hibernate et mysql

saluts à tous.
merci de m'aider sur cette erreur.

%%%% Error Creating SessionFactory %%%%
org.hibernate.InstantiationException: could not instantiate test objectEmployee
at org.hibernate.engine.UnsavedValueFactory.instantiate(UnsavedValueFactory.java:25)
at org.hibernate.engine.UnsavedValueFactory.getUnsavedIdentifierValue(UnsavedValueFactory.java:44)
at org.hibernate.tuple.PropertyFactory.buildIdentifierProperty(PropertyFactory.java:41)
at org.hibernate.tuple.EntityMetamodel.<init>(EntityMetamodel.java:104)
at org.hibernate.persister.entity.BasicEntityPersister.<init>(BasicEntityPersister.java:398)
at org.hibernate.persister.entity.SingleTableEntityPersister.<init>(SingleTableEntityPersister.java:104)
at org.hibernate.persister.PersisterFactory.createClassPersister(PersisterFactory.java:55)
at org.hibernate.impl.SessionFactoryImpl.<init>(SessionFactoryImpl.java:199)
at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1043)
at business.HibernateSessionFactory.currentSession(HibernateSessionFactory.java:51)
at business.LibraryTest.setUp(LibraryTest.java:68)
at business.LibraryTest.main(LibraryTest.java:24)
java.lang.NullPointerException
at business.HibernateSessionFactory.currentSession(HibernateSessionFactory.java:58)
at business.LibraryTest.setUp(LibraryTest.java:68)
at business.LibraryTest.main(LibraryTest.java:24)

mes fichiers sont:
hibernate.cfg.xml:
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE hibernate-configuration
PUBLIC "-//Hibernate/Hibernate Configuration DTD//EN"
"http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">

<hibernate-configuration>

<session-factory >

<property name="hibernate.connection.url">jdbc:mysql://localhost/sampledb</property>
<property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
<property name="hibernate.connection.username">root</property>
<property name="hibernate.connection.password"></property>


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

<property name="hibernate.show_sql">false</property>
<property name="hibernate.transaction.factory_class">org.hibernate.transaction.JDBCTransactionFactory</property>
<mapping resource="business/Employee.hbm.xml"/>
</session-factory>
</hibernate-configuration>

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

<hibernate-mapping>
<class name="Employee" table="employee" schema="public">
<id name="id" type="java.lang.Integer" column="id">
<generator class="increment" />
</id>

<property name="firstname" type="java.lang.String">
<column name="firstname" />
</property>
<property name="lastname" type="java.lang.String">
<column name="lastname" />
</property>

</class>
</hibernate-mapping>

Test:

package business;

import java.util.Iterator;
import java.util.List;

import org.apache.log4j.Logger;
import org.hibernate.Session;
import org.hibernate.Transaction;

public class LibraryTest {
private Session session;

private Logger log;

public static void main(String[] args) {
/*
* hibernate needs log4j. Either create a log4j.properties file in the
* source directory * or alternatively make the following to create a
* standard configuration BasicConfigurator.configure();
*/
LibraryTest libraryTest = new LibraryTest();
try {
libraryTest.setUp();
libraryTest.testCreateDomains();
libraryTest.listEmployees();
libraryTest.tearDown();
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}

private void testCreateDomains() {
Transaction tx = session.beginTransaction();
Employee emp = new Employee();
emp.setId(4);
emp.setFirstname("Hatem");
emp.setLastname("SFAXI");
session.save(emp);
tx.commit();

}

protected void setUp() throws Exception {
session = HibernateSessionFactory.currentSession();
log = Logger.getLogger(this.getClass());
}

protected void tearDown() throws Exception {
HibernateSessionFactory.closeSession();
}

private void listEmployees() {
log.info("####### list employees");

Transaction tx = session.beginTransaction();
List employees = session.createQuery("select c from Employee as c")
.list();
for (Iterator iter = employees.iterator(); iter.hasNext();) {
Employee element = (Employee) iter.next();
//Set books = element.getBooks();
System.out.println(element);
log.info(element);
}
tx.commit();

}

/**
* @return Returns the session.
*/
public Session getSession() {
return session;
}

/**
* @param session
* The session to set.
*/
public void setSession(Session session) {
this.session = session;
}
}