org.hibernate.MappingException: Unknown entity: bean.Students
at org.hibernate.internal.SessionFactoryImpl.getEntityPersister(SessionFactoryImpl.java:776)
at org.hibernate.internal.SessionImpl.getEntityPersister(SessionImpl.java:1533)
at org.hibernate.event.internal.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:104)
at org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.saveWithGeneratedOrRequestedId(DefaultSaveOrUpdateEventListener.java:192)
at org.hibernate.event.internal.DefaultSaveEventListener.saveWithGeneratedOrRequestedId(DefaultSaveEventListener.java:38)
at org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.entityIsTransient(DefaultSaveOrUpdateEventListener.java:177)
at org.hibernate.event.internal.DefaultSaveEventListener.performSaveOrUpdate(DefaultSaveEventListener.java:32)
at org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.onSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:73)
at org.hibernate.internal.SessionImpl.fireSave(SessionImpl.java:682)
at org.hibernate.internal.SessionImpl.save(SessionImpl.java:674)
at org.hibernate.internal.SessionImpl.save(SessionImpl.java:669)
at test.HibernateTest.main(HibernateTest.java:30)

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-configuration SYSTEM "http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
    <session-factory>
        <property name="connection.driver_class">oracle.jdbc.driver.OracleDriver</property>
        <property name="connection.url">jdbc:oracle:thin:@localhost:1521:xe</property>
        <property name="connection.username">system</property>
        <property name="connection.password">admin</property>
        <property name="connection.pool_size">1</property>
        <property name="dialect">org.hibernate.dialect.Oracle10gDialect</property>
        <property name = "show_sql">true</property>
        <property name = "hbm2ddl.auto">create</property>
        <mapping class = "bean.Students"/>


    </session-factory>
</hibernate-configuration>

package bean;

import javax.persistence.Entity;
import javax.persistence.Id;

@Entity
public class Students {

    @Id
    private int studentId;
    private String studentLastName;
    private String studentFirstName;
    private int studentAge;
    private String studentSex;

    public int getStudentId() {
        return studentId;
    }
    public void setStudentId(int studentId) {
        this.studentId = studentId;
    }
    public String getStudentLastName() {
        return studentLastName;
    }
    public void setStudentLastName(String studentLastName) {
        this.studentLastName = studentLastName;
    }
    public String getStudentFirstName() {
        return studentFirstName;
    }
    public void setStudentFirstName(String studentFirstName) {
        this.studentFirstName = studentFirstName;
    }
    public int getStudentAge() {
        return studentAge;
    }
    public void setStudentAge(int studentAge) {
        this.studentAge = studentAge;
    }
    public String getStudentSex() {
        return studentSex;
    }
    public void setStudentSex(String studentSex) {
        this.studentSex = studentSex;
    }
}

package test;

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

import bean.Students;

import org.hibernate.boot.registry.StandardServiceRegistryBuilder;

public class HibernateTest {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Students newStud = new Students();
        newStud.setStudentId(4);
        newStud.setStudentLastName("Danulushyn");
        newStud.setStudentFirstName("Petro");
        newStud.setStudentAge(18);
        newStud.setStudentSex("male");

        Configuration conf = new Configuration().configure();
        ServiceRegistry sr = new StandardServiceRegistryBuilder().
                                    applySettings(conf.getProperties()).build();
        SessionFactory factory = conf/*.addAnnotatedClass(Students.class)*/.buildSessionFactory(sr);
        Session session = factory.openSession();
        try{
            session.beginTransaction();
            session.save(newStud);
            session.getTransaction().commit();
        }catch(Exception e){
            session.getTransaction().rollback();
            e.printStackTrace();
        }finally{
            session.close();
            factory.close();
        }

    }

}

SessionFactory factory = conf.addAnnotatedClass(Students.class).buildSessionFactory(sr);