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Hibernate
Hibernate Many-to-Many Mapping

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A Many-to-Many relationship exists when multiple records in one table are associated with multiple records in another table.

For example:

  • A Student can enroll in multiple Courses.
  • A Course can have multiple Students.

✅ This requires a Join Table to link the two entities.

  • Student Entity (Many-to-Many with Course)
  • Course Entity (Many-to-Many with Student)
  • Join Table (student_course) for mapping.

Person Entity:

  • @ManyToMany → A student can enroll in many courses.
  • @JoinTable → Defines the join table person_course.
  • Bidirectional Relationship → We add course.getPersons().add(this);.
Person.java
import jakarta.persistence.*;
import java.util.HashSet;
import java.util.Set;

@Entity
@Table(name = "persons")
public class Person {

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
private String name;

@ManyToMany
@JoinTable(
name = "person_course",
joinColumns = @JoinColumn(name = "person_id"),
inverseJoinColumns = @JoinColumn(name = "course_id")
)
private Set<Course> courses = new HashSet<>();

public Person() {}

public Person(String name) {
this.name = name;
}

// Getters & 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 Set<Course> getCourses() { return courses; }
public void setCourses(Set<Course> courses) { this.courses = courses; }

public void addCourse(Course course) {
courses.add(course);
course.getPersons().add(this);
}
}

Course Entity:

  • mappedBy = "courses" → This refers to the courses field in the Student entity.
  • Bidirectional Relationship → A course also maintains a list of students.
Course.java
import jakarta.persistence.*;
import java.util.HashSet;
import java.util.Set;

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

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;

private String title;

@ManyToMany(mappedBy = "courses")
private Set<Person> persons = new HashSet<>();

public Course() {}

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

// Getters & Setters
public Long getId() { return id; }
public void setId(Long id) { this.id = id; }
}

Session Create:

Create a separate Hibernate utility class to manage SessionFactory:

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(Person.class)
.addAnnotatedClass(Course.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
}
}

CRUD Operations in Many-to-Many:

Insert Data (Adding Students and Courses)

  • Hibernate automatically inserts records in the student_course table.


import org.hibernate.Session;
import org.hibernate.Transaction;

public class Main {
public static void main(String[] args) {
Session session = HibernateUtil.getSession();
Transaction transaction = session.beginTransaction();

// Create Courses
Course java = new Course("Java Programming");
Course python = new Course("Python Basics");

// Create Students
Student alice = new Student("Alice");
Student bob = new Student("Bob");

// Enroll Students in Courses
alice.addCourse(java);
alice.addCourse(python);

bob.addCourse(java);

// Save Data
session.persist(java);
session.persist(python);
session.persist(alice);
session.persist(bob);

transaction.commit();
session.close();
}
}

Fetch Data (Get Student and Courses):

Session session = HibernateUtil.getSession();
Person person = session.get(Person.class, 1L);

System.out.println("Person: " + person.getName());
for (Course course : person.getCourses()) {
System.out.println("Enrolled in: " + course.getTitle());
}

session.close();
Output
Student: Alice
Enrolled in: Java Programming
Enrolled in: Python Basics

Update Data (Add Course to Existing Student):

  • Automatically updates the person_course table.
Session session = HibernateUtil.getSession();
Transaction transaction = session.beginTransaction();
Person person = session.get(Person.class, 1L);
Course course = session.get(Course.class, 3L);
if (person != null && course != null) {
person.addCourse(course);
session.merge(person);
}
transaction.commit();
session.close();

Delete Student (Cascade Deletion):

Session session = HibernateUtil.getSession();
Transaction transaction = session.beginTransaction();
Person person = session.get(Person.class, 1L);
if (person != null) {
session.remove(person);
}
transaction.commit();
session.close();

Database Table Structure:

Table name : person

id name
1 Alice
2 Bob

Table name : course

id title
1 Java Programming
2 Python Programming

Join Table: student_course

student_id course_id
1 2
1 2
2 1

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