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Tags/Keywords : Hibernate Mapping, hibernate, Mapping, Example Code, Tutorial, Article,Persist
Author : Amit
Date (Year/Month/Date): 2009-02-18
An example to show how class Hierarchy can be persisted in Database using Hibernate

Please be informed that NONE of the design/code from this
page is claiming to be some sort of best practices and we DO NOT expect
any of our visitor/reader of this page to assume this as some sort of
best practice for any context and should not be using this 
as it is without appropriate evaluation to their, so to say, 
specific programming context.

This page intends only to provide bit and piece of known ways  for
doing some sort of example and may not be fit for any other purpose.


Hibernate Qustion explained as follows:

Have you ever used a parent and child object hierarchy of domain objects
with Hibernate for persisting into database?

Yes, it is very much possible by using joined-subclass tag in HBM file.

Hibernate has a provision to map object hierarchy to different tables
with primary and foreign key relationship.

For example, let us discuss a very simple example to demonstrate this
very solution to the above question.



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Suppose a person can have a single contact and each contact can have
single address. And the domain representation to this case study is
as follows:

Person is the super class, Contact extends Person and Address extends
Contact.

Database tables are as follows (for HSQLDB as database):

create table Person
( person_id varchar(10) not null,
  name varchar(100) not null,
  age integer not null,
  passport_number varchar(10),
  primary key (person_id));

create table Contact
( contact_id varchar(10) not null,
  phone varchar(10) not null,
  email_id varchar(20) not null,
  person_id varchar(10),
  primary key(contact_id),
  foreign key (person_id)
  references Person(person_id));

create table Address
( address_id varchar(10) not null,
  addr_line_1 varchar(50) not null,
  addr_line_2 varchar(50) not null,
  addr_line_3 varchar(50) not null,
  city varchar(20) not null,
  state varchar(20) not null,
  pin integer,
  contact_id varchar(10),
  primary key(address_id),
  foreign key(contact_id)
  references Contact(contact_id));


Suppose domain Java class files for this case study are as follows:

Person.java
 
package demo.profile;

import java.io.Serializable;

public class Person implements Serializable{
private String personId;
private String name;
private int age;
private String passportNumber;
public int getAge() {
	return age;
}
public void setAge(int age) {
	this.age = age;
}
public String getName() {
	return name;
}
public void setName(String name) {
	this.name = name;
}
public String getPassportNumber() {
	return passportNumber;
}
public void setPassportNumber(String passportNumber) {
	this.passportNumber = passportNumber;
}
public String getPersonId() {
	return personId;
}
public void setPersonId(String personId) {
	this.personId = personId;
}
}
 
This Person class is the Super class in the domain hierarchy.

Other class files are Contact and Address, Contact extends Person
and Address extends Contact, as seen below:

Contact.java
 
package demo.profile;

import java.io.Serializable;

public class Contact extends Person implements Serializable {

	/**
	 *
	 */
	private String contactId;
	private int phoneNumber;
	private String emailId;
	public String getEmailId() {
		return emailId;
	}
	public void setEmailId(String emailId) {
		this.emailId = emailId;
	}
	public int getPhoneNumber() {
		return phoneNumber;
	}
	public void setPhoneNumber(int phoneNumber) {
		this.phoneNumber = phoneNumber;
	}
	public String getContactId() {
		return contactId;
	}
	public void setContactId(String contactId) {
		this.contactId = contactId;
	}
}
 

Address class extends Contact :

Address.java
 
package demo.profile;

import java.io.Serializable;

public class Address extends Contact implements Serializable {

	/**
	 *
	 */
	private String addressId;
	private String addrLine1;
	private String addrLine2;
	private String addrLine3;
	private String city;
	private String state;
	private int pin;
	public String getAddrLine1() {
		return addrLine1;
	}
	public void setAddrLine1(String addrLine1) {
		this.addrLine1 = addrLine1;
	}
	public String getAddrLine2() {
		return addrLine2;
	}
	public void setAddrLine2(String addrLine2) {
		this.addrLine2 = addrLine2;
	}
	public String getAddrLine3() {
		return addrLine3;
	}
	public void setAddrLine3(String addrLine3) {
		this.addrLine3 = addrLine3;
	}
	public String getCity() {
		return city;
	}
	public void setCity(String city) {
		this.city = city;
	}
	public int getPin() {
		return pin;
	}
	public void setPin(int pin) {
		this.pin = pin;
	}
	public String getState() {
		return state;
	}
	public void setState(String state) {
		this.state = state;
	}
	public String getAddressId() {
		return addressId;
	}
	public void setAddressId(String addressId) {
		this.addressId = addressId;
	}
}

 

All these above mentioned Java class files are plain POJO
with private fields and public getters and setters.

Now we have table and POJO Java files ready with us. Let us
start to write mapping files or the HBM file for Hibernate
to load while creation of Hibernate SessionFactory.

