PrepayRequest prepayRequest = new PrepayRequest();
Amount amount = new Amount();
amount.setCurrency("CNY");
amount.setTotal(payOrderExist.getPayFee());
prepayRequest.setAmount(amount);

prepayRequest.setAppid("xx");///这个是什么appid？？

prepayRequest.setMchid(MERCHANT_ID);
prepayRequest.setDescription(desc);
prepayRequest.setNotifyUrl(serverHost + "/api/order/app/pay/callback");
prepayRequest.setOutTradeNo(String.valueOf(payOrderExist.getId()));
PrepayResponse prepayResponse = appService.prepay(prepayRequest)