Using optional facelet tag libraries - jsf

Question

I have a web application that uses optional modules. The modules are implemented as Web Fragment projects, their jars may or may not be deployed with the war depending on the build profile.
A module can contain it's own module.taglib.xml with a http://company.com/module namespace and some tags.
The war xhtml templates use module tags like this:
<ui:composition ... xmlns:mod="http://company.com/module">
<c:if test="#{moduleDeployed}">
<mod:someTag />
</c:if>
Problems.
When the module is not deployed, the war pages work fine, but in ProjectStage.Development I get FacesMessage warnings:
Warning: This page calls for XML namespace
http://company.com/module declared with prefix mod but no
taglibrary exists for that namespace.
As far as I can see, JSF specification doesn't define what happens, when a template uses a nonexistent tag library. So with my current approach war pages can stop working after an upgrade or a switch to a different JSF implementation.
Questions.
Is there a (not very ugly) way to disable this specific warning?
Is there a better approach to using optional facelet tag libraries?
As of now I plan to disable the warning anyway I can: e.g. override Messages renderer and check message string if I have to. If the problem 2 manifests, make the build supply placeholder taglib.xml files for not deployed modules.

Answer 1

Even though placeholder taglibs seemed like a pretty good solution, they also seemed harder to implement and maintain.
So in the end I went with filtering the messages. This is likely Mojarra specific: the message text, the fact that the iterator allows removal (this isn't forbidden by the spec, but it's not required either). It's known to work with Mojarra 2.2.8 to 2.2.13.
public class SuppressNoTaglibraryExistsFacesMessage implements SystemEventListener {
private static final Pattern PTTRN_NO_TAGLIBRARY_EXISTS_FOR_NAMESPACE =
Pattern.compile("Warning: This page calls for XML namespace \\S+ declared with "
+ "prefix \\S+ but no taglibrary exists for that namespace.");
#Override
public void processEvent(SystemEvent event) {
Iterator<FacesMessage> messages = FacesContext.getCurrentInstance().getMessages();
while (messages.hasNext()) {
String messageSummary = messages.next().getSummary();
if (PTTRN_NO_TAGLIBRARY_EXISTS_FOR_NAMESPACE.matcher(messageSummary).matches()) {
messages.remove();
}
}
}
#Override
public boolean isListenerForSource(Object source) {
return true;
}
}
Bind the listener only in Development project stage.
public class SubscribeListenersAfterApplicationPostConstructListener
implements SystemEventListener {
#Override
public void processEvent(SystemEvent event) throws AbortProcessingException {
Application application = (Application) event.getSource();
if (ProjectStage.Development.equals(application.getProjectStage())) {
application.subscribeToEvent(PostAddToViewEvent.class, UIViewRoot.class,
new SuppressNoTaglibraryExistsFacesMessage());
}
}
#Override
public boolean isListenerForSource(Object source) {
return source instanceof Application;
}
}
And in faces-config.xml:
<system-event-listener>
<system-event-listener-class><packages>.SubscribeListenersAfterApplicationPostConstructListener</system-event-listener-class>
<system-event-class>javax.faces.event.PostConstructApplicationEvent</system-event-class>
</system-event-listener>