code: private String getRequestUrl(RequestParametersHolder requestHolder) throws AlipayApiException {
          StringBuffer urlSb = new StringBuffer(serverUrl);
          try {
              String sysMustQuery = WebUtils.buildQuery(requestHolder.getProtocalMustParams(),
                  charset);
              String sysOptQuery = WebUtils.buildQuery(requestHolder.getProtocalOptParams(), charset);
  
              urlSb.append("?");
              urlSb.append(sysMustQuery);
              if (sysOptQuery != null & sysOptQuery.length() > 0) {
                  urlSb.append("&");
                  urlSb.append(sysOptQuery);
              }
          } catch (IOException e) {
              throw new AlipayApiException(e);
          }
  
          return urlSb.toString();
      }
docs: 获取POST请求的base url
  
  @param requestHolder
  @return
  @throws AlipayApiException