What And How In Java: Hibernate Many-To-Many Mapping Using Annotations Tutorial

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();

        }

 

    }

}