Inheritance mapping in hibernate : Single Table strategy

Hibernate provides the following approaches to mapping inheritance hierarchies into the database.

• One table for each class hierarchy
• One table for each subclass
• One table for each concrete class implementation 
• Implicit polymorphism

Single Table :

In this approach all attributes of all the classes are available in a single table as its columns. A discriminator field is used to identify classes. In this strategy  we have to ensure that no one field must be conflict with field of super class. Because if there is a filed named x in super class as well as in subclass, hibernate Will not be able to map them to table. so we have to rename such type of filed these are conflicting with the same name.

In this example, I am taking Employee class as a base class and two subclasses FullTimeEmployee and PartTimeEmploye of Employee class.
Here I am mapping the all field of all classes Hierarchy into a single table using JPA annotation.

Employee class
In this class I have to provide the @Inheritance  and @DiscriminatorColumn annotations. Rest of annotation remains as before.
@Inheritance(strategy=InheritanceType.SINGLE_TABLE)
    This annotation specify that this class is being inherited in other classes. The data mapping strategy type is single table.
@DiscriminatorColumn(name="EMP_TYPE" , discriminatorType=DiscriminatorType.STRING)
    In this annotation we have to provide the name of extra column to be created to mark the row of table to which it belongs. This field is used to differentiate the data for subclass.

Annotation in sub class :
Subclass is annotated with @DiscriminatorValue annotation spiffing the value to be placed in the discrimination column (i.e. EMP_TYPE) to prove that if any column has this value in discriminator column, the data of that row belongs to this class.
Example :

@DiscriminatorValue("PT")
public class PartTimeEmployee extends Employee{

Example Source Code :

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package domain;

import javax.persistence.*;
@Enti
@Inheritance(strategy=InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name="EMP_TYPE" , discriminatorType=DiscriminatorType.STRING)
public class Employee{
    private int empId;
    private String name;
    private String email;
    @Id @GeneratedValue
    public int getEmpId() {
        return empId;
    }
    public void setEmpId(int empId) {
        this.empId = empId;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getEmail() {
        return email;
    }
    public void setEmail(String email) {
        this.email = email;
    }
    
}


package domain;
import javax.persistence.*;
@Entity
@DiscriminatorValue("FT")
public class FullTimeEmployee extends Employee{
    private String designation;
    private String empCode;
    public String getDesignation() {
        return designation;
    }
    public void setDesignation(String designation) {
        this.designation = designation;
    }
    public String getEmpCode() {
        return empCode;
    }
    public void setEmpCode(String empCode) {
        this.empCode = empCode;
    }
    @Override
    public String toString() {
        return "FullTimeEmployee [designation=" + designation + ", empCode="
                + empCode + ", getEmpId()=" + getEmpId() + ", getName()="
                + getName() + ", getEmail()=" + getEmail() + "]";
    }
    
}


package domain;
import javax.persistence.*;
@Entity
@DiscriminatorValue("PT")
public class PartTimeEmployee extends Employee{
    private int duration;
    private String reportTo;
    public int getDuration() {
        return duration;
    }
    public void setDuration(int duration) {
        this.duration = duration;
    }
    public String getReportTo() {
        return reportTo;
    }
    public void setReportTo(String reportTo) {
        this.reportTo = reportTo;
    }
    @Override
    public String toString() {
        return "PartTimeEmployee [duration=" + duration + ", reportTo="
                + reportTo + ", getEmpId()=" + getEmpId() + ", getName()="
                + getName() + ", getEmail()=" + getEmail() + "]";
    }
     
}

Main class to be used to test

import org.hibernate.Session;
import org.hibernate.cfg.AnnotationConfiguration;

import domain.FullTimeEmployee;
import domain.PartTimeEmployee;

public class Operations {
    public static void create() {
        System.out.println("############## Create ##############");
        Session session = new AnnotationConfiguration().configure()
                .buildSessionFactory().openSession();
        session.beginTransaction();
        PartTimeEmployee pe = new PartTimeEmployee();
        pe.setName("abc");
        pe.setEmail("abc@rmail.com");
        pe.setDuration(4);
        pe.setReportTo("PM");
        session.save(pe);

        FullTimeEmployee fe = new FullTimeEmployee();
        fe.setName("ssd");
        fe.setDesignation("manager");
        fe.setEmail("mail@email.com");
        fe.setEmpCode("86789");
        session.save(fe);

        session.getTransaction().commit();
    }

    public static void read() {
        System.out.println("############## Read ##############");
        Session session = new AnnotationConfiguration().configure()
                .buildSessionFactory().openSession();
        session.beginTransaction();
        System.out.println("########### load contact and person");
        PartTimeEmployee pe = (PartTimeEmployee) session.load(
                PartTimeEmployee.class, 1);
        FullTimeEmployee fe = (FullTimeEmployee) session.load(
                FullTimeEmployee.class, 2);
        System.out.println(pe);
        System.out.println(fe);
        session.getTransaction().commit();
    }

    public static void update() {
        System.out.println("############## Update ##############");
        Session session = new AnnotationConfiguration().configure()
                .buildSessionFactory().openSession();
        // TODO
        session.getTransaction().commit();
    }

    public static void delete() {
        System.out.println("############## Delete ##############");
        Session session = new AnnotationConfiguration().configure()
                .buildSessionFactory().openSession();
        session.beginTransaction();
        // TODO
        session.getTransaction().commit();
    }

    public static void main(String[] args) {
        create();
        read();
        // update();
        // delete();
        // Exceptions
    }
}

<?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">com.mysql.jdbc.Driver</property>
        <property name="connection.url">jdbc:mysql://localhost:3306/test</property>
        <property name="connection.username">root</property>
        <property name="connection.password" />
        <property name="connection.pool_size">1</property>
        <property name="dialect">org.hibernate.dialect.MySQLDialect</property>
        <property name="current_session_context_class">thread</property>
        <property name="cache.provider_class">org.hibernate.cache.NoCacheProvider</property>
        <property name="show_sql">true</property>
        <property name="hbm2ddl.auto" >update</property>
        <mapping class="domain.Employee"/>
        <mapping class="domain.PartTimeEmployee"/>
        <mapping class="domain.FullTimeEmployee"/>
    </session-factory>
</hibernate-configuration>

Employee Table