springboot下载文件 范围下载

关键词：springboot，download，Range，Content-Range，Content-Length，http code 206 Partial Content

下载文件的一部分，我们在request header：Range 中指定要获取的文件的字节范围。要注意http response header：Content-Length 一定要与Range中所表示的获取bytes的范围长度相等。

如果Content-Length大于要Range所表示的bytes长度，那么客户端在发起请求后，下载完相应的Range范围的bytes后，连接不会自动关闭，直到连接超时，抛出异常。

如果Content-Length小于要Range所表示的bytes长度，那么发起请求后，只会获得Content-Length所指定的较小范围的bytes，导致获取的数据不完整。

controller：

    @SneakyThrows
    @GetMapping("/download")
    public void download(HttpServletResponse response,
                         @RequestParam String versionNum,
                         @RequestHeader(value = HttpHeaders.RANGE, required = false) String range) {
        if (StringUtils.isEmpty(versionNum)) {
            throw new ServiceException("版本号不能为空");
        }
        // jar包的上一级目录
        String canonicalPath = FileUtil.getCanonicalPath(new File(".."));
        String filePath = canonicalPath + "/device-upgrade-pack-repo/" + versionNum + "/" + versionNum + ".tar";
        long fileSize = FileUtil.file(filePath).length();

        // 获得文件流
        BufferedInputStream inputStream = FileUtil.getInputStream(filePath);

        // offset表示从哪个字节开始读取，length表示读取多少个字节
        long offset = 0;
        long length;
        // 如果range为空，那么就是读取整个文件
        if (StringUtils.isEmpty(range)) {
            length = fileSize;
        } else {
            // 根据range计算offset和length
            Pair<Long, Long> pair = getOffset(range);
            offset = pair.getLeft();
            length = pair.getRight();
            // length == -1 表示 end range 为最后一个字节
            if (length == -1) {
                length = fileSize - offset + 1;
            }
            // 跳过文件的offset个字节
            inputStream.skip(offset);
        }

        response.setContentType("application/octet-stream");
        response.setHeader("Content-disposition",
                "attachment;filename=" + versionNum + ".tar");
        response.setHeader("Content-Length", String.valueOf(length));
        response.setHeader("Content-Range", "bytes" + " " +
                offset + "-" + (offset + length - 1) + "/" + fileSize);
        response.setStatus(HttpServletResponse.SC_PARTIAL_CONTENT);
        ServletOutputStream os = response.getOutputStream();

        byte[] buf = new byte[1024];
        int bytesRead;
        // sum 是当前读取的字节数
        int sum = 0;
        while (sum < length && (bytesRead = inputStream.read(buf, 0, (length - sum) <= buf.length ? (int) (length - sum) : buf.length)) != -1) {
            os.write(buf, 0, bytesRead);
            sum += bytesRead;
        }

        inputStream.close();
        os.close();
    }

解析range：

    /**
     * left is offset, right is length
     *
     * @param range
     * @return
     */
    private Pair<Long, Long> getOffset(String range) {
        if (!range.contains("bytes=") || !range.contains("-")) {
            throw new ServiceException("range 格式错误");
        }
        range = range.substring(range.lastIndexOf("=") + 1).trim();
        String[] split = range.split("-");
        if (split.length == 0 || split.length > 2) {
            throw new ServiceException("range 格式错误");
        }
        if (split.length == 1) {
            if (range.startsWith("-")) {
                return Pair.of(0L, Long.parseLong(split[0]) + 1);
            } else {
                // -1 表示length无限制
                return Pair.of(Long.parseLong(split[0]), -1L);
            }
        } else {
            return Pair.of(Long.parseLong(split[0]), Long.parseLong(split[1]) - Long.parseLong(split[0]) + 1);
        }
    }

测试下载接口：

    @Test
    void downloadObject() {
        OkHttpClient httpClient = HttpUtil.getHttpClient();
        Request request = new Request.Builder()
                .get()
                .url("http://localhost:8081/device/version/download?versionNum=1.1.1.2023")
                .header("Range", "bytes=0-10")
                .build();

        try (Response response = httpClient.newCall(request).execute()) {
            InputStream inputStream = response.body().byteStream();
            byte[] buf = new byte[5];
            int readBytes;

            while ((readBytes = inputStream.read(buf)) != -1) {
                if (readBytes < buf.length) {
                    buf = Arrays.copyOf(buf, readBytes);
                }
                System.out.println(Arrays.toString(buf) + "   " + readBytes);
            }
            inputStream.close();
            response.close();
            System.out.println("end");
        } catch (IOException e) {
            throw new ServiceException("请求异常");
        }
    }

输出结果：
[49, 46, 49, 46, 49]   5
[46, 50, 48, 50, 51]   5
[46]   1
end