Using table per class hierarchy

When using this inheritance strategy, the class hierarchy is represented by multiple classes combined into one de-normalized DB table. A discriminator column identifies the type and the subclass.
Example:

package com.sample
public class Vehicle {

    long id;
    int noOfTyres;
    private String colour;
   // Constructors and getters/setters
}
package com.sample
public class Car extends Vehicle {


    private String licensePlate;
    long price;
    int speed;
    // Constructors and getters/setters
}

And here’s the Database view which contains the DISCRIMINATOR field which will contain either “C” or “V” depending if you are inserting a new Car or a new Vehicle.

Here’s the hibernate configuration:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
 
<hibernate-mapping package="com.sample">
 
    <class name="Vehicle" table="VEHICLE" discriminator-value="V">
        <id name="id" column="VEHICLE_ID">
            <generator class="native" />
        </id>
 
        <discriminator column="DISCRIMINATOR" type="string" />
 
        <property name="noOfTyres" type="integer" column="NO_OF_TYRES" />
        <property name="colour" type="string"  />
 
        <subclass name="Car" extends="Vehicle" discriminator-value="C">
                <property name="licensePlate" column="LICENSE_PLATE" />
                <property name="price" type="long"   />
                <property name="speed" type="integer"   />
        </subclass>
    </class>
</hibernate-mapping>

This is the same, using JPA Annotations:

import javax.persistence.Column;
import javax.persistence.DiscriminatorColumn;
import javax.persistence.DiscriminatorType;
import javax.persistence.DiscriminatorValue;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.Inheritance;
import javax.persistence.InheritanceType;
import javax.persistence.Table;
 
 
@Entity
@Table(name = "VEHICLE")
@Inheritance(strategy=InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(
    name="discriminator",
    discriminatorType=DiscriminatorType.STRING
)
@DiscriminatorValue(value="V")
public class Vehicle {
 
    @Id
    @GeneratedValue
    @Column(name = "VEHICLE_ID")
    private Long id;
     
    @Column(name="NO_OF_TYRES")
    int noOfTyres;
     
    @Column
    private String colour;
     
    // Constructors and Getter/Setter methods,
}

 
import javax.persistence.Column;
import javax.persistence.DiscriminatorValue;
import javax.persistence.Entity;
import javax.persistence.Table;
 
@Entity
@Table(name="VEHICLE")
@DiscriminatorValue("C")
public class Car extends Vehicle {

    @Column(name="license_plate")
    private String licensePlate;

    @Column
    long price;

    @Column
    int speed;


    // Constructors and Getter/Setter methods,
}

In the following example we will insert at first a Vehicle class (will into the VEHICLE table using the discriminator “V”), then we will insert a Car Object into the VEHICLE table (using the discriminator “C”)

 
        SessionFactory sf = HibernateUtil.getSessionFactory();
        Session session = sf.openSession();
        session.beginTransaction();
 
        // Will use discriminator V 
        Vehicle v = new Vehicle(1, 4, "red");
        session.save(v);
        // Will use discriminator C
        Car car = new Car("AB123456", 15000l, 200);
        session.save(c);
         
        session.getTransaction().commit();
        session.close();