HIBERNATE 4U: Hibernate Mapping Many-to-Many

In this example you will learn how to map many-to-many relationship using Hibernate. 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 and COURSE tables you need to create the following hibernate mapping files.
Student.hbm.xml is used to create the STUDENT and STUDENT_COURSE table.

xml  version="1.0"?>

"-//Hibernate/Hibernate  Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
    <class  name="com.vaannila.student.Student" table="STUDENT">
        <meta  attribute="class-description">This  class contains student details.meta>
        <id name="studentId" type="long" column="STUDENT_ID">
            <generator class="native" />
        id>
        <property name="studentName" type="string" length="100" not-null="true" column="STUDENT_NAME" />
        <set  name="courses" table="STUDENT_COURSE" cascade="all">
            <key column="STUDENT_ID" />
            <many-to-many column="COURSE_ID"  class="com.vaannila.student.Course"  />
        set>
    class>
hibernate-mapping>

We use many-to-many element to create the many-to-many relationship between the Student and Course entities. Since a student can enroll in any number of courses we use a collection to hold the values. In this case we use Set.
Course.hbm.xml is used to create the COURSE table.

xml  version="1.0"?>

    "-//Hibernate/Hibernate Mapping DTD  3.0//EN"
    "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
<class name="com.vaannila.student.Course"  table="COURSE">
    <meta  attribute="class-description">
        This class contains course  details.
    meta>
    <id name="courseId" type="long" column="COURSE_ID">
        <generator  class="native"/>
    id>
    <property name="courseName" type="string" column="COURSE_NAME"/>  
class>
hibernate-mapping>

Now create the hibernate configuration file and add all the mapping files.

"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
        "com/vaannila/student/Student.hbm.xml"/>
        "com/vaannila/student/Course.hbm.xml"/>
    


After creating the configuration file, generate java class files using Hibernate Tools.(To generate code using Hibernate Tools refer this example )
The following classes will be generated.

package com.vaannila.student;
 
// Generated May 30, 2009 6:48:40 AM by Hibernate Tools  3.2.4.GA
 
import java.util.HashSet;
import java.util.Set;
 
/**
 * This class contains the student details.
 */
public class Student implements java.io.Serializable {
 
    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;
    }
 
    public long  getStudentId() {
        return this.studentId;
    }
 
    public void  setStudentId(long  studentId) {
        this.studentId  = studentId;
    }
 
    public String  getStudentName() {
        return this.studentName;
    }
 
    public void  setStudentName(String studentName) {
        this.studentName  = studentName;
    }
 
    public Set  getCourses() {
        return this.courses;
    }
 
    public void  setCourses(Set courses) {
        this.courses  = courses;
    }
 
}

package com.vaannila.student;
 
// Generated May 30, 2009 6:48:40 AM by Hibernate Tools  3.2.4.GA
 
/**
 *  This class contains the course details.
 *  
 */
public class Course implements java.io.Serializable {
 
    private long  courseId;
    private String  courseName;
 
    public Course()  {
    }
 
    public Course(String  courseName) {
        this.courseName = courseName;
    }
 
    public long  getCourseId() {
        return this.courseId;
    }
 
    public void  setCourseId(long  courseId) {
        this.courseId  = courseId;
    }
 
    public String  getCourseName() {
        return this.courseName;
    }
 
    public void  setCourseName(String courseName) {
        this.courseName  = courseName;
    }
 
}

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();
        }
 
    }
}