h:commandLink f:ajax action not called when page has f:viewParam required="true"

In my web app, JSF2 (mojarra 2.1.20), there is a strange behaviour on a page having a component with the following snippet:

<h:commandLink ... action="#{cc.attrs.bean.next}">
    <f:ajax execute="@this" render=":#{cc.clientId}" />
</h:commandLink>

The action next() is not always called. I've found out that the problem is related for some reason to the url parameter id:

<f:metadata>
    <f:viewParam name="id" value="#{torneoBean.idParam}" required="true" />
</f:metadata>

When I insert the viewParam above the problem occurs. But I cannot get rid of it. Debugging ajax request it seems that start, success, complete event sequence is called (no errors), the components to render are correctly rendered but the action is not fired. Is there a way to debug this situation and find out what is the root cause of the problem?

Solution

Apparently you haven't attached a <h:message(s)> to the <f:viewParam id="torneo_id"> and/or you aren't updating it on every ajax request. You would otherwise have seen a required validation error on it. The concrete problem is essentially covered by point 3 of commandButton/commandLink/ajax action/listener method not invoked or input value not updated.

As to the cause, the <f:viewParam> is executed on every single HTTP request, also on ajax postbacks. When the ajax request is sent, the desired request parameter is not present anymore in the current request and hence <f:viewParam> fails the required validation.

There are several solutions to this:

Retain the request parameter for the subsequent request by <f:param> in the command component.

<h:commandLink ...>
    ...
    <f:param name="id" value="#{param.id}" />
</h:commandLink>

Make it required during a non-postback only. This works only if the page is initially opened by a GET request. If it's however opened by a non-redirect navigation, then it would still fail.
```
<f:viewParam ... required="#{not facesContext.postback}" />
```
Replace <f:viewParam> by the one provided by JSF utility library OmniFaces, the <o:viewParam>. It implicitly turns off required="true" on postbacks so that you kan keep using the same attributes.
```
<o:viewParam ... required="true" />
```