HIBERNATE 4U: Hibernate Mapping Many-to-Many 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  courses = new HashSet(0);
 
    public Student()  {
    }
 
    public Student(String  studentName) {
        this.studentName =  studentName;
    }
 
    public Student(String  studentName, Set 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  getCourses() {
        return this.courses;
    }
 
    public void  setCourses(Set 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.

"1.0" encoding="UTF-8"?>

        "-//Hibernate/Hibernate  Configuration DTD 3.0//EN"
        "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">

    
        "hibernate.connection.driver_class"> org.hsqldb.jdbcDriver
        "hibernate.connection.url">  jdbc:hsqldb:hsql://localhost
        "hibernate.connection.username">sa
        "connection.password">
        "connection.pool_size">1
        "hibernate.dialect">  org.hibernate.dialect.HSQLDialect
        "show_sql">true
        "hbm2ddl.auto">create-drop
        class="com.vaannila.student.Student" />
        class="com.vaannila.student.Course" />
    


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  courses = new HashSet();
            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();
        }
 
    }
}