HIBERNATE 4U: Hibernate Mapping Many-to-One

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

According to the relationship many students can have the same address.
To create this relationship you need to have a STUDENT and ADDRESS table. The relational model is shown below.

To create the STUDENT and ADDRESS table you need to create the following hibernate mapping files.
Student.hbm.xml is used to create the STUDENT 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" />
        <many-to-one  name="studentAddress" class="com.vaannila.student.Address"  column="STUDENT_ADDRESS" cascade="all" not-null="true" />
    class>
hibernate-mapping>

The many-to-one element is used to create the many-to-one relationship between the Student and Address entities. The cascade option is used to cascade the required operations to the associated entity. If the cascade option is set to all then all the operations will be cascaded. For instance when you save a Student object, the associated Address object will also be saved automatically.
Address.hbm.xml is used to create the ADDRESS 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.Address" table="ADDRESS">
        <meta  attribute="class-description">This  class contains the student's address
            details.meta>
        <id  name="addressId" type="long" column="ADDRESS_ID">
            <generator class="native" />
        id>
        <property name="street" column="ADDRESS_STREET" type="string" length="250" />
        <property name="city" column="ADDRESS_CITY" type="string" length="50" />
        <property name="state" column="ADDRESS_STATE" type="string" length="50" />
        <property name="zipcode" column="ADDRESS_ZIPCODE" type="string" length="10" />
    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/Address.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 7:20:37 PM by Hibernate Tools  3.2.4.GA
 
/**
 * This class contains student details.
 */
public class Student implements java.io.Serializable {
 
    private long  studentId;
    private String  studentName;
    private Address  studentAddress;
 
    public Student()  {
    }
 
    public Student(String  studentName, Address studentAddress) {
        this.studentName  = studentName;
        this.studentAddress =  studentAddress;
    }
 
    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 Address  getStudentAddress() {
        return this.studentAddress;
    }
 
    public void  setStudentAddress(Address studentAddress) {
        this.studentAddress  = studentAddress;
    }
 
}

package com.vaannila.student;
 
// Generated Sep 3, 2009 7:20:37 PM by Hibernate Tools  3.2.4.GA
 
/**
 * This class contains the student's address
 *          details.
 */
public class Address implements java.io.Serializable {
 
    private long  addressId;
    private String  street;
    private String city;
    private String  state;
    private String zipcode;
 
    public Address()  {
    }
 
    public Address(String  street, String city, String state, String zipcode) {
        this.street  = street;
        this.city = city;
        this.state  = state;
        this.zipcode = zipcode;
    }
 
    public long  getAddressId() {
        return this.addressId;
    }
 
    public void  setAddressId(long  addressId) {
        this.addressId  = addressId;
    }
 
    public String  getStreet() {
        return this.street;
    }
 
    public void  setStreet(String street) {
        this.street  = street;
    }
 
    public String  getCity() {
        return this.city;
    }
 
    public void  setCity(String city) {
        this.city  = city;
    }
 
    public String  getState() {
        return this.state;
    }
 
    public void  setState(String state) {
        this.state  = state;
    }
 
    public String  getZipcode() {
        return this.zipcode;
    }
 
    public void  setZipcode(String zipcode) {
        this.zipcode  = zipcode;
    }
 
}

Create the Main class to run the example.

package com.vaannila.student;
 
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();
            Address  address = new Address("OMR Road", "Chennai", "TN", "600097");
            //By using  cascade=all option the address need not be saved explicitly when the  student object is persisted the address will be automatically saved.
            //session.save(address);
            Student  student1 = new Student("Eswar", address);
            Student  student2 = new Student("Joe", address);
            session.save(student1);
            session.save(student2);
            transaction.commit();
        } catch  (HibernateException e) {
            transaction.rollback();
            e.printStackTrace();
        } finally  {
            session.close();
        }
 
    }
 
}

On executing the Main class you will see the following output.
The Student table has two records.

The Address table has one record.

Both the student records points to the same address record, this illustrates the many-to-one mapping.
The folder structure of the example is shown below.

Hibernate Mapping Many-to-One

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