JSF Converters - Validation error: value is not valid

Apparently, the error occurs because you have to override the equals(Object object)and methods hashCode()in the bean on which the Converter is applied.

On the other hand, in the xhtml file, I have modified the value of the attribute itemValue="#{producto}", so that the solution is as follows:

xhtml:

<h:panelGrid cellpadding = "5" columns = "4" >
    <p:commandButton value="Guardar"
        update=":dmContadoresConsumoForm:edicionContadorConsumoPnl"
        disabled="#{!contadoresConsumoBean.contadorCentroEditable}"
        actionListener="#{contadoresConsumoBean.guardarContador}"/>
    <h:outputText value = "Producto" />
    <h:selectOneListbox
        id = "productoSel"
        size = "1"
        value = "#{contadoresConsumoBean.productoSeleccionado}"
        disabled = "#{!contadoresConsumoBean.contadorCentroEditable}"
        style = "width:100%" >
        <f:selectItem itemValue = "0"
            itemLabel = "" / >

        <!-- Cambio. Antiguo: itemValue = "#{producto.id}"-->
        <f:selectItems
            value = "#{contadoresConsumoBean.productos}" 
            var = "producto"
            itemValue = "#{producto}"
            itemLabel = "#{producto.nombre}" />
    </h:selectOneListbox>

<!-- ... -->

</h:panelGrid>

ProductConverter.java

The converter didn't have any errors, so it's still the same:

@ManagedBean(name = "productoConverter")
@SessionScoped
@FacesConverter(forClass = Producto.class)
public class ProductoConverter implements Converter {

    @Override
    public Object getAsObject(FacesContext context, UIComponent component, String value) {
        long id = -1;
        try{
            id = Long.parseLong(value);
        }
        catch(Exception e){
            return null;
        }
        Transaction t = HibernateUtil.getSessionFactory().getCurrentSession().beginTransaction();
        try{
            Producto e = new ProductoServiceImpl().find(id);
            t.commit();
            return e;
        }
        catch(Exception e){
            t.rollback();
            e.printStackTrace();
            return null;
        }
    }

    @Override
    public String getAsString(FacesContext context, UIComponent component, Object value) {
        if (value instanceof Producto){
            return ((Producto)value).getId().toString();
        }
        return null;
    }
}

Product.java

In this class I override the methods equals(Object object)andhashCode()

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

    // Propiedades

    // Getters y Setters

    // Constructores

    // Métodos

    @Override
    public int hashCode() {
        int hash = 0;
        hash += (id != null ? id.hashCode() : 0);
        return hash;
    }

    @Override
    public boolean equals(Object object) {
        if (!(object instanceof Producto)) {
            return false;
        }
        Producto other = (Producto) object;
        if ((this.id == null && other.id != null) || (this.id != null && !this.id.equals(other.id))) {
            return false;
        }
        return true;
    }
}