Why does p:commandButton onerror not fire when I explicitly make a 500 error response?

Question

I'm trying to make a simple dialog alert for when a user tries to delete something. But my problem is with PrimeFaces. The response content is as expected a 500 error with the message I want to show but it doesn't show the message, the dialog appear and is empty.

Here is my the code of my backing bean:

FacesContext.getCurrentInstance().addMessage(null, new FacesMessage(FacesMessage.SEVERITY_ERROR, "Error!", "No se ha podido eliminar el periodo"));
ExternalContext context = FacesContext.getCurrentInstance().getExternalContext();
HttpServletResponse response = (HttpServletResponse) context.getResponse();
response.setStatus(500);

Here is the command button:

<p:commandButton value="Eliminar"
                 action="#{cicloEscolarWiz.doConfirmarEliminarPeriodoLectivo}"
                 ajax="true"
                 onerror="messages.show();"
                 update=":dlgMessages"/>

And here is my dialog:

<p:dialog id="dlgMessages"
          showEffect="explode"
          closable="true"
          header="Mensajes"
          widgetVar="messages"
          modal="true"
          style="font-size: 12px;">

    <h:panelGroup id="pnlGrpMessages">

        <p:panel>
            <h:panelGrid columns="1" cellpadding="0" cellspacing="0">

                <p:messages id="deletePeriodoLectivoMessages"
                            showDetail="true"
                            autoUpdate="true"
                            closable="true" />
            </h:panelGrid>
        </p:panel>

        <p:spacer height="10"/>
        <p:separator />
    </h:panelGroup>
</p:dialog>

So what I want to achieve is when I hit the command button make a response of a 500 error then the dialog must appear and appears but empty, there's no message.

Answer 1

Just put this on top of your page, use style for position.

<p:panel styleClass="message" >      
  <p:messages styleClass="message" showDetail="true" autoUpdate="true" closable="true" />  
  </p:panel> 

In your doConfirmarEliminarPeriodoLectivo method set the message when error occurs

FacesContext.getCurrentInstance().addMessage(null, new FacesMessage(FacesMessage.SEVERITY_ERROR, "Error!", "No se ha podido eliminar el periodo"));

And your commandButton

<p:commandButton value="Eliminar"
                 action="#{cicloEscolarWiz.doConfirmarEliminarPeriodoLectivo}" />