hibernate session.get(class,serialid) 方法为空值的原因？

package hibernate.test;

import hibernate.test.pojo.Person;

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;
import org.hibernate.service.ServiceRegistry;
import org.hibernate.service.ServiceRegistryBuilder;

public class Test {
    public static void main(String[] args) {
		// TODO Auto-generated method stub
    Configuration config=new Configuration().configure();
    ServiceRegistry sr=new ServiceRegistryBuilder().applySettings(config.getProperties()).buildServiceRegistry();

    SessionFactory sFactory=config.buildSessionFactory(sr);
    Session session=sFactory.openSession();
    Transaction tx=session.beginTransaction();
    Person person=(Person)session.get(Person.class, 12);//老是说空值，也就是没有获得对象，为什么呢？？？
    person.setName("oscasfsfsfr");
    person.setPassword("sfasf1ddd");
   //Person person=new Person();//临时对象
    session.merge(person);
    tx.commit();
    session.close();
	}

}

Exception in thread "main" java.lang.NullPointerException

at hibernate.test.Test.main(Test.java:23)

person.java

package hibernate.test.pojo;

public class Person {
private Integer id;
private String name;
private String password;
public Integer getId() {
	return id;
}
public void setId(Integer id) {
	this.id = id;
}
public String getName() {
	return name;
}
public void setName(String name) {
	this.name = name;
}
public String getPassword() {
	return password;
}
public void setPassword(String password) {
	this.password = password;
}
}