Hibernate - Table Per Subclass Example using XML File

Table Per Subclass mapping in Hibernate stores each class in the hierarchy in a separate table, linked through primary key–foreign key relationships. It ensures normalized data storage by avoiding duplication and maintaining clear parent–child associations.

Separate tables for each class with relationships maintained via foreign keys
Uses <joined-subclass> in XML mapping to define subclass relationships
No data duplication, as subclass tables store only their specific fields and link to parent table

Example for Table Per Subclass strategy (XML mapping)

Let's suppose we have a class Employee with subclasses as P_Employee and C_Employee. Following the class diagram and relationship of these classes.

Steps of Implementation

Step 1: Create Maven Project

Open IntelliJ IDEA -> New Project -> Maven
Provide GroupId and ArtifactId

Add dependencies(pom.xml)

XML

<dependencies>

    <dependency>
        <groupId>org.hibernate</groupId>
        <artifactId>hibernate-core</artifactId>
        <version>5.2.6.Final</version>
    </dependency>

    <dependency>
        <groupId>mysql</groupId>
        <artifactId>mysql-connector-java</artifactId>
        <version>6.0.5</version>
    </dependency>

</dependencies>

project Structure

Step 2: Create Database Tables

There are three tables: Employee, P_Employee, and C_Employee. Common data is stored in the Employee table, while subclass-specific data is stored in their respective tables, linked via foreign keys.

Creating Database Table to persist Concrete classes:

CREATE TABLE `Employee` (
`id` BIGINT(20) NOT NULL AUTO_INCREMENT,
`name` VARCHAR(50) NOT NULL DEFAULT '0',
`age` BIGINT(3) NOT NULL DEFAULT '0',
PRIMARY KEY (`id`)
)
CREATE TABLE `P_Employee` (
`id` BIGINT(20) NOT NULL AUTO_INCREMENT,
`salary` BIGINT(11) NULL DEFAULT NULL,
PRIMARY KEY (`id`)
CONSTRAINT `ForK_Employee` FOREIGN KEY (`id`) REFERENCES `Employee` (`id`)
)
CREATE TABLE `C_Employee` (
`id` BIGINT(20) NOT NULL AUTO_INCREMENT,
`hourlyrate` BIGINT(11) NULL DEFAULT NULL,
`duration` BIGINT(11) NULL DEFAULT NULL,
PRIMARY KEY (`id`)
CONSTRAINT `ForK_Employee2` FOREIGN KEY (`id`) REFERENCES `Employee` (`id`)
)

Step 3: Create Model Classes

Creating the Employee, P_Employee, and C_Employee classes for the above hierarchy.

Inheritance is implemented using extends
Classes are simple POJO classes
No annotations (XML mapping is used)

Below is the implementation of the Employee.java file:

Java

package com.exploit.model;

public class Employee {
    private int id;
    private String name;
    private int age;

    public int getId() { return id; }

    public void setId(int id) { this.id = id; }

    public String getName() { return name; }

    public void setName(String name) { this.name = name; }

    public int getAge() { return age; }

    public void setAge(int age) { this.age = age; }
}

Implementation of the P_Employee.java file:

Java

package com.exploit.model;

public class P_Employee extends Employee {
    private double salary;

    public double getSalary() { return salary; }

    public void setSalary(double salary)
    {
        this.salary = salary;
    }
}

Implementation of the C_Employee.java file:

Java

package com.exploit.model;

public class C_Employee extends Employee {
    private double hourlyRate;
    private double duration;

    public double getDuration() { return duration; }

    public void setDuration(double duration)
    {
        this.duration = duration;
    }

    public double getHourlyRate() { return hourlyRate; }

    public void setHourlyRate(double hourlyRate)
    {
        this.hourlyRate = hourlyRate;
    }
}

Step 4: Create Mapping File

Creating the mapping file for the Persistent class

<class> defines the base class
<joined-subclass> is used for Table Per Subclass strategy
<key> defines foreign key relationship
Data is split across multiple tables

