ManyToMany example

In this example we will see how to establish many to many relationship in hibernate. To explain this, lets take two classes Student and Course. A student can enroll in any number of courses and a course can be enrolled by any number of students. So this makes a many to many relationship. To achieve this we can make use of the annotation "@ManyToMany". Below code is sample program of the many to many relation ship.

<?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="connection.driver_class">oracle.jdbc.driver.OracleDriver</property>
        <property name="connection.url">jdbc:oracle:thin:@localhost:1521:xe</property>
        <property name="connection.username">hr</property>
        <property name="connection.password">hr</property>

        <!-- JDBC connection pool (use the built-in) -->
        <property name="connection.pool_size">1</property>

        <!-- SQL dialect -->
        <property name="dialect">org.hibernate.dialect.OracleDialect</property>

        <!-- Disable the second-level cache  -->
        <property name="cache.provider_class">org.hibernate.cache.NoCacheProvider</property>

        <!-- Echo all executed SQL to stdout -->
        <property name="show_sql">true</property>

        <!-- Drop and re-create the database schema on startup -->
        <property name="hbm2ddl.auto">create</property>

        <!-- Names the annotated entity class -->
        <mapping class="com.ram.dao.Student"/>
        <mapping class="com.ram.dao.Course"/>

    </session-factory>

</hibernate-configuration>

File: com.ram.dao.Student.java

package com.ram.dao;

import java.util.ArrayList;
import java.util.Collection;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.ManyToMany;

@Entity
public class Student {
 @Id @GeneratedValue
 private int studentId;
 private String studentName;
 @ManyToMany
 private Collection course = new ArrayList();
 
 public Collection getCourse() {
  return course;
 }
 public void setCourse(Collection course) {
  this.course = course;
 }
 public int getStudentId() {
  return studentId;
 }
 public void setStudentId(int studentId) {
  this.studentId = studentId;
 }
 public String getStudentName() {
  return studentName;
 }
 public void setStudentName(String studentName) {
  this.studentName = studentName;
 }
}

File: com.ram.dao.Course.java

package com.ram.dao;

import java.util.ArrayList;
import java.util.Collection;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.ManyToMany;

@Entity
public class Course {
 @Id @GeneratedValue
 private int courseId;
 private String courseName;
 private int courseDays;
 @ManyToMany
 private Collection student = new ArrayList();
 
 public Collection getStudent() {
  return student;
 }
 public void setStudent(Collection student) {
  this.student = student;
 }
 public int getCourseId() {
  return courseId;
 }
 public void setCourseId(int courseId) {
  this.courseId = courseId;
 }
 public String getCourseName() {
  return courseName;
 }
 public void setCourseName(String courseName) {
  this.courseName = courseName;
 }
 public int getCourseDays() {
  return courseDays;
 }
 public void setCourseDays(int courseDays) {
  this.courseDays = courseDays;
 }
}

File: com.ram.hibernate.ManyToManyExample.java

package com.ram.hibernate;

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;

import com.ram.dao.Course;
import com.ram.dao.Student;

public class ManyToManyExample {
 public static void main(String[] args) {
  Student student1 = new Student();
  student1.setStudentName("Ram");
  
  Student student2 = new Student();
  student2.setStudentName("Veena");
  
  Course course1 = new Course();
  course1.setCourseName("Hibernate3");
  course1.setCourseDays(10);
  
  Course course2 = new Course();
  course2.setCourseName("HTML5");
  course2.setCourseDays(5);
  
  course1.getStudent().add(student1);
  course1.getStudent().add(student2);
  course2.getStudent().add(student1);
  course2.getStudent().add(student2);
  
  student1.getCourse().add(course1);
  student1.getCourse().add(course2);
  student2.getCourse().add(course1);
  student2.getCourse().add(course2);
  
  SessionFactory sessionFactory = new Configuration().configure().buildSessionFactory();
  Session session = sessionFactory.openSession();
  
  session.beginTransaction();
  session.save(student1);
  session.save(student2);
  session.save(course1);
  session.save(course2);
  
  session.getTransaction().commit();
  session.close();
 }
}

Execute the ManyToManyExample.java and you get the below output.