code: public static String generateChooseWXPayJson(String prepay_id,String appId,String key){
  		String json = PayUtil.generateMchPayJsRequestJson(prepay_id, appId, key);
  		json = json.replaceAll("\"timeStamp\"","\"timestamp\"")
  				   .replaceAll(",?\"appId\":\"[^\"]*\",?","");
  		return json;
  	}
docs: 生成微信支付JSON
  @since 2.8.1
  @param prepay_id	预支付订单号
  @param appId appId
  @param key 商户支付密钥
  @return json