Hibernate Many-To-Many Mapping Example Part - 1

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.

01.<?xml version="1.0"?>

02.<!DOCTYPE hibernate-mapping PUBLIC

03."-//Hibernate/Hibernate Mapping DTD 3.0//EN"

04."http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

05.<hibernate-mapping>

06.<class name="com.vaannila.student.Student" table="STUDENT">

07.<meta attribute="class-description">This class contains student details.</meta>

08.<id name="studentId" type="long" column="STUDENT_ID">

09.<generator class="native" />

10.</id>

11.<property name="studentName" type="string" length="100" not-null="true" column="STUDENT_NAME" />

12.<set name="courses" table="STUDENT_COURSE" cascade="all">

13.<key column="STUDENT_ID" />

14.<many-to-many column="COURSE_ID" class="com.vaannila.student.Course" />

15.</set>

16.</class>

17.</hibernate-mapping>

We use many-to-many element to create the many-to-many relationship between the Student andCourse 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.

01.<?xml version="1.0"?>

02.<!DOCTYPE hibernate-mapping PUBLIC

03."-//Hibernate/Hibernate Mapping DTD 3.0//EN"

04."http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

05.<hibernate-mapping>

06.<class name="com.vaannila.student.Course" table="COURSE">

07.<meta attribute="class-description">

08.This class contains course details.

09.</meta>

10.<id name="courseId" type="long" column="COURSE_ID">

11.<generator class="native"/>

12.</id>

13.<property name="courseName" type="string" column="COURSE_NAME"/> 

14.</class>

15.</hibernate-mapping>

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

<?xml version="1.0" encoding="UTF-8"?>

02.<!DOCTYPE hibernate-configuration PUBLIC

03."-//Hibernate/Hibernate Configuration DTD 3.0//EN"

04."http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">

05.<hibernate-configuration>

06.<session-factory>

07.<property name="hibernate.connection.driver_class"> org.hsqldb.jdbcDriver</property>

08.<property name="hibernate.connection.url"> jdbc:hsqldb:hsql://localhost<;/property>

09.<property name="hibernate.connection.username">sa</property>

10.<property name="connection.password"></property>

11.<property name="connection.pool_size">1</property>

12.<property name="hibernate.dialect"> org.hibernate.dialect.HSQLDialect</property>

13.<property name="show_sql">true</property>

14.<property name="hbm2ddl.auto">create-drop</property>

15.<mapping resource="com/vaannila/student/Student.hbm.xml"/>

16.<mapping resource="com/vaannila/student/Course.hbm.xml"/>

17.</session-factory>

18.</hibernate-configuration>

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.

01.package com.vaannila.student;

02. 

03.// Generated May 30, 2009 6:48:40 AM by Hibernate Tools 3.2.4.GA

04. 

05.import java.util.HashSet;

06.import java.util.Set;

07. 

08./**

09.* This class contains the student details.

10.*/

11.public class Student implements java.io.Serializable {

12. 

13.private long studentId;

14.private String studentName;

15.private Set<Course> courses = new HashSet<Course>(0);

16. 

17.public Student() {

18.}

19. 

20.public Student(String studentName) {

21.this.studentName = studentName;

22.}

23. 

24.public Student(String studentName, Set<Course> courses) {

25.this.studentName = studentName;

26.this.courses = courses;

27.}

28. 

29.public long getStudentId() {

30.return this.studentId;

31.}

32. 

33.public void setStudentId(long studentId) {

34.this.studentId = studentId;

35.}

36. 

37.public String getStudentName() {

38.return this.studentName;

39.}

40. 

41.public void setStudentName(String studentName) {

42.this.studentName = studentName;

43.}

44. 

45.public Set<Course> getCourses() {

46.return this.courses;

47.}

48. 

49.public void setCourses(Set<Course> courses) {

50.this.courses = courses;

51.}

52. 

53.}

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