Implementation of the employee.hbm.xml file:

XML

<?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.exploit.model">
    <class name="Employee" table="Employee">
        <id name="id" column="Id">
            <generator class="native"></generator>
        </id>
        <property name="name" column="Name"></property>
        <property name="age" column="Age"></property>

        <joined-subclass name="com.exploit.model.P_Employee" table="P_Employee">
            <key column="Id"/>
            <property name="salary" column="Salary"></property>
        </joined-subclass>

        <joined-subclass name="com.exploit.model.C_Employee" table="C_Employee">
            <key column="Id"/>
            <property name="hourlyRate" column="HourlyRate"></property>
            <property name="duration" column="Duration"></property>
        </joined-subclass>

    </class>

</hibernate-mapping>

Step 5: Configure Hibernate

Adding the mapping of hbm.xml file in the hibernate configuration file.

Configuration file connects Hibernate to database
Mapping file must be registered
SQL queries can be logged

Implementation of the hibernate.cfg.xml file:

XML

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-configuration PUBLIC
        "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
        "http://hibernate.org/dtd/hibernate-configuration-3.0.dtd">

<hibernate-configuration>

    <session-factory>

        <!-- Database connection properties -->
        <property name="connection.driver_class">com.mysql.jdbc.Driver</property>
        <property name="connection.url">jdbc:mysql://localhost/javainsimpleway</property>
        <property name="connection.username">root</property>
        <property name="connection.password">root</property>

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

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

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

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

        <!-- Format the generated Sql -->
        <property name="format_sql">true</property>

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

        <mapping resource="com/exploit/mapping/employee.hbm.xml"/>

    </session-factory>

</hibernate-configuration>

Step 6: Create Main Class

Creating the class that stores the persistent object.

Parent data stored in Employee table
Subclass data stored in respective tables
Hibernate manages relationships automatically

Below is the implementation of the Main.java file:

Java

package com.exploit.db;

import com.exploit.model.C_Employee;
import com.exploit.model.Employee;
import com.exploit.model.P_Employee;
import com.exploit.util.HibernateUtil;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;

public class Main {
    public static void main(String[] args)
    {
        // Get session factory using Hibernate Util class
        SessionFactory sessionFactory
            = HibernateUtil.getSessionFactory();
      
        // Get session from Session factory
        Session session = sessionFactory.openSession();

        // Begin transaction
        Transaction transaction
            = session.beginTransaction();

        // Creating Employee base class record
        Employee employee = new Employee();
        employee.setName("ChikkoRita");
        employee.setAge(19);

        // Creating Permanent Employee subclass record
        P_Employee permanentEmployee = new P_Employee();
        permanentEmployee.setName("Saili.H");
        permanentEmployee.setAge(18);
        permanentEmployee.setSalary(30000);

        // Creating Contract Employee subclass record
        C_Employee contractEmployee = new C_Employee();
        contractEmployee.setName("KirikoChan");
        contractEmployee.setAge(19);
        contractEmployee.setHourlyRate(2000);
        contractEmployee.setDuration(8.5);

        // persist all the employee records
        session.persist(employee);
        session.persist(permanentEmployee);
        session.persist(contractEmployee);

        // Commit the transaction and close the session
        transaction.commit();
        session.close();
        System.out.println(
            "Employee records successfully persisted");
    }
}

We have defined only one hibernate mapping (hbm) file Employee.hbm.xml. Both C_Employee and P_Employee model classes are defined within one hbm.xml file. This is the usual way of mapping Table Per Subclass using XML.

Hibernate - Table Per Subclass Example using XML File

Example for Table Per Subclass strategy (XML mapping)

Steps of Implementation

Step 1: Create Maven Project

Step 2: Create Database Tables

Step 3: Create Model Classes

Step 4: Create Mapping File

Step 5: Configure Hibernate

Step 6: Create Main Class

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