官网：
https://pay.weixin.qq.com/wiki/doc/api/index.html

/**
 * Create By CodeCow on 2020/7/19.
 */
@Slf4j
@RestController
@RequestMapping("/pay")
public class WeiXinPayController {

    @Resource
    private WeiXinPayService weiXinPayService;

    @ApiOperation(value = "微信支付（公众号/小程序）", response = WechatPayMiniRespDTO.class)
    @ApiResponses({@ApiResponse(code = 0, message = "Success"), @ApiResponse(code = 500, message = "failed")})
    @PostMapping(value = "/miniPay", consumes = "application/json", produces = "application/json")
    public RespResult<WechatPayMiniRespDTO> weChatMiniBookOrderPay(@RequestBody @Valid WechatPayReqDTO wechatPayReqDTO) {
        try {
            return weiXinPayService.weChatBookOrderPay(wechatPayReqDTO);
        } catch (Exception e) {
            log.error("weChatBookOrderPay error: ", e);
            return RespResult.error(RespResultEnum.WEIXIN_PAY_ERROR.getCode(),
                    RespResultEnum.WEIXIN_PAY_ERROR.getMessage() + e.getMessage());
        }
    }
}

/**
 * Create By CodeCow on 2020/7/19.
 */
public interface WeiXinPayService {

    RespResult weChatBookOrderPay(WechatPayReqDTO wechatPayReqDTO);
}

/**
 * Create By CodeCow on 2020/7/19.
 */
@Slf4j
@Service
public class WeiXinPayServiceImpl implements WeiXinPayService {

    @Resource
    private UserMapper userMapper; //用户mapper

    @Resource
    private WechatConfig wechatConfig; //微信配置（在最后）

    @Resource
    private OrderMapper orderMapper; //订单mapper

    @Resource
    private CommonTemplateService templateService; // 自己封装的restTemplate(在最后)

    @Override
    public RespResult weChatBookOrderPay(WechatPayReqDTO wechatPayReqDTO) {
        UserDO userDO = userMapper.selectById(wechatPayReqDTO.getUserId());
        Long orderCode = TimeUtil.getTimestamp();    //生成随机订单号
        WechatBookOrderReqDTO wechatBookOrderReqDTO  //封装微信下单请求参数（此方法在下面）
                = getBookOrderReqDTO(wechatPayReqDTO, orderCode, userDO);
        String sign =     //生成签名（签名工具类在最后）
                WeChatSignUtil.getSign(wechatBookOrderReqDTO, wechatConfig.getApiKey());
        wechatBookOrderReqDTO.setSign(sign);
        String reqXML = XstreamUtil.objectToXml(wechatBookOrderReqDTO); //把请求参数转化成 xml 格式
        log.info(">>>> reqXML >>>>:\n " + reqXML);  //打印日志
        ResponseEntity<String> responseEntity =     //使用自己封装的restTemplate向微信发起请求
                templateService.executePost(reqXML, wechatConfig.getBookOrderUrl());
        if (responseEntity.getStatusCode().value() != HttpStatus.OK.value()) { //判断请求是否成功
            return RespResult.error(RespResultEnum.ORDER_ERROR.getCode(),
                    RespResultEnum.ORDER_ERROR.getMessage() + responseEntity.getBody());
        }
        WechatBookOrderRespDTO wechatBookOrderRespDTO = // 把微信返回的参数封装到自己DTO中（DTO在最后）
                XstreamUtil.xmlToObject(responseEntity.getBody(), WechatBookOrderRespDTO.class);
        if (wechatBookOrderRespDTO.getXmlReturnCode().equals(WechatRespEnum.WECHAT_RETURN_CODE_FAIL.getCode())) { //判断微信响应结果
            return RespResult.error(RespResultEnum.ORDER_RETURN_ERROR.getCode(),
                    RespResultEnum.ORDER_RETURN_ERROR.getMessage() + wechatBookOrderRespDTO.getXmlReturnMsg());
        } else if (wechatBookOrderRespDTO.getXmlResultCode().equals(WechatRespEnum.WECHAT_RESULT_CODE_FAIL.getCode())) {
            return RespResult.error(RespResultEnum.ORDER_RESULT_ERROR.getCode(),
                    RespResultEnum.ORDER_RESULT_ERROR.getMessage() + wechatBookOrderRespDTO.getXmlErrCodeDes());
        }
        insertRefuelOrder(userDO, wechatPayReqDTO, orderCode); //向数据库保存订单信息（方法在下面）
        String nonceStr = WeChatSignUtil.getNonceStr();
        long timestamp = TimeUtil.getTimestamp();
        String paySign = WeChatPaySignUtil.getPaySign(wechatConfig.getAppId(), nonceStr, // 重新生成签名（前段调起支付需要）
                wechatBookOrderRespDTO.getXmlPrepayId(), wechatConfig.getSignType(), String.valueOf(timestamp), wechatConfig.getApiKey());
        WechatPayMiniRespDTO wechatPayMiniRespDTO =  // 返回前段支付需要的DTO（DTO在最后）
                getPayMiniRespDTO(paySign, nonceStr, timestamp, orderCode, wechatBookOrderRespDTO);
        return RespResult.success(wechatPayMiniRespDTO); // 返回数据
    }

