//第1步：拼接对应的参数
Map<String, String> param = new TreeMap<>();
param.put("appid", Config.MINI_PROGRAM_APP_ID);  //像这些值，最好有个微信config类来存
param.put("mch_id", Config.MCH_ID);
param.put("openid", openId);
param.put("attach", orderDesc);
param.put("body", orderDesc);
param.put("detail", orderDesc);
param.put("limit_pay", Config.NO_CREDIT_LIMIT_PAY);
param.put("nonce_str", nonceStr);
param.put("notify_url", Config.UNIFIED_ORDER_NOTIFY_RELATIVE_URL);
param.put("out_trade_no", outTradeNo);
param.put("spbill_create_ip", clientIp);
param.put("total_fee", String.valueOf(totalFee));
param.put("trade_type", tradeType);
param.put("sign_type","MD5");
//第2步：生成签名
StringBuilder urlParam = new StringBuilder();
for (String paramKey : param.keySet()) {   //排序
    urlParam.append(paramKey).append("=").append(param.get(paramKey)).append("&");
}
String preParam = urlParam.toString().substring(0, urlParam.toString().length() - 1);  //拼接的urlParam最后有一个&，删除
String finalParam = preParam + "&key=" + WeiXinConfig.PRIVATE_KEY;   //key不参与排序
String sign = MD5.sign(finalParam, Const.Charset.UTF8).toUpperCase();  //排序后的参数+key 进行MD5签名，并且全部大写

//第三步：方便后续生成xml格式请求体，简易用一个类来封装对应的参数，然后返回
request.setAppId(param.get("appid"));
request.setSign(sign);
//request.set...其他参数，但要额外加一个成员变量存储sign

//第1步：拼接对应的参数
Map<String, String> param = new TreeMap<>();
String nonceStr = WeiXinPayUtils.generateNonceStr();  //专门写了个方法来生成随机字符串
param.put("appId", Config.MINI_PROGRAM_APP_ID);
param.put("package", "prepay_id="+prepayId);   //从统一下单响应的结果中取的参数
param.put("nonceStr", nonceStr);
param.put("timeStamp", String.valueOf(System.currentTimeMillis() / 1000));  //秒级别的时间戳，应该毫秒也OK
param.put("signType","MD5");   //小程序，这里和统一下单的签名类型保持一致
//第2步：再次生成签名
StringBuilder urlParam = new StringBuilder();
for (String paramKey : param.keySet()) {
    urlParam.append(paramKey).append("=").append(param.get(paramKey)).append("&");
}
String preParam = urlParam.toString().substring(0, urlParam.toString().length() - 1);
String finalParam = preParam + "&key=" + Config.PRIVATE_KEY;
String sign = MD5.sign(finalParam, Const.Charset.UTF8).toUpperCase();
//同理，这里建议做一个响应参数给前端
需要返回给前端的参数有：
response.setSign(sign);
response.setAppId(param.get("appId"));
response.setPrepayId(weiXinUnifiedOrderResponse.getPrepayId);  //统一下单的响应数据
response.setNonceStr(param.get("nonceStr"));
response.setTimeStamp(param.get("timeStamp"));
response.setPayWay(Const.PayWay.WEIXIN);
response.setAmount(amount);
response.setSignType("MD5");
response.setOrderNo(outTradeNo);
response.setCodeUrl(weiXinUnifiedOrderResponse.getCodeUrl());//统一下单的响应数据