Hibernate Many to Many Mapping Example

Let us see an example on this many to many relationship in hibernate.  Actually here there is no question of unidirectional, only Bi-Directional.

Applying many to many relationship between two pojo class objects is nothing but applying one to many relationship on both sides, which tends to Bi-Directional i mean many to many.

Example:

Let us see this, if we apply many to many association between two pojo class objects student and course, provided the relationship is one student may joined in multiple courses and one course contains lot of students (joined by multiple students)

Remember, when ever we are applying many to many relationship between two pojo class objects, on both sides  we need a collection property [As we are applying one to many from both the sides]

Note Points:

• While applying many to many relationship between pojo classes,  a mediator table is mandatory in the database, to store primary key as foreign key both sides, we call this table as Join table
• In many to many relationship join table contain foreign keys only

Many To Many Relationship Example

files required…..

• Student.java
• Course.java
• Course.hbm.xml
• Student.hbm.xml
• OurLogic.java
• hibernate.cfg.xml

Student.java

package str;

import java.util.Set;

public class Student {

	private int studentId;
	private String studentName;
	private int marks;

	private Set courses;

	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;
	}

	public int getMarks() {
		return marks;
	}

	public void setMarks(int marks) {
		this.marks = marks;
	}

	public Set getCourses() {
		return courses;
	}

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

}

Course.java

package str;

import java.util.Set;

public class Course {

	private int courseId;
	private String courseName;
	private int duration;

	private Set students;

	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 getDuration() {
		return duration;
	}

	public void setDuration(int duration) {
		this.duration = duration;
	}

	public Set getStudents() {
		return students;
	}

	public void setStudents(Set students) {
		this.students = students;
	}	

}

Student.hbm.xml

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping>
<class name="str.Student" table="student">

<id name="studentId" column="studentid" />

<property name="studentName" column="studentname" length="20"/>
<property name="marks" />

<set name="courses" cascade="all" table="students_courses">

<key column="student_id "/>
<many-to-many class="str.Course" column="course_id" />

</set>

</class>

</hibernate-mapping>

Course.hbm.xml

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping>
<class name="str.Course" table="courses">

<id name="courseId" column="courseid" />

<property name="courseName" column="coursename" length="20"/>
<property name="duration" />

<set name="students" inverse="false" cascade="all" table="students_courses">

<key column="course_id" />
<many-to-many class="str.Student" column="student_id "/>

</set>

</class>

</hibernate-mapping>

OurLogic.java

package str;

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

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

public class OurLogic {

	public static void main(String args[])
	{

		Configuration cfg = new Configuration();
		cfg.configure("hibernate.cfg.xml"); 

		SessionFactory factory = cfg.buildSessionFactory();
		Session session = factory.openSession();		

		Student s1=new Student();
		s1.setStudentId(100);
		s1.setStudentName("James");
		s1.setMarks(98);

		Student s2=new Student();
		s2.setStudentId(101);
		s2.setStudentName("Lee");
		s2.setMarks(99);

		Course c1=new Course();
		c1.setCourseId(500);
		c1.setCourseName("Hibernate");
		c1.setDuration(7);

		Course c2=new Course();
		c2.setCourseId(501);
		c2.setCourseName("Java");
		c2.setDuration(30);

		Set s =new HashSet();
		      s.add(c1);
		      s.add(c2);

		s1.setCourses(s);
		s2.setCourses(s);

		    Transaction tx = session.beginTransaction();

		                      session.save(s1);
		                      session.save(s2);

		    tx.commit();

		    session.close();
		    System.out.println("Many To Many Bi-Directional is Done..!!");
		    factory.close();

	}
}

hibernate.cfg.xml

<?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="connection.driver_class">oracle.jdbc.driver.OracleDriver
</property>
<property name="connection.url">jdbc:oracle:thin:@www.java4s.com:1521:XE</property>
<property name="connection.username">system</property>
<property name="connection.password">admin</property>

<property name="dialect">org.hibernate.dialect.OracleDialect</property>
<property name="show_sql">true</property>
<property name="hbm2ddl.auto">update</property>

<mapping resource="Student.hbm.xml"></mapping>
<mapping resource="Course.hbm.xml"></mapping>
</session-factory>
</hibernate-configuration>

Output

Just see the mapping files by seeing this output file….

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::. About the Author .::
		Sivateja Kandula - Java/J2EE Full Stack Developer Founder of Java4s - Get It Yourself, A popular Java/J2EE Programming Blog, Love Java and UI frameworks. You can sign-up for the Email Newsletter for your daily dose of Java tutorials.