Hibernate Many-To-Many Mapping Using Annotations

In this example you will learn how to map many-to-many relationship using Hibernate Annotations. Consider the following relationship between Student and Course entity.

According to the relationship a student can enroll in any number of courses and the course can have any number of students.

To create this relationship you need to have a STUDENT, COURSE and STUDENT_COURSE table. The relational model is shown below.

To create the STUDENT, COURSE and STUDENT_COURSE table you need to create the following Java Class files.

Student class is used to create the STUDENT and STUDENT_COURSE table.

package com.vaannila.student;

import java.util.HashSet;
import java.util.Set;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.JoinTable;
import javax.persistence.ManyToMany;
import javax.persistence.Table;

@Entity
@Table(name = "STUDENT")
public class Student {

 private long studentId;
 private String studentName;
 private Set<Course> courses = new HashSet<Course>(0);

 public Student() {
 }

 public Student(String studentName) {
  this.studentName = studentName;
 }

 public Student(String studentName, Set<Course> courses) {
  this.studentName = studentName;
  this.courses = courses;
 }

 @Id
 @GeneratedValue
 @Column(name = "STUDENT_ID")
 public long getStudentId() {
  return this.studentId;
 }

 public void setStudentId(long studentId) {
  this.studentId = studentId;
 }

 @Column(name = "STUDENT_NAME", nullable = false, length = 100)
 public String getStudentName() {
  return this.studentName;
 }

 public void setStudentName(String studentName) {
  this.studentName = studentName;
 }

 @ManyToMany(cascade = CascadeType.ALL)
 @JoinTable(name = "STUDENT_COURSE", joinColumns = { @JoinColumn(name = "STUDENT_ID") }, inverseJoinColumns = { @JoinColumn(name = "COURSE_ID") })
 public Set<Course> getCourses() {
  return this.courses;
 }

 public void setCourses(Set<Course> courses) {
  this.courses = courses;
 }

}

The @ManyToMany annotation is used to create the many-to-many relationship between the Student and Course entities. The @JoinTable annotation is used to create the STUDENT_COURSE link table and @JoinColumn annotation is used to refer the linking columns in both the tables.

Course class is used to create the COURSE table.

package com.vaannila.student;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.Table;

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

 private long courseId;
 private String courseName;

 public Course() {
 }

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

 @Id
 @GeneratedValue
 @Column(name="COURSE_ID")
 public long getCourseId() {
  return this.courseId;
 }

 public void setCourseId(long courseId) {
  this.courseId = courseId;
 }

 @Column(name="COURSE_NAME", nullable=false)
 public String getCourseName() {
  return this.courseName;
 }

 public void setCourseName(String courseName) {
  this.courseName = courseName;
 }

}

Now create the hibernate configuration file with the Student and Course class mapping.

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-configuration PUBLIC
  "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
  "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
    <session-factory>
        <property name="hibernate.connection.driver_class"> org.hsqldb.jdbcDriver</property>
        <property name="hibernate.connection.url"> jdbc:hsqldb:hsql://localhost</property>
        <property name="hibernate.connection.username">sa</property>
        <property name="connection.password"></property>
        <property name="connection.pool_size">1</property>
        <property name="hibernate.dialect"> org.hibernate.dialect.HSQLDialect</property>
        <property name="show_sql">true</property>
        <property name="hbm2ddl.auto">create-drop</property>
        <mapping class="com.vaannila.student.Student" />
        <mapping class="com.vaannila.student.Course" />
    </session-factory>
</hibernate-configuration>

Create the Main class to run the example.

package com.vaannila.student;

import java.util.HashSet;
import java.util.Set;

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

import com.vaannila.util.HibernateUtil;

public class Main {

 public static void main(String[] args) {

  Session session = HibernateUtil.getSessionFactory().openSession();
  Transaction transaction = null;
  try {
   transaction = session.beginTransaction();

   Set<Course> courses = new HashSet<Course>();
   courses.add(new Course("Maths"));
   courses.add(new Course("Computer Science"));

   Student student1 = new Student("Eswar", courses);
   Student student2 = new Student("Joe", courses);
   session.save(student1);
   session.save(student2);

   transaction.commit();
  } catch (HibernateException e) {
   transaction.rollback();
   e.printStackTrace();
  } finally {
   session.close();
  }

 }
}

On executing the Main class you will see the following output.

The STUDENT table has two records.

The COURSE table has two records.

The STUDENT_COURSE table has four records to link the student and courses.

Each student has enrolled in the same two courses, this illustrates the many-to-many mapping.

The folder structure of the example is shown below.