    private WechatBookOrderReqDTO getBookOrderReqDTO(WechatPayReqDTO wechatPayReqDTO, Long orderCode, UserDO userDO) {
        return WechatBookOrderReqDTO.builder()
                .appid(wechatConfig.getAppId())
                .mch_id((wechatConfig.getMchId()))
                .openid(userDO.getOpenid())
                .trade_type(TradeTypeEnum.WECHAT_JSAPI.getType())
                .nonce_str(WeChatSignUtil.getNonceStr())
                .sign_type(wechatConfig.getSignType())
                .body(wechatPayReqDTO.getDescription())
                .out_trade_no(String.valueOf(orderCode))
                .total_fee(wechatPayReqDTO.getTotalFee())
                .spbill_create_ip(wechatPayReqDTO.getIp())
                .time_start(String.valueOf(TimeUtil.getTimestamp()))
                .notify_url(wechatConfig.getNotifyUrl())
                .build();
    }

    private void insertRefuelOrder(UserDO userDO, WechatPayReqDTO wechatPayReqDTO, Long orderCode) {
        OrderDO orderDO = OrderDO.builder()
                .id(TimeUtil.getTimestamp())
                .userId(String.valueOf(userDO.getUserId()))
                .orderCode(String.valueOf(orderCode))
                .status("0")
                .totalFee(new BigDecimal(wechatPayReqDTO.getTotalFee()))
                .payType("1") //支付类型(1-微信，2-支付宝)
                .detail(wechatPayReqDTO.getDescription())
                .createTime(TimeUtil.now())
                .build();
        orderMapper.insert(orderDO);
    }

    private WechatPayMiniRespDTO getPayMiniRespDTO(String paySign, String nonceStr, long timestamp,
                                                   Long orderCode, WechatBookOrderRespDTO wechatBookOrderRespDTO) {
        return WechatPayMiniRespDTO.builder()
                .paySign(paySign)
                .nonceStr(nonceStr)
                .outTradeNo(String.valueOf(orderCode))
                .ppackage(ColumnNames.PARAM_PREPAY_ID + wechatBookOrderRespDTO.getXmlPrepayId())
                .signType(wechatConfig.getSignType())
                .timeStamp(String.valueOf(timestamp))
                .tradeType(wechatBookOrderRespDTO.getXmlTradeType())
                .build();
    }
}

/**
 * Create By CodeCow on 2020/7/19.
 */
@Data
@Builder
@ApiModel("微信小程序/公众号支付返回参数")
public class WechatPayMiniRespDTO {

    @ApiModelProperty(value = "时间戳从1970年1月1日00:00:00至今的秒数,即当前的时间", required = true)
    private String timeStamp;

    @ApiModelProperty(value = "随机字符串，不长于32位", required = true)
    private String nonceStr;

    @ApiModelProperty(value = "统一下单接口返回的 prepay_id 参数值", required = true)
    @JsonProperty(value = "package")
    private String ppackage;

    @ApiModelProperty(value = "MD5", required = true)
    private String signType;

    @ApiModelProperty(value = "前端调用微信支付功能所需签名", required = true)
    private String paySign;

    @ApiModelProperty(value = "交易类型", required = true)
    private String tradeType;

    @ApiModelProperty(value = "订单号")
    private String outTradeNo;

}