apollo-configservice/src/main/java/com/ctrip/framework/apollo/configservice/controller/ConfigFileController.java/ConfigFileController/queryConfigAsProperties
class ConfigFileController:  @GetMapping(value = "/{appId}/{clusterName}/{namespace:.+}")
  public ResponseEntity<String> queryConfigAsProperties(@PathVariable String appId,
                                                        @PathVariable String clusterName,
                                                        @PathVariable String namespace,
                                                        @RequestParam(value = "dataCenter", required = false) String dataCenter,
                                                        @RequestParam(value = "ip", required = false) String clientIp,
                                                        @RequestParam(value = "label", required = false) String clientLabel,
                                                        HttpServletRequest request,
                                                        HttpServletResponse response)
      throws IOException {

    String result =
        queryConfig(ConfigFileOutputFormat.PROPERTIES, appId, clusterName, namespace, dataCenter,
            clientIp, clientLabel, request, response);

    if (result == null) {
      return NOT_FOUND_RESPONSE;
    }

    return new ResponseEntity<>(result, propertiesResponseHeaders, HttpStatus.OK);
  }