后端开发场景集 — 5个真实工作场景

// 退款流程 — 微信支付最容易出问题的环节
@Service
public class WxRefundService {

    @Autowired
    private OrderMapper orderMapper;

    /**
     * 申请退款
     * 常见坑：
     * 1. 重复退款 — 同一笔订单退款两次，钱扣两次！必须用 out_refund_no 做幂等
     * 2. 部分退款 — 一笔订单退一部分，剩余金额要正确计算
     * 3. 退款回调 — 退款是异步的，不能只看申请返回，必须等回调确认
     */
    public RefundResult refund(String orderId, Integer refundAmount, String reason) {
        Order order = orderMapper.selectById(orderId);

        // 1. 幂等检查：是否已退过
        if (order.getRefundStatus() != null && order.getRefundStatus().equals("REFUNDED")) {
            return RefundResult.alreadyRefunded(orderId);
        }

        // 2. 金额校验：退款不能超过支付金额
        if (refundAmount > order.getPayAmount()) {
            throw new BusinessException("退款金额不能超过支付金额");
        }

        // 3. 调微信退款API
        WxRefundRequest request = new WxRefundRequest()
            .setOutTradeNo(orderId)                    // 原支付单号
            .setOutRefundNo("RF_" + orderId)           // 退款单号（幂等键！）
            .setTotalAmount(order.getPayAmount())       // 原支付金额（分）
            .setRefundAmount(refundAmount)             // 退款金额（分）
            .setReason(reason);

        WxRefundResponse response = wxPayService.refund(request);

        // 4. 注意：退款是异步的！这里只是"申请成功"，真正到账等回调
        if (response.isSuccess()) {
            order.setRefundStatus("REFUNDING");
            orderMapper.updateById(order);
            return RefundResult.processing(orderId);
        }

        throw new BusinessException("退款申请失败: " + response.getMsg());
    }

    // 退款回调 — 微信异步通知退款结果
    @PostMapping("/callback/refund")
    public String handleRefundCallback(@RequestBody String body) {
        WxRefundCallback callback = wxPayService.parseRefundCallback(body);

        Order order = orderMapper.selectById(callback.getOutTradeNo());

        if (callback.isSuccess()) {
            order.setRefundStatus("REFUNDED");
            order.setRefundAmount(callback.getRefundAmount());
        } else {
            order.setRefundStatus("REFUND_FAILED");
            order.setRefundFailReason(callback.getFailReason());
        }

        orderMapper.updateById(order);
        return "SUCCESS";  // 告诉微信别再通知了
    }
}
  

// 断点续传：查询已上传的分片列表
@GetMapping("/chunks")
public R<List<Integer>> getUploadedChunks(
        @RequestParam String fileHash,
        @RequestParam int totalChunks) {

    // 根据文件hash查询已上传的分片号
    List<Integer> uploadedChunks = uploadService.getUploadedChunks(fileHash);

    // 前端拿到已上传列表后，只上传缺失的分片
    return R.ok(uploadedChunks);
}

// UploadService 实现
public List<Integer> getUploadedChunks(String fileHash) {
    // 方案1：查数据库
    // List<ChunkRecord> records = chunkMapper.selectByFileHash(fileHash);
    // return records.stream().map(ChunkRecord::getChunkNumber).toList();

    // 方案2：查临时目录（分片先存临时目录，合并后删除）
    String tempDir = "/tmp/upload/" + fileHash;
    File dir = new File(tempDir);
    if (!dir.exists()) return Collections.emptyList();

    return Arrays.stream(dir.listFiles())
        .map(f -> Integer.parseInt(f.getName().replace("chunk_", "")))
        .sorted()
        .toList();
}