docs: public String getFileTokenUrl(String filename) {
          try {
              String baseUrl = conf.get(PROP_HTTP_TOKEN_BASE_URL, "");
              String secretKey = conf.get(PROP_HTTP_SECRET_KEY, "");
              if (Strings.isBlank(filename) || Strings.isBlank(secretKey) || Strings.isBlank(baseUrl)) {
                  log.info("[FastdfsService] filename & http_token_base_url & http_secret_key must not empty");
                  return "";
              }
              int pos = filename.indexOf(FILENAME_SEPERATOR);
              String tokenFilename = filename.substring(pos + 1);
              long ts = Times.getTS();
              StringBuilder sb = new StringBuilder();
              sb.append(baseUrl);
              if (!baseUrl.endsWith(FILENAME_SEPERATOR))
                  sb.append(FILENAME_SEPERATOR);
              sb.append(filename);
              sb.append("?token=");
              sb.append(getToken(tokenFilename, ts, secretKey));
              sb.append("&ts=");
              sb.append(ts);
              return sb.toString();
          } catch (Exception e) {
              log.error(e);
          }
          return "";
      }