code: private static String wrapSystemId(String systemId) {
          if (systemId == null) {
              return "jstl:";
          } else if (UrlUtil.isAbsoluteUrl(systemId)) {
              return systemId;
          } else {
              return ("jstl:" + systemId);
          }
      }
docs: Wraps systemId with a "jstl:" prefix to prevent the parser from
  thinking that the URI is truly relative and resolving it against
  the current directory in the filesystem.