Hibernate Table per concrete class is throwing org.hibernate.NonUniqueObjectException:

[VenkataPrasad Regula]

I have an abstract class and two concrete class which extend these abstract class. I have mapped all the three in a employee.hbm.xml file. It is throwing the following error. My db is mysql

Exception in thread "main" org.hibernate.NonUniqueObjectException: a different object with the same identifier value was already associated with the session: [temp.ContractEmployee#0]

.
Could someone kindly look into it.

Abstract Class

package temp;

public abstract class Employee {
	public long id;
	public String employeeName;
	public String location;
	
	public String getEmployeeName() 
	{
		return employeeName;
	}
	public void setEmployeeName(String employeeName) 
	{
		this.employeeName = employeeName;
	}
	
	public String getLocation() 
	{
		return location;
	}
	
	public void setLocation(String location) 
	{
		this.location = location;
	}
	
	public long getId() 
	{
		return id;
	}
	
	public void setId(long id) 
	{
		this.id = id;
	}
}

Extended Class 1

package temp;

public class RegularEmployee extends Employee {
	public double salary;
	public double bonus;
	public double getSalary() {
		return salary;
	}
	public void setSalary(double salary) {
		this.salary = salary;
	}
	public double getBonus() {
		return bonus;
	}
	public void setBonus(double bonus) {
		this.bonus = bonus;
	}
}

Extended Class 2

package temp;

public class ContractEmployee extends Employee {
	public double hourlyRate;
	public double perDiem;
	public double contractPeriod;
	
	public double getHourlyRate() {
		return hourlyRate;
	}
	public void setHourlyRate(double hourlyRate) {
		this.hourlyRate = hourlyRate;
	}
	public double getPerDiem() {
		return perDiem;
	}
	public void setPerDiem(double perDiem) {
		this.perDiem = perDiem;
	}
	public double getContractPeriod() {
		return contractPeriod;
	}
	public void setContractPeriod(double contractPeriod) {
		this.contractPeriod = contractPeriod;
	}
}

MainClass

package temp;

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;

public class Main {

	public static void main(String[] args) 
	{
		SessionFactory sessionFactory;
		sessionFactory = new Configuration().configure().buildSessionFactory();
		Session session = sessionFactory.openSession();
		session.beginTransaction();
		RegularEmployee regular = new RegularEmployee();
		regular.setEmployeeName("Prasad");
		regular.setLocation("Hyderabad");
		regular.setBonus(500.00);
		regular.setSalary(20000.00);
		
		ContractEmployee cemployee = new ContractEmployee();
		cemployee.setEmployeeName("Sai");
		cemployee.setLocation("Hyderabad");
		cemployee.setContractPeriod(12);
		cemployee.setHourlyRate(50.00);
		cemployee.setPerDiem(300.00);
		session.clear();
		session.save(regular);
		session.save(cemployee);
		
		session.getTransaction().commit();
		session.flush();
		session.close();
	}

}

employee.hbm.xml file

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping>
	<class name="temp.Employee" abstract="true">
		<id name="id" column="EMP_ID" type="long">
			<generator class="assigned"></generator>
		</id>
		<property name="employeeName" column="EMP_NAME" type="string"/>
		<property name="location" column="LOCATION" type="string"/>
		<union-subclass name="temp.RegularEmployee" table="REG_EMP">
			<property name="salary" column="SALARY"/>
			<property name="bonus" column="BONUS"/>
		</union-subclass>
			
		<union-subclass name="temp.ContractEmployee" table="CON_EMP">
			<property name="hourlyRate" column="RATE"/>
			<property name="perDiem" column="PER_DIEM"/>
			<property name="contractPeriod" column="CON_PERIOD"/> 
		</union-subclass>
	</class>
</hibernate-mapping>

[Arun Kumarr]

your primary key is "assigned"?
Where are you setting your primary key?