Hibernate – Table Per Hierarchy using XML File

Hibernate is capable of storing the inherited properties of an object along with its new properties in its database when an object is saved in the database. In Hibernate, inheritance between POJO classes is applied when multiple POJO classes of a module contain some common properties. In a real-time application, POJO classes of Hibernate are designed based on database table design. If more than one POJO class has some common properties then those common properties are separated into a POJO class called the base class and the properties which are un-common are stored in the derived class. This is the concept of Hibernate Inheritance mechanism.

Hibernate Inheritance Mapping

Object-oriented can model both “is a” and “has a” relationship. The relational model supports only the “has a” relationship between two entities. Hibernate helps in mapping such Objects with relational tables. There are three inheritance mapping strategies defined in the Hibernate.

• Table Per Hierarchy
• Table Per Concrete class
• Table Per Subclass

Table Per Hierarchy (XML mapping)

Table per Hierarchy is one of the inheritance strategies in hibernate. In this strategy, the entire hierarchy is mapped to a single table. All attributes of all the classes in the hierarchy are stored in a single table. In a table per Hierarchy strategy:

• Only one table is created in the database for storing data of the entire class hierarchy.
• Hibernate stores a string in the database apart from the data of an object for indicating which derived class object has inserted this data of a row.
• Hibernate needs an additional column in the table called a discriminator column for placing or storing the discriminator value of a subclass object.
• To inform the Hibernate we configure <sub-class> tag under <class> tag in hbm.xml file.

In this strategy, null values will be stored in the table for which there is no column applicable. A discriminator uniquely identifies the base type of the class hierarchy.

Example for Table per Hierarchy Strategy

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.

Hierarchy of Classes

We have 3 tables Employee, P_Employee, and C_Employee. The Employee is the superclass for P_Employee and C_Employee classes. We will store all the attributes of all 3 classes in a single table.

Creating  Database Table to persist class Hierarchy:

CREATE TABLE `Employee` (

       `id` BIGINT(20) NOT NULL AUTO_INCREMENT,

      `name` VARCHAR(50) NOT NULL DEFAULT ‘0’,

      `age` BIGINT(3) NOT NULL DEFAULT ‘0’,

     `salary` BIGINT(11) NULL DEFAULT NULL,

     `hourlyrate` BIGINT(11) NULL DEFAULT NULL,

    `duration` BIGINT(11) NULL DEFAULT NULL,

    `discriminator` VARCHAR(20) NOT NULL,

     PRIMARY KEY (`id`)

)

The Employee table will store objects of all three classes Employee, P_Employee and C_Employee.

Project Structure (IntelliJ IDEA)  for XML mapping:

Project Structure

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

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; }
}

Below is the 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;
    }
}

Below is the 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;
    }
}

Creating the mapping file for the Persistent class:

Below is the 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="Employee1" discriminator-value="E">
        <id name="id">
            <generator class="increment"></generator>
        </id>

        <discriminator column="discriminator" type="string"></discriminator>
        <property name="name"></property>
        <property name="age"></property>

        <subclass name="com.exploit.model.P_Employee" extends="Employee" discriminator-value="PE">
            <property name="salary"></property>
        </subclass>

        <subclass name="com.exploit.model.C_Employee" extends="Employee" discriminator-value="CE">
            <property name="hourlyRate"></property>
            <property name="duration"></property>
        </subclass>

    </class>

</hibernate-mapping>

We have defined only one hibernate mapping (hbm) file employee.hbm.xml, both P_Employee and C_Employee model classes are defined within one hbm.xml file. The <subclass> tag is used to map both the subclasses. We have defined discriminators as:  

• “E” to identify that record belongs to Employee class.
• “PE” to identify that record belongs to P_Employee class.
• “CE” to identify that record belongs to C_Employee class.

Hibernate will put the appropriate discriminator values based on the record we are persisting.

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

Below is the 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://www.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">toor</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>

Following are the dependencies used in pom.xml file:

XML

<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0
                             http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>TablePerHierarchyXML</groupId>
    <artifactId>TablePerHierarchyXML</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <packaging>jar</packaging>

    <name>TablePerHierarchyXML</name>
    <url>http://maven.apache.org</url>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    </properties>

    <dependencies>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>3.8.1</version>
            <scope>test</scope>
        </dependency>

        <!-- https://mvnrepository.com/artifact/org.hibernate/hibernate-core -->
        <dependency>
            <groupId>org.hibernate</groupId>
            <artifactId>hibernate-core</artifactId>
            <version>5.2.6.Final</version>
        </dependency>

        <!-- https://mvnrepository.com/artifact/mysql/mysql-connector-java -->
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>6.0.5</version>
        </dependency>
    </dependencies>
</project>

Creating the class that stores the persistent object:

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("KirikoChan");
        employee.setAge(19);

        // Creating Permanent Employee subclass record
        P_Employee permanentEmployee = new P_Employee();
        permanentEmployee.setName("Saili.H");
        permanentEmployee.setAge(20);
        permanentEmployee.setSalary(30000);

        // Creating Contract Employee subclass record
        C_Employee contractEmployee = new C_Employee();
        contractEmployee.setName("ChikkoRita");
        contractEmployee.setAge(18);
        contractEmployee.setDuration(8.5);
        contractEmployee.setHourlyRate(2000);

        // 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.");
    }
}

This is the usual way of mapping Table Per Hierarchy using XML.