code: private static Map<String, String> __getRequestHeaderMap(HttpServletRequest req) {
          Map<String, String> reqHeaders = new LinkedHashMap<>();
          Enumeration<String> headerNames = req.getHeaderNames();
          if (headerNames != null) {
              while (headerNames.hasMoreElements()) {
                  String headerName = headerNames.nextElement();
                  Enumeration<String> headerValues = req.getHeaders(headerName);
  
                  if (headerValues != null) {
                      StringBuilder headerValueBuilder = new StringBuilder();
                      int i = 0;
                      while (headerValues.hasMoreElements()) {
                          if (i > 0) {
                              headerValueBuilder.append(", ");
                          }
  
                          headerValueBuilder.append(headerValues.nextElement());
                          i++;
                      }
  
                      reqHeaders.put(headerName, headerValueBuilder.toString());
                  } else {
                      reqHeaders.put(headerName, "");
                  }
              }
          }
          return reqHeaders;
      }
docs: Retrive a map containing the request headers associated with given request.