code: protected boolean handleErrorAttribute(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
          final String attributeKey = ERROR_EXCEPTION_ATTRIBUTE_KEY;
          final Object errorObj = request.getAttribute(attributeKey);
          if (errorObj != null && errorObj instanceof Throwable) {
              if (errorObj instanceof RuntimeException) {
                  throw (RuntimeException) errorObj;
              } else if (errorObj instanceof Throwable) {
                  String msg = "Found the exception in " + attributeKey;
                  throw new ServletException(msg, (Throwable) errorObj);
              }
              sendInternalServerError(request, response, null);
              final String msg = "The error attribute exists but unknown type: " + errorObj;
              if (errorLogging) {
                  logger.error(msg);
              } else {
                  logger.debug(msg);
              }
              return true;
          }
          return false;
      }
docs: ===============