Table Per Concreteclass Example Hibernate

Table-per-concreteclass Hierarchy

Note: Before going through this application, it is advised to read Hibernate Mapping Tutorial and also Table per class hierarchy.

In this Table Per Concreteclass style, two tables for each subclass are created. The super class variables are placed in each subclass. Observe the screenshots at the end of the notes.

The three tables of Table-per-subclass do not exist. Here tables change (only two tables exist). Let hbm2ddl.auto create the required tables.

The same three programs of Table per class hierarchy Employee, PermanentEmployee and TemporaryEmployee are used and also the same client program is used. Configuration file does not change but mapping file changes.

Following is the Hierarchy of classes involved. Output screens are shown the end of this notes.

Let us create Java classes for the above hierarchy.

1. File Name: Employee.java

public class Employee {
	
  private short empId;
  private String empName;

  public Employee() {
    // TODO Auto-generated constructor stub
  }
  public short getEmpId() {
    return empId;
  }
  public void setEmpId(short empId) {
    this.empId = empId;
  }
  public String getEmpName() {
    return empName;
  }
  public void setEmpName(String empName) {
    this.empName = empName;
  }   
}

2. File Name: PermanentEmployee.java

public class PermanentEmployee extends Employee {
  private String designation;
  private String department;

  public PermanentEmployee() {
    // TODO Auto-generated constructor stub
  }
  public String getDesignation() {
    return designation;
  }
  public void setDesignation(String designation) {
    this.designation = designation;
  }
  public String getDepartment() {
    return department;
  }
  public void setDepartment(String department) {
    this.department = department;
  }   
}

3. File Name: TemporaryEmployee.java

public class TemporaryEmployee extends Employee {
	
  private short workingDays;
  String contractorName;

  public TemporaryEmployee() {
    // TODO Auto-generated constructor stub
  }
  public short getWorkingDays() {
    return workingDays;
  }
  public void setWorkingDays(short workingDays) {
    this.workingDays = workingDays;
  }
  public String getContractorName() {
    return contractorName;
  }
  public void setContractorName(String contractorName) {
    this.contractorName = contractorName;
  }    
} 

4. File Name: hibernate.cfg.xml

  org.hibernate.dialect.Oracle9Dialect
  jdbc:oracle:thin:@localhost:1521:orcl1
  scott
  tiger
  oracle.jdbc.driver.OracleDriver
  SNRaoConnection
  update
  true

  

5. File Name: Employee.hbm.file

Observe, the element used in table-per-concrete class is <union-subclass>

6. Client Program – TablePerConcreteClass.java

import org.hibernate.*;
import org.hibernate.cfg.*;

public class TablePerConcreteClass 
{
  public static void main(String[] args) 
  {
    Configuration cfg = new Configuration();
    cfg.configure("hibernate.cfg.xml"); 
		 
    SessionFactory sf = cfg.buildSessionFactory();
    Session session = sf.openSession();

    PermanentEmployee p1 = new PermanentEmployee();
    p1.setEmpId((short)100);
    p1.setEmpName("S N Rao");
    p1.setDesignation("Chemist");
    p1.setDepartment("Drugs");
	        
    PermanentEmployee p2 = new PermanentEmployee();
    p2.setEmpId((short)101);
    p2.setEmpName("Sridhar");
    p2.setDesignation("Foreman");
    p2.setDepartment("Chemicals");

    TemporaryEmployee t1 = new TemporaryEmployee();
    t1.setEmpId((short)102);
    t1.setEmpName("Jyostna");
    t1.setWorkingDays((short)28);
    t1.setContractorName("Raju");
		        
    TemporaryEmployee t2 = new TemporaryEmployee();
    t2.setEmpId((short)103);
    t2.setEmpName("Jyothi");
    t2.setWorkingDays((short)22);
    t2.setContractorName("Pratap");
     		        
    Transaction tx = session.beginTransaction();
    session.save(p1);  session.save(t1);  
    session.save(p2);  session.save(t2);

    tx.commit();   session.close();   sf.close();   
    System.out.println("Objects saved");
  }  
}

Observe, for 4 records 4 times SQL statement is executed.

Important observation

Important observation here is the super class Employee variables empId and empName are added to each subclass table. When they are added, obviously we do not require super class table. Here, no foreign key maintained. But in table-per-subclass, three tables existed and the primary key of super class is associated as foreign key in each subclass.

Limitations of Table Per Concreteclass strategy

The identity should not be used with union subclass inheritance. The primary key and other variables (like empName) of the super class have to be shared by all unioned subclasses in the hierarchy.

Another client program to retrieve records (same that of previous table-per-class hierarchy).

import java.util.List;  
import org.hibernate.Query;  
import org.hibernate.Session;  
import org.hibernate.SessionFactory;  
import org.hibernate.cfg.Configuration;

public class RetrievingRecords 
{
  public static void main(String[] args)
  {
    Configuration c= new Configuration();
    c.configure("/hibernate.cfg.xml");
    SessionFactory sf=c.buildSessionFactory();
    Session s=sf.openSession();
	
    Query q = s.createQuery("FROM PermanentEmployee");
    List myList = q.list();
    for(PermanentEmployee p1 :myList) 
    {
      System.out.println(p1.getEmpId()+ ", " + p1.getEmpName() + ", " + p1.getDesignation() + ", " + p1.getDepartment());
    }

    Query q1 = s.createQuery("FROM TemporaryEmployee");
    List yourList = q1.list();
    for(TemporaryEmployee t1 : yourList)
    {
      System.out.println(t1.getEmpId()+ ", " + t1.getEmpName() + ", " + t1.getWorkingDays() + ", " + t1.getContractorName());
    }
  }    
}

One question for you.

Can we use all three mapping in one program?

No, Hibernate does not mingle all <subclass>, <joined-subclass> and <union-subclass> mappings in one <class> element. However, it is permitted to combine <subclass> and <joined-subclass> elements in one <class>.