HIBERNATE 4U: Hibernate Mapping One-to-Many

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.

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" not-null="true" length="100" column="STUDENT_NAME" />
        <set  name="studentPhoneNumbers" table="STUDENT_PHONE" cascade="all">
            <key column="STUDENT_ID" />
            <many-to-many column="PHONE_ID" unique="true" class="com.vaannila.student.Phone" />
        set>
    class>
hibernate-mapping>

We use many-to-many element to create the one-to-many relationship between the Student and Phone 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.

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.Phone" table="PHONE">
        <meta attribute="class-description">This  class contains student's phone number
            details.meta>
        <id  name="phoneId" type="long" column="PHONE_ID">
            <generator class="native" />
        id>
        <property name="phoneType" type="string" length="10" column="PHONE_TYPE" />
        <property name="phoneNumber" type="string" length="15" column="PHONE_NUMBER" />
    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/Phone.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 Sep 3, 2009 8:47:06 PM by Hibernate Tools  3.2.4.GA
 
import java.util.HashSet;
import java.util.Set;
 
/**
 * This class contains student details.
 */
public class Student implements java.io.Serializable {
 
    private long  studentId;
    private String  studentName;
    private Set  studentPhoneNumbers = new HashSet(0);
 
    public Student()  {
    }
 
    public Student(String  studentName) {
        this.studentName =  studentName;
    }
 
    public Student(String  studentName, Set studentPhoneNumbers) {
        this.studentName  = studentName;
        this.studentPhoneNumbers =  studentPhoneNumbers;
    }
 
    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  getStudentPhoneNumbers() {
        return this.studentPhoneNumbers;
    }
 
    public void  setStudentPhoneNumbers(Set  studentPhoneNumbers) {
        this.studentPhoneNumbers =  studentPhoneNumbers;
    }
 
}

package com.vaannila.student;
 
// Generated Sep 3, 2009 8:47:06 PM by Hibernate Tools  3.2.4.GA
 
/**
 * This class contains student's phone number
 *          details.
 */
public class Phone implements java.io.Serializable {
 
    private long  phoneId;
    private String  phoneType;
    private String phoneNumber;
 
    public Phone()  {
    }
 
    public Phone(String  phoneType, String phoneNumber) {
        this.phoneType  = phoneType;
        this.phoneNumber =  phoneNumber;
    }
     
    public long  getPhoneId() {
        return this.phoneId;
    }
 
    public void  setPhoneId(long  phoneId) {
        this.phoneId  = phoneId;
    }
 
    public String  getPhoneType() {
        return this.phoneType;
    }
 
    public void  setPhoneType(String phoneType) {
        this.phoneType  = phoneType;
    }
 
    public String  getPhoneNumber() {
        return this.phoneNumber;
    }
 
    public void  setPhoneNumber(String phoneNumber) {
        this.phoneNumber  = phoneNumber;
    }
 
}

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  phoneNumbers = new HashSet();
            phoneNumbers.add(new Phone("house","32354353"));
            phoneNumbers.add(new Phone("mobile","9889343423"));
             
            Student  student = new Student("Eswar", phoneNumbers);
            session.save(student);
             
            transaction.commit();
        } catch  (HibernateException e) {
            transaction.rollback();
            e.printStackTrace();
        } finally  {
            session.close();
        }
 
    }
 
}