Just look at the following HBM file and we shall go through
all these tags one by one.

profile.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 package="demo.profile">
  <class name="Person" table="Person">
        <id name="personId" access="property" column="person_id"/>

        <property name="name" column="name"/>
        <property name="age" column="age"/>
        <property name="passportNumber" column="passport_number"/>


  <joined-subclass name="Contact" table="Contact">
          <key column="person_id"/>
          <property name="contactId" column="contact_id"/>
          <property name="phoneNumber" column="phone"/>
          <property name="emailId" column="email_id"/>

          <joined-subclass name="Address" table="Address">
                <key column="contact_id"/>
                <property name="addressId" column="address_id"/>
                <property name="addrLine1" column="addr_line_1"/>
                <property name="addrLine2" column="addr_line_2"/>
                <property name="addrLine3" column="addr_line_3"/>
                <property name="city" column="city"/>
                <property name="state" column="state"/>
                <property name="pin" column="pin"/>

          </joined-subclass>

  </joined-subclass>
  </class>
</hibernate-mapping>
 

If you look at the above set of Tags, there is a class tag,
and within class tag, there is joined-subclass and within joined-subclass
there is another joined-subclass tag to accomodate all the three
POJO class files. Each class file has a table of its own, with a primary
and a foreign key references.

Once these domain Java files are compiled and the test client is written
to test this mapping, we may take reference of following client class:

Client.java
 
package demo.profile;

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



public class Client {

    private static final SessionFactory sessionFactory;

    static {
        try {
            // Create the SessionFactory from hibernate.cfg.xml
            sessionFactory = new Configuration().configure().buildSessionFactory();
        } catch (Throwable ex) {
            // Make sure you log the exception, as it might be swallowed
            System.err.println("Initial SessionFactory creation failed." + ex);
            throw new ExceptionInInitializerError(ex);
        }
    }

    public static SessionFactory getSessionFactory() {
        return sessionFactory;
    }
    public static void createRecord()
    {
		System.out.println(getSessionFactory());
		Session session = getSessionFactory().openSession();
		Transaction trx = session.beginTransaction();
		trx.begin();

		Address address = new Address();
		address.setPersonId("P004");
		address.setName("girish");
		address.setAge(23);
		address.setPassportNumber("PPP");
		address.setContactId("P004");
		address.setPhoneNumber(234323);
		address.setEmailId("test@test.com");

		address.setAddressId("P004");
		address.setAddrLine1("123 street");
		address.setAddrLine2("JB Sawant Marg");
		address.setAddrLine3("street marg");
		address.setCity("Pune");
		address.setState("Maharastra");
		session.save(address);

		trx.commit();
		session.close();
    }
    public static void readRecord()
    {
		Session session = getSessionFactory().openSession();
		Contact contact = (Contact) session.get(Contact.class,"P003");
		System.out.println(((Person)contact).getPassportNumber());
		session.close();
    }
    public static void deleteRecord()
    {
		Session session = getSessionFactory().openSession();
		Transaction trx = session.beginTransaction();
		trx.begin();

		Contact contact = (Contact) session.get(Contact.class,"P001");
		session.delete(contact);
		trx.commit();
		session.close();
		System.out.println("deleted...");

    }
	/**
	 * @param args
	 */
	public static void main(String[] args) {

		//createRecord();
		readRecord();
		//deleteRecord();
	}

}

 
This client is having three methods, one for the creation of records,
second is to read record and third one is to delete appropriate record.
One has to change appropriate values at many places to be able to
run and see this case study working.

I have placed the hibernate.cfg.xml file for your reference only.
One has to change certain settings like database, file path to HBM file
etc.

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>

        <!-- Database connection settings -->
        <property name="connection.driver_class">
                  org.hsqldb.jdbcDriver
        </property>
        <property name="connection.url">
                  jdbc:hsqldb:file:D:/hibernate-sample-local/project/db/demo
        </property>
        <property name="connection.username">sa</property>
        <property name="connection.password"></property>

        <!-- JDBC connection pool (use the built-in) -->
        <property name="connection.pool_size">1</property>

        <!-- SQL dialect -->
        <property name="dialect">
                  org.hibernate.dialect.HSQLDialect
        </property>

        <!-- Enable Hibernate's automatic session context management -->
        <property name="current_session_context_class">
                  thread
        </property>

        <!-- Disable the second-level cache  -->
        <property name="cache.provider_class">
                  org.hibernate.cache.NoCacheProvider
        </property>

        <!-- Echo all executed SQL to stdout -->
        <property name="show_sql">true</property>

        <!-- Drop and re-create the database schema on startup
        <property name="hbm2ddl.auto">create</property> -->

        <mapping resource="demo/profile/profile.hbm.xml"/>

    </session-factory>

</hibernate-configuration>



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