上传图片到Mongo数据库；从Mongo数据库下载图片显示到前端

/**
     * 上传一个图片文件，保存到mongo数据库中
     * @param doc
     * @param parent  文件所在的目录
     * @param request
     * @return
     */
    @PostMapping("/uploaddoc")
    public ErrorMessage uploaddoc(MultipartFile doc, @RequestParam String parent, HttpServletRequest request) throws IOException {
        ErrorMessage result = new ErrorMessage();
        MongoDatabase db = mongoTemplate.getDb();
        GridFSBucket bucket = GridFSBuckets.create(db, "docs"); // 打开一个对象，不存在时创建
        GridFSUploadStream uploadStream = bucket.openUploadStream(doc.getOriginalFilename());
        uploadStream.write(doc.getBytes());
        System.out.println(uploadStream.getObjectId());
        uploadStream.close();
    　 return result;
    }   

/**
 * 下载一个图片文件，显示在前端<img>中
 */
@GetMapping("/downloadImage/{uuid}")
public ErrorMessage downloadImage(@PathVariable String uuid, HttpServletResponse response) throws IOException {
    MongoDatabase db = mongoTemplate.getDb();
    GridFSBucket bucket = GridFSBuckets.create(db, "docs"); // 打开一个已存在的对象
    // uuid为图片文件在Mongo数据库中的Id
    GridFSDownloadStream downloadStream;
    try {
        downloadStream = bucket.openDownloadStream(new ObjectId(uuid));
    } catch (Exception e) {
        response.setStatus(412);
        return ErrorMessage.create("指定图片不存在！");
    }
    OutputStream outputStream = response.getOutputStream();
    IOUtils.copy(downloadStream, outputStream);
    downloadStream.close();
    outputStream.close();
    return null;
}

// 前端部分代码
// http://localhost:8080/downloadImage/5dca4f1d911b52478896430e 是访问后端的接口

// 5dca4f1d911b52478896430e是GridFs对象的ObjectId

<img id="img1" src="">
<button onclick="showImage()">get Image</button>
<script>
    function showImage() {
        document.getElementById('img1').src='http://localhost:8080/downloadImage/5dca4f1d911b52478896430e';
    }
</script>