Table per Class Hierarchy Using Annotation

 
In our previous tutorial we got introduced to Inheritance Mapping In Hibernate, In this article I will explain you about Table per Class Hierarchy Inheritance mapping. By this inheritance strategy, we can map the whole hierarchy in a single table. Here, an extra column otherwise known as discriminator column is created in the table to identify the class. In the table, for each record some columns will be empty; those columns for which the particular Java class does not have fields.
 

 
The above is the Hierarchy of classes involved. Here Employee is the super class for PermanentEmployee and ContractEmployee classes. Now Let us create Java classes for the above hierarchy to implement
 

Entity class

 
File: Employee.java

package entity;

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 = "EMPLOYEE")
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name = "DTYPE", discriminatorType = DiscriminatorType.STRING)
@DiscriminatorValue(value = "")
public class Employee {

	@Id
	@GeneratedValue
	@Column(name = "EMPID")
	private int empID;

	@Column(name = "EMP_NAME")
	private String empName;

	public int getEmpID() {
		return empID;
	}

	public String getEmpName() {
		return empName;
	}

	public void setEmpID(int empID) {
		this.empID = empID;
	}

	public void setEmpName(String empName) {
		this.empName = empName;
	}
}

 
In case of table per class hierarchy a discriminator column is added by the hibernate framework that specifies the type of the record. It is mainly used to identify which derived class object have been saved in the table (see Database screen shot for better understanding). To specify this, following annotation where used in Super class entity.
 
@Inheritance – Defines the inheritance strategy to be used for an entity class hierarchy. It is specified on the entity class that is the root of the entity class hierarchy.

@DiscriminatorColumn – Is used to define the discriminator column for the SINGLE_TABLE and JOINED inheritance mapping strategies. The strategy and the discriminator column are only specified in the root of an entity class hierarchy or sub hierarchy in which a different inheritance strategy is applied

If the @DiscriminatorColumn annotation is missing, and a discriminator column is required, the name of the discriminator column defaults to “DTYPE” and the discriminator type to DiscriminatorType.STRING.

@DiscriminatorValue – Is used to specify the value of the discriminator column for entities of the given type. The DiscriminatorValue annotation can only be specified on a concrete entity class. If the DiscriminatorValue annotation is not specified and a discriminator column is used, a provider-specific function will be used to generate a value representing the entity type. If the DiscriminatorType is STRING, the discriminator value default is the entity name.

The inheritance strategy and the discriminator column are only specified in the root of an entity class hierarchy or subhierarchy in which a different inheritance strategy is applied. The discriminator value, if not defaulted, should be specified for each entity class in the hierarchy.
 
File: PermanentEmployee.java

package entity;

import javax.persistence.Column;
import javax.persistence.DiscriminatorValue;
import javax.persistence.Entity;
import javax.persistence.Table;

@Entity
@Table(name = "EMPLOYEE")
@DiscriminatorValue("Permanent Employee")
public class PermanentEmployee extends Employee {

	@Column(name = "COMPANY_NAME")
	private String companyName;

	public String getCompanyName() {
		return companyName;
	}

	public void setCompanyName(String companyName) {
		this.companyName = companyName;
	}
}

 
PermanentEmployee class is child of Employee class. Thus while specifying the mappings, we used @DiscriminatorValue to specify discriminator value. In our case “Permanent Employee” will be persisted in discriminator column.
 
File: ContractEmployee.java

package entity;

import javax.persistence.Column;
import javax.persistence.DiscriminatorValue;
import javax.persistence.Entity;
import javax.persistence.Table;

@Entity
@Table(name = "EMPLOYEE")
@DiscriminatorValue("Contract Employee")
public class ContractEmployee extends Employee {

	@Column(name = "CONTRACTOR_NAME")
	private String contractorName;

	public String getContractorName() {
		return contractorName;
	}

	public void setContractorName(String contractorName) {
		this.contractorName = contractorName;
	}
}

Hibernate Configuration file

	
	oracle.jdbc.driver.OracleDriver
	system
	admin
	jdbc:oracle:thin:@xxx.x.x.x:1521:XE

	
	org.hibernate.dialect.Oracle10gDialect

	
	true

	
	create
	
       
       
       

 
The hbm2ddl.auto property is defined for creating automatic table in the database.
 

Client program

 
Create the class that stores the persistent object in this class, we are simply storing the employee objects in the database.
 

package util;

import entity.ContractEmployee;
import entity.PermanentEmployee;

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.boot.registry.StandardServiceRegistryBuilder;
import org.hibernate.cfg.Configuration;
import org.hibernate.service.ServiceRegistry;

public class HibernateUtil {

	public static void main(String[] args) {

		Configuration cf = new Configuration().configure("hibernate.cfg.xml");

		StandardServiceRegistryBuilder srb = new StandardServiceRegistryBuilder();
		srb.applySettings(cf.getProperties());
		ServiceRegistry sr = srb.build();
		SessionFactory sf = cf.buildSessionFactory(sr);

		Session session = sf.openSession();

		PermanentEmployee p1 = new PermanentEmployee();
		p1.setEmpID(1);
		p1.setEmpName("Ameer");
		p1.setCompanyName("CTS");

		PermanentEmployee p2 = new PermanentEmployee();
		p2.setEmpID(2);
		p2.setEmpName("Lourde");
		p2.setCompanyName("TCS");

		// create two objects of ContractEmployee
		ContractEmployee t1 = new ContractEmployee();
		t1.setEmpID(3);
		t1.setEmpName("Prabhu");
		t1.setContractorName("ABD Consultancy");

		ContractEmployee t2 = new ContractEmployee();
		t2.setEmpID(4);
		t2.setEmpName("Badru");
		t2.setContractorName("MN Consultancy");

		Transaction tx = session.beginTransaction();

		session.save(p1);
		session.save(p2);
		session.save(t1);
		session.save(t2);

		tx.commit();
		System.out.println("Object saved successfully !");
		session.close();
		sf.close();
	}
}

Eclipse Console

Database output