code: public static WstxInputSource resolveEntityUsing
          (WstxInputSource refCtxt, String entityName,
           String publicId, String systemId,
           XMLResolver resolver, ReaderConfig cfg, int xmlVersion)
          throws IOException, XMLStreamException
      {
          URL ctxt = (refCtxt == null) ? null : refCtxt.getSource();
          if (ctxt == null) {
              ctxt = URLUtil.urlFromCurrentDir();
          }
          Object source = resolver.resolveEntity(publicId, systemId, ctxt.toExternalForm(), entityName);
          return (source == null) ? null : sourceFrom(refCtxt, cfg, entityName, xmlVersion, source);
      }
docs: A very simple utility expansion method used generally when the
  only way to resolve an entity is via passed resolver; and where
  failing to resolve it is not fatal.