private String getExtension() {
   String extension = RequestInstanceHolder.getExtension();
   if (StringUtil.isBlank(extension) || !isExtFit(extension)) {
     return viewExtFileName;
   }
   return extension;
 }
 protected Object getCacheKey(String viewName, Locale locale) {
   String extension = RequestInstanceHolder.getExtension();
   if (StringUtil.isBlank(extension)) {
     return super.getCacheKey(viewName, locale);
   }
   return newViewName(viewName, extension) + "_" + locale;
 }