调用微信退款接口https://api.mch.weixin.qq.com/secapi/pay/refund

返回了一个null回来导致报错

代码如下所示：

public static void main(String[] args) {
    try {
        // 设置API请求参数
        SortedMap<String, Object> params = new TreeMap<String, Object>();
        params.put("appid", WxPayConfig.appid);
        params.put("mch_id", WxPayConfig.mch_id);
        params.put("nonce_str", System.currentTimeMillis() + "");
        params.put("out_trade_no", 2057);//订单编号
        params.put("total_fee", 1);
        params.put("refund_fee", 1);
        params.put("out_refund_no", 2057);//订单退款编号
        params.put("notify_url", WxPayConfig.notify_refund_url+2057);
        params.put("trade_type", "JSAPI");
        params.put("openid", "openId");
        //获取sign
        String sign = PayCommonUtil.createSign("UTF-8", params, WxPayConfig.apiKey);//最后这个是自己设置的32位密钥
        params.put("sign", sign);
        log.error("params:"+params);


        //转成XML
        String requestXML = PayCommonUtil.getRequestXml(params);
        log.error(requestXML);
        //得到含有prepay_id的XML
        String resXml = HttpUtil.postData(WxPayConfig.refund_url, requestXML);
        log.error("resXml:"+resXml);
        //解析XML存入Map
        Map map = XMLUtil.doXMLParse(resXml);
        System.out.println("map:"+map);

    } catch (Exception e) {
        e.printStackTrace();
    }
}