#支付系统设计

// 支付幂等实现：基于订单号的唯一约束

public class PaymentService {

    @Transactional
    public PaymentResult pay(String orderId, String channel, BigDecimal amount) {
        // Step 1: 查询支付流水
        PaymentDO existing = paymentDAO.selectByOrderId(orderId);
        if (existing != null) {
            // 已存在支付流水，直接返回
            return PaymentResult.from(existing);
        }

        // Step 2: 创建支付流水（订单号唯一索引，重复插入报错）
        PaymentDO payment = new PaymentDO();
        payment.setOrderId(orderId);
        payment.setChannel(channel);
        payment.setAmount(amount);
        payment.setStatus(PaymentStatus.PENDING);
        payment.setCreateTime(new Date());
        paymentDAO.insert(payment);

        // Step 3: 调用支付渠道
        ChannelResponse response = channelService.pay(payment);
        if (response.isSuccess()) {
            payment.setStatus(PaymentStatus.SUCCESS);
            payment.setChannelTradeNo(response.getTradeNo());
        } else {
            payment.setStatus(PaymentStatus.FAILED);
            payment.setErrorCode(response.getErrorCode());
        }
        paymentDAO.update(payment);

        return PaymentResult.from(payment);
    }
}

// 统一支付接口
public interface PaymentChannel {
    /**
     * 统一下单
     */
    ChannelPayResponse pay(PayRequest request);

    /**
     * 异步回调处理
     */
    ChannelCallbackResult callback(CallbackRequest request);

    /**
     * 退款
     */
    ChannelRefundResponse refund(RefundRequest request);

    /**
     * 查账
     */
    ChannelQueryResponse query(OrderQueryRequest request);
}

// 支付宝通道实现
@Service
public class AlipayChannel implements PaymentChannel {
    @Override
    public ChannelPayResponse pay(PayRequest request) {
        // 调用支付宝统一下单接口
        AlipayTradePayRequest alipayRequest = new AlipayTradePayRequest();
        alipayRequest.setOutTradeNo(request.getOrderId());
        alipayRequest.setTotalAmount(request.getAmount().toString());
        alipayRequest.setSubject(request.getSubject());
        // ... 设置其他参数
        AlipayTradePayResponse response = alipayClient.execute(alipayRequest);
        return ChannelPayResponse.from(response);
    }
}

// 微信支付通道实现
@Service
public class WechatPayChannel implements PaymentChannel {
    @Override
    public ChannelPayResponse pay(PayRequest request) {
        // 调用微信统一下单接口
        WxPayUnifiedOrderRequest wxRequest = new WxPayUnifiedOrderRequest();
        wxRequest.setOutTradeNo(request.getOrderId());
        wxRequest.setTotalFee(request.getAmount().multiply(new BigDecimal(100)).intValue());
        // ... 设置其他参数
        WxPayUnifiedOrderResponse response = wxPayService.unifiedOrder(wxRequest);
        return ChannelPayResponse.from(response);
    }
}

public class RefundService {

    @Transactional
    public RefundResult refund(String orderId, BigDecimal refundAmount, String reason) {
        // Step 1: 检查原支付是否成功
        PaymentDO payment = paymentDAO.selectByOrderId(orderId);
        if (payment == null || payment.getStatus() != PaymentStatus.SUCCESS) {
            throw new BusinessException("原支付未成功，无法退款");
        }

        // Step 2: 检查退款金额
        BigDecimal refundedAmount = refundDAO.sumRefundedAmount(orderId);
        BigDecimal canRefundAmount = payment.getAmount().subtract(refundedAmount);
        if (refundAmount.compareTo(canRefundAmount) > 0) {
            throw new BusinessException("退款金额超出可退金额");
        }

        // Step 3: 创建退款记录（退款单号唯一）
        RefundDO refund = new RefundDO();
        refund.setRefundId(genRefundId());
        refund.setOrderId(orderId);
        refund.setAmount(refundAmount);
        refund.setStatus(RefundStatus.PENDING);
        refundDAO.insert(refund);

        // Step 4: 调用渠道退款
        try {
            ChannelRefundResponse response = channelService.refund(refund);
            if (response.isSuccess()) {
                refund.setStatus(RefundStatus.SUCCESS);
                refund.setChannelRefundNo(response.getRefundNo());
                // 更新账户余额
                accountService.unfreezeAndRefund(payment.getUserId(), refundAmount);
            } else {
                refund.setStatus(RefundStatus.FAILED);
                refund.setErrorMsg(response.getErrorMsg());
            }
        } catch (Exception e) {
            refund.setStatus(RefundStatus.FAILED);
            refund.setErrorMsg(e.getMessage());
        }
        refundDAO.update(refund);

        return RefundResult.from(refund);
    }
}