Hibernate One-To-Many Mapping Example Part -1

In this example you will learn how to map one-to-many relationship using Hibernate. Consider the following relationship between Student and Phone entity.

According to the relationship a student can have any number of phone numbers.

To create this relationship you need to have a STUDENT, PHONE and STUDENT_PHONE table. The relational model is shown below.

To create the STUDENT and PHONE table you need to create the following hibernate mapping files.

Student.hbm.xml is used to create the STUDENT and STUDENT_PHONE 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" not-null="true"length="100" column="STUDENT_NAME" />

12.<set name="studentPhoneNumbers" table="STUDENT_PHONE" cascade="all">

13.<key column="STUDENT_ID" />

14.<many-to-many column="PHONE_ID" unique="true"class="com.vaannila.student.Phone" />

15.</set>

16.</class>

17.</hibernate-mapping>

We use many-to-many element to create the one-to-many relationship between the Student andPhone entities. Since a student can have any number of phone numbers we use a collection to hold the values. In this case we use Set. Many-to-many element is usually used to create many-to-many relationship, here we place the unique constraint on the PHONE_ID column, this makes the relationship one-to-many.

Phone.hbm.xml is used to create the PHONE 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.Phone" table="PHONE">

07.<meta attribute="class-description">This class contains student's phone number

08.details.</meta>

09.<id name="phoneId" type="long" column="PHONE_ID">

10.<generator class="native" />

11.</id>

12.<property name="phoneType" type="string" length="10"column="PHONE_TYPE" />

13.<property name="phoneNumber" type="string" length="15"column="PHONE_NUMBER" />

14.</class>

15.</hibernate-mapping>

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

01.<?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.<propertyname="hibernate.connection.driver_class">org.hsqldb.jdbcDriver </property>

08.<propertyname="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/Phone.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 Sep 3, 2009 8:47:06 PM by Hibernate Tools 3.2.4.GA

04. 

05.import java.util.HashSet;

06.import java.util.Set;

07. 

08./**

09.* This class contains student details.

10.*/

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

12. 

13.private long studentId;

14.private String studentName;

15.private Set<Phone> studentPhoneNumbers = new HashSet<Phone>(0);

16. 

17.public Student() {

18.}

19. 

20.public Student(String studentName) {

21.this.studentName = studentName;

22.}

23. 

24.public Student(String studentName, Set<Phone> studentPhoneNumbers) {

25.this.studentName = studentName;

26.this.studentPhoneNumbers = studentPhoneNumbers;

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<Phone> getStudentPhoneNumbers() {

46.return this.studentPhoneNumbers;

47.}

48. 

49.public void setStudentPhoneNumbers(Set<Phone> studentPhoneNumbers) {

50.this.studentPhoneNumbers = studentPhoneNumbers;

51.}

52. 

53.}

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