Hibernate não atualiza dados

//bean
@Entity
@Table(name = "CATEGORIA")
public class Categoria implements Serializable {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "id_categoria", nullable = false, length = 20)
    private Integer id_categoria;
    @Column(name = "descricao", nullable = false, length = 40)
    private String descricao;
    @OneToMany
    private Collection<Subcategoria> subcategoria;

    public Integer getId_categoria() {
        return id_categoria;
    }

    public void setId_categoria(Integer id_categoria) {
        this.id_categoria = id_categoria;
    }

    public String getDescricao() {
        return descricao;
    }

    public void setDescricao(String descricao) {
        this.descricao = descricao;
    }
}

//AbstractDAO
@SuppressWarnings("serial")
public abstract class AbstractDAO extends HibernateUtil implements Serializable {

    public void  salvarOuAtualizar(Object obj) {
        Session session = null;
        Transaction transaction = null;
        try {
            session = HibernateUtil.getSession();
            transaction = session.beginTransaction();
            session.saveOrUpdate(obj);
            session.flush();
            transaction.commit();
            session.close();
        } catch (HibernateException e) {
            e.printStackTrace();
            transaction.rollback();
            throw new HibernateException("Falha ao salvar o "
                    + "objeto : "
                    + obj.toString()
                    + "(" + e.getMessage() + ")",
                    e.getCause());
        }
    }

    public void excluir(Object obj) {
        Session session = null;
        Transaction transaction = null;

        try {
            session = HibernateUtil.getSession();
            transaction = session.beginTransaction();
            session.delete(obj);
            session.flush();
            transaction.commit();
        } catch (HibernateException e) {
            transaction.rollback();
            throw new HibernateException("Falha"
                    + " ao excluir o objeto : "
                    + obj.toString() + "("
                    + e.getMessage() + ")",
                    e.getCause());
        }
    }

    public ArrayList<Object> buscarTodos(Class classe) {
        Criteria criteria = null;
        ArrayList<Object> dados = null;
        try {
            criteria = getSession().createCriteria(classe);

            dados = (ArrayList<Object>) criteria.list();
        } catch (Exception e) {
            throw new HibernateException("Method: CONSULTA_TODOS, erro: " + e.getLocalizedMessage());
        } finally {
        }
        return dados;
    }
    
    
}

//Hibernate
public class HibernateUtil {

    private static final SessionFactory sessionFactory = buildSessionFactory();
    private static Session session = null;

    private static SessionFactory buildSessionFactory() {
        return new AnnotationConfiguration().configure().buildSessionFactory();
    }

    public static Session getSession() {
        if (session == null || !session.isOpen()) {
            session = sessionFactory.openSession();
        }

        return session;
    }
}

//hibernate.cfg
?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.bytecode.use_reflection_optimizer">false</property>
        <property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>

        <!--  localhost || ip do servidor     -->                                  
        <property name="hibernate.connection.url">jdbc:mysql://localhost:3306/despesas</property>
        <property name="hibernate.connection.username">root</property>
        <property name="hibernate.connection.password">***********</property>

        <property name="hbm2ddl.auto">validate</property>
        
       <!--<property name="hbm2ddl.auto">update</property>-->
       <!--<property name="hbm2ddl.auto">validate</property>-->
        <property name="hibernate.dialect">org.hibernate.dialect.MySQL5InnoDBDialect</property>
        
        <!-- Mapear todos as classes persistentes ou seja os beans-->
        <mapping class="br.com.despesas.bean.Categoria"/>
        <mapping class="br.com.despesas.bean.Subcategoria"/>
        <mapping class="br.com.despesas.bean.Despesa"/>
        
                        
    </session-factory>
</hibernate-configuration>