code: public SC registerServlet(Class<? extends HttpServlet> servletClass, String urlPattern) {
          __throwIfInitialized();
  
          if (servletMap.containsKey(urlPattern) && servletMap.get(urlPattern) != servletClass)
              throw new IllegalArgumentException(String.format("URL pattern is already associated with another servlet class: %s => %s", urlPattern, servletMap.get(urlPattern)));
  
          servletMap.put(urlPattern, servletClass);
          return (SC) this;
      }
docs: Registers a servlet.
  
  @param servletClass class to be registered.
  @param urlPattern url pattern to be associated with given class.
  @return this