[Hibernate] org.hibernate.MappingException

package exec;
import Entidade.Aluno;
import conf.HibernateUtil;
import org.hibernate.Session;


public class Main {

    public static void main(String[] args) {

        Aluno a = new Aluno (1, "Bruno", 22);
        Session session = HibernateUtil.getSession();
        session.beginTransaction();
        session.save(a);
        session.getTransaction().commit();
        System.out.println(a.toString());

    }

}

package Entidade;

import java.io.Serializable;
import javax.persistence.*;

@Entity
@Table (name="Aluno")
public class Aluno implements Serializable {

    @Id
    private int Id;
    @Column (nullable=false, length=200)
    private String nome;
    @Column (nullable=false, length=10)
    private int Idade;

    public Aluno() {}

    public Aluno(int Id, String nome, int Idade) {
        this.Id = Id;
        this.nome = nome;
        this.Idade = Idade;
    }

    /**
     * @return the Id
     */
    //@Id @GeneratedValue(strategy=GenerationType.SEQUENCE, generator="SEQ_STORE")
    public int getId() {
        return Id;
    }

    /**
     * @param Id the Id to set
     */
    public void setId(int Id) {
        this.Id = Id;
    }

    /**
     * @return the nome
     */
    public String getNome() {
        return nome;
    }

    /**
     * @param nome the nome to set
     */
    public void setNome(String nome) {
        this.nome = nome;
    }

    /**
     * @return the Idade
     */
    public int getIdade() {
        return Idade;
    }

    /**
     * @param Idade the Idade to set
     */
    public void setIdade(int Idade) {
        this.Idade = Idade;
    }

    @Override
    public String toString() {
        return "Meu nome é " + getNome() + " tenho " + getIdade() + " anos.";
    }



}

package conf;

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


public class HibernateUtil {

    private static final SessionFactory sessionFactory;

    static {
        try {
            sessionFactory = new AnnotationConfiguration().configure().buildSessionFactory();
        } catch (Throwable ex) {
            throw new ExceptionInInitializerError(ex);
        }
    }

    public static Session getSession()
            throws HibernateException {
        return sessionFactory.openSession();
    }
    
}

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-configuration PUBLIC "-//Hibernate/Hibernate Configuration DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
  <session-factory>
    <property name="hibernate.dialect">org.hibernate.dialect.DerbyDialect</property>
    <property name="hibernate.connection.driver_class">org.apache.derby.jdbc.ClientDriver</property>
    <property name="hibernate.connection.url">jdbc:derby://localhost:1527/sample</property>
    <property name="hibernate.connection.username">app</property>
    <property name="hibernate.connection.password">app</property>
    <property name="show_sql">true</property>
    <property name="hbm2ddl.auto">create</property>
  </session-factory>
</hibernate-configuration>

run:
log4j:WARN No appenders could be found for logger (org.hibernate.cfg.annotations.Version).
log4j:WARN Please initialize the log4j system properly.
Exception in thread "main" org.hibernate.MappingException: Unknown entity: Entidade.Aluno
        at org.hibernate.impl.SessionFactoryImpl.getEntityPersister(SessionFactoryImpl.java:628)
        at org.hibernate.impl.SessionImpl.getEntityPersister(SessionImpl.java:1366)
        at org.hibernate.event.def.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:121)
        at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.saveWithGeneratedOrRequestedId(DefaultSaveOrUpdateEventListener.java:210)
        at org.hibernate.event.def.DefaultSaveEventListener.saveWithGeneratedOrRequestedId(DefaultSaveEventListener.java:56)
        at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.entityIsTransient(DefaultSaveOrUpdateEventListener.java:195)
        at org.hibernate.event.def.DefaultSaveEventListener.performSaveOrUpdate(DefaultSaveEventListener.java:50)
        at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.onSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:93)
        at org.hibernate.impl.SessionImpl.fireSave(SessionImpl.java:563)
        at org.hibernate.impl.SessionImpl.save(SessionImpl.java:551)
        at org.hibernate.impl.SessionImpl.save(SessionImpl.java:547)
        at exec.Main.main(Main.java:14)
Java Result: 1