Secure File Upload, Download, and Preview with MinIO and SpringBoot

Preface

The previous article exposed the MinIO address when previewing files; this guide improves security by using file streams, token validation, and base64‑encoded URLs.

1. File Upload

Upload service implementation:

public void uploadFile(MultipartFile file) throws Exception {
    String fileName = System.currentTimeMillis() + "-" + file.getOriginalFilename();
    PutObjectArgs args = PutObjectArgs.builder()
        .bucket(minioConfig.getBucketName())
        .object(fileName)
        .stream(file.getInputStream(), file.getSize(), -1)
        .contentType(file.getContentType())
        .build();
    client.putObject(args);
}

Upload controller:

@PostMapping("upload")
public RestResult
upload(MultipartFile file) {
    try {
        sysFileService.uploadFile(file);
    } catch (Exception e) {
        log.error("上传文件失败", e);
        return RestResult.fail(e.getMessage());
    }
}

2. File Download

Download service implementation:

public void download(String filename, HttpServletResponse response) throws ServiceException {
    try {
        InputStream inputStream = client.getObject(GetObjectArgs.builder()
            .bucket(minioConfig.getBucketName())
            .object(filename)
            .build());
        response.setContentType("application/octet-stream;charset=UTF-8");
        response.setCharacterEncoding("UTF-8");
        response.setHeader("Content-Disposition", "attachment;filename=" + URLEncoder.encode(filename, "UTF-8"));
        OutputStream outputStream = response.getOutputStream();
        byte[] buffer = new byte[4096];
        int bytesRead;
        while ((bytesRead = inputStream.read(buffer)) != -1) {
            outputStream.write(buffer, 0, bytesRead);
        }
        inputStream.close();
        outputStream.close();
    } catch (Exception e) {
        log.error("文件下载失败：" + e.getMessage());
        throw new ServiceException("文件下载失败");
    }
}

Download controller (token validation required):

@ApiOperation("文件下载")
@GetMapping("/download/{token}/{filename}")
public void getDownload(@PathVariable("token") String token,
                        @PathVariable("filename") String filename,
                        HttpServletResponse response) {
    tokenUtils.validateToken(token);
    sysFileService.download(filename, response);
}

Key notes:

The @GetMapping("/download/{token}/{filename}") path must place filename last.

Token validation should be performed in an interceptor; the endpoint must be whitelisted for kkfile preview requests.

3. File Preview URL Generation

Service that creates a preview URL (token is appended but not validated here):

public String getPreviewUrl(String filename) throws UnsupportedEncodingException {
    ServletRequestAttributes sra = (ServletRequestAttributes) RequestContextHolder.getRequestAttributes();
    HttpServletRequest request = sra.getRequest();
    if (request == null || StringUtils.isBlank(request.getHeader(TokenConstants.AUTHENTICATION))) {
        throw new ServiceException("未获取到有效token");
    }
    String previewUrl = filePreviewUrl + FileUploadUtils.base64UrlEncode(fileDownloadUrl + "/" + token + "/" + filename);
    return previewUrl + "&fullfilename=" + URLEncoder.encode(filename, "UTF-8");
}

Utility for base64 URL encoding:

public static String base64UrlEncode(String url) throws UnsupportedEncodingException {
    String base64Url = Base64.getEncoder().encodeToString(url.getBytes(StandardCharsets.UTF_8));
    return URLEncoder.encode(base64Url, "UTF-8");
}

Controller to obtain the preview URL:

@GetMapping("/getPreviewUrl")
public RestResult
getPreviewUrl(String filename) throws UnsupportedEncodingException {
    return RestResult.ok(sysFileService.getPreviewUrl(filename));
}

Test Scenario

Assumptions:

File service address: http://file-server

kkfile service address: http://kkfile-server

File name: xxxx.docx

Resulting preview URL example:

http://kkfile-server/onlinePreview?url=aHR0cDovLzE3Mi4xNi41MC4y....&fullfilename=xxxx.docx

The aHR0cDovLzE3Mi4xNi41MC4y.... part is produced by:

FileUploadUtils.base64UrlEncode("http://file-server" + "/" + token + "/" + filename);