code: @SuppressWarnings("unchecked")
      public static ParamExtractor makeParamExtractor(HttpServletRequest req,
                                                      Object refer) {
          if (refer != null && Map.class.isAssignableFrom(refer.getClass())) {
              return new MapParamExtractor(req, (Map<String, Object>) refer);
          }
          return new BaseParamExtractor(req);
      }
docs: 构造参数提取器