How to obtain ExtHttpService from OSGI registry

Hi

I'm developing a WAB application where I want to register Servlet Filters in BundleActivator.start() method. I'm getting null references when I try to locate ExtHttpService:

ServiceReference sRef = bc.getServiceReference(ExtHttpService.class.getName());

However, the HttpService reference is correctly bound, but it doesn't allows filter registration.

Thanks in advance.

- Cristian