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Hibernate One-to-Many Mapping
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 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 getEmployees() { return employees; }
public void setEmployees(List 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 aDepartmentis saved/deleted, all relatedEmployeeswill be affected.orphanRemoval = true→ If anEmployeeis removed from the list, it is also deleted from the database.- Helper Methods (
addEmployee,removeEmployee) → Ensure consistency betweenDepartmentandEmployee.
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; }
public void setDepartment(Department department) { this.department = 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
