Hibernate One-to-Many Mapping

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Hibernate One-to-Many Mapping

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A One-to-Many relationship occurs when one entity is associated with multiple entities.

For example:

One Department has many Employees.
One Customer has many Orders.

Department.java

import jakarta.persistence.*;

import java.util.List;

@Entity

@Table(name = "department")

public class Department {

@Id

@GeneratedValue(strategy = GenerationType.IDENTITY)

private Long id;

private String name;

@OneToMany(mappedBy = "department", cascade = CascadeType.ALL, orphanRemoval = true)

private List<Employee> employees;

// Constructors

public Department() {}

public Department(String name) {

this.name = name;

}

// Getters and Setters

public Long getId() { return id; }

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

public String getName() { return name; }

public void setName(String name) { this.name = name; }

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

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

// Add Helper Method

public void addEmployee(Employee employee) {

employees.add(employee);

employee.setDepartment(this);

}

public void removeEmployee(Employee employee) {

employees.remove(employee);

employee.setDepartment(null);

}

@OneToMany(mappedBy = "department") → Defines the One-to-Many relationship.
cascade = CascadeType.ALL → If a Department is saved/deleted, all related Employees will be affected.
orphanRemoval = true → If an Employee is removed from the list, it is also deleted from the database.
Helper Methods (addEmployee, removeEmployee) → Ensure consistency between Department and Employee.

Employee.java

import jakarta.persistence.*;

@Entity

@Table(name = "employee")

public class Employee {

@Id

@GeneratedValue(strategy = GenerationType.IDENTITY)

private Long id;

private String name;

@ManyToOne

@JoinColumn(name = "department_id", nullable = false)

private Department department;

// Constructors

public Employee() {}

public Employee(String name) {

this.name = name;

}

// Getters and Setters

public Long getId() { return id; }

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

public String getName() { return name; }

public void setName(String name) { this.name = name; }

public Department getDepartment() { return department; }

Main.java

import org.hibernate.Session;

import org.hibernate.SessionFactory;

import org.hibernate.Transaction;

import org.hibernate.cfg.Configuration;

import java.util.Arrays;

public class Main {

public static void main(String[] args) {

// Create Hibernate session

SessionFactory sessionFactory = new Configuration().configure("hibernate.cfg.xml")

.addAnnotatedClass(Department.class).addAnnotatedClass(Employee.class).buildSessionFactory();

Session session = sessionFactory.openSession();

Transaction transaction = session.beginTransaction();

// Create Department

Department department = new Department("IT");

// Create Employees

Employee emp1 = new Employee("Alice");

Employee emp2 = new Employee("Bob");

// Add Employees to Department

department.setEmployees(Arrays.asList(emp1, emp2));

emp1.setDepartment(department);

emp2.setDepartment(department);

session.persist(user); // Hibernate will generate an INSERT SQL statement

transaction.commit();

session.close();

sessionFactory.close();

System.out.println("Insert data successfully!");

}

Output

Table name : department

id	name
1	IT

Table name : employee

id	name	department_id
1	Alice	1
2	Bob	1

Create Read Update Delete (CRUD):

In this guide, we will implement CRUD (Create, Read, Update, Delete) operations for a One-to-Many Mapping using Hibernate.

Department and Employee class mentioned above

HibernateUtil.java

import org.hibernate.Session;

import org.hibernate.SessionFactory;

import org.hibernate.cfg.Configuration;

public class HibernateUtil {

private static final SessionFactory sessionFactory = buildSessionFactory();

private static SessionFactory buildSessionFactory() {

try {

return new Configuration()

.configure("hibernate.cfg.xml")

.addAnnotatedClass(Employee.class)

.addAnnotatedClass(Department.class)

.buildSessionFactory();

} catch (Throwable ex) {

System.err.println("SessionFactory creation failed: " + ex);

throw new ExceptionInInitializerError(ex);

}

public static SessionFactory getSessionFactory() {

return sessionFactory;

}

public static Session getSession() {

return sessionFactory.openSession(); // Opens a new session when needed

}

public static void shutdown() {

sessionFactory.close(); // Close SessionFactory when the application shuts down

}

Main.java

import org.hibernate.Session;

import org.hibernate.Transaction;

public class Main {

public static void main(String[] args) {

// Create

Session session = HibernateUtil.getSession();

Transaction transaction = session.beginTransaction();

// Create Department

Department department = new Department("IT");

// Create Employees

Employee emp1 = new Employee("Alice");

Employee emp2 = new Employee("Bob");

// Add Employees to Department

department.setEmployees(Arrays.asList(emp1, emp2));

emp1.setDepartment(department);

emp2.setDepartment(department);

// Save Department (Employees will also be saved due to CascadeType.ALL)

session.persist(department);

transaction.commit();

session.close();

// Read

session = HibernateUtil.getSession();

department = session.get(Department.class, 1L);

System.out.println("Department: " + department.getName());

for (Employee emp : department.getEmployees()) {

System.out.println("Employee: " + emp.getName());

}

session.close();

// Update

session = HibernateUtil.getSession();

transaction = session.beginTransaction();

department = session.get(Department.class, 1L);

if (department != null) {

department.setName("Engineering");

session.merge(department);

}

transaction.commit();

session.close();

// Delete

session = HibernateUtil.getSession();

transaction = session.beginTransaction();

department = session.get(Department.class, 1L);

if (department != null) {

session.remove(department); // This will also delete employees due to CascadeType.ALL

}

transaction.commit();

session.close();

HibernateUtil.shutdown(); // Close sessionFactory

}

Output

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