employeeName;
		}
	
		public Department getDepartment() {
			return department;
		}
	
		public void setDepartment(Department department) {
			this.department = department;
		}
	
	}
	

HibernateUtil.java

	package com.hareeshsoni.blog.hibernate.mapping.util;
	
	
	import org.hibernate.SessionFactory;
	import org.hibernate.cfg.Configuration;
	
	public class HibernateUtil {
	private static SessionFactory sessionFactory;
		
		private static SessionFactory buildSessionFactory() {
			Configuration configuration = new Configuration();
	    	configuration.configure("hibernate.cfg.xml");
	    	System.out.println("Hibernate configuration file loaded");
	    	SessionFactory sessionFactory = configuration.buildSessionFactory();
	        return sessionFactory;
	    }
		
		public static SessionFactory getSessionFactory() {
			if(sessionFactory == null) 
				sessionFactory = buildSessionFactory();
	        return sessionFactory;
	    }
	}
		

hibernate.cfg.xml

In hibernate.cfg.xml file provide correct username and password.

Create database name "one_to_many_mapping" inside mysql database.

<?xml version='1.0' encoding='utf-8'?>
	<!DOCTYPE hibernate-configuration PUBLIC
		"-//Hibernate/Hibernate Configuration DTD 3.0//EN"
		"http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">
	
	<hibernate-configuration>
		<session-factory>
			<!-- Database connection settings -->
			<property name="hibernate.connection.driver_class">com.mysql.cj.jdbc.Driver</property>
			<property name="hibernate.connection.url">jdbc:mysql://localhost:3306/one_to_many_mapping</property>
			<property name="hibernate.connection.username">root</property>
			<property name="hibernate.connection.password">########</property>
			<property name="hibernate.dialect">org.hibernate.dialect.MySQL5Dialect</property>
			<property name="show_sql">true</property>
			<property name="hbm2ddl.auto">update</property>
			<mapping class="com.hareeshsoni.blog.hibernate.mapping.model.Employee" />
			<mapping class="com.hareeshsoni.blog.hibernate.mapping.model.Department" />
		</session-factory>
	</hibernate-configuration>	
		

MainApp.java

  
	package com.hareeshsoni.blog.hibernate.mapping;
	
	import org.hibernate.Session;
	import org.hibernate.SessionFactory;
	
	import com.hareeshsoni.blog.hibernate.mapping.model.Department;
	import com.hareeshsoni.blog.hibernate.mapping.model.Employee;
	import com.hareeshsoni.blog.hibernate.mapping.util.HibernateUtil;
	
	public class MainApp {
	
		public static void main(String[] args) {
	
			SessionFactory sessionFactory = HibernateUtil.getSessionFactory();
			Session session = sessionFactory.openSession();
	
			session.beginTransaction();
	
			Department department = new Department("Human Resource");
	
			Employee employee1 = new Employee("Mohan");
			employee1.setDepartment(department);
			session.save(employee1);
	
			Employee employee2 = new Employee("Rohan");
			employee2.setDepartment(department);
			session.save(employee2);
	
			session.getTransaction().commit();
			session.close();
			sessionFactory.close();
		}
	
	}
	

Now run MainApp.java

Output

Database tables:

Department Table

Employee Table

Download Project Code: