public class FileUploadUtil {
    public static final List<String> ALLOW_TYPES = Arrays.asList(
            "image/jpg","image/jpeg","image/png","image/gif"
    );
//文件重命名
    public static String rename(String fileName){
        int i = fileName.lastIndexOf(".");
        String str = fileName.substring(i);
        return new Date().getTime()+""+ new Random().nextInt(99999999) +str;
    }
//校验文件类型是否是被允许的
    public static boolean allowUpload(String postfix){
        return ALLOW_TYPES.contains(postfix);
    }
}

//自己写的一个使用例子
private void uploadGoodsImg(
            MultipartFile[] imageFiles,
            String goodsId,
            String realPath,
            String resourcePath,
            String type
    ) throws IOException {
        if (imageFiles != null) {
            for (int i = 0; i < imageFiles.length; i++) {
                MultipartFile imageFile = imageFiles[i];
//先检验文件类型是否被允许
                if (FileUploadUtil.allowUpload(imageFile.getContentType())) {
//                获得文件名称后重命名
                    String fileName = goodsId + FileUploadUtil.rename(imageFile.getOriginalFilename());
                    File file = new File(realPath + resourcePath + fileName);
                    imageFile.transferTo(file);
                    //缩略图
                    Thumbnails.of(file)
                            .size(160, 160)
                            .keepAspectRatio(false)
                            .outputFormat("jpg")
                            .toFile(new File(realPath + resourcePath, fileName + "_160x160.jpg"));
                    String imgUrl = resourcePath + fileName;
                    GoodsImg goodsImg = new GoodsImg();
                    goodsImg.setId(UUID.randomUUID().toString());
                    goodsImg.setGoodsId(goodsId);
                    goodsImg.setUrl(imgUrl);
                    goodsImg.setSort(i + 1);
                    goodsImgDao.insert(goodsImg);
                }
                else {
                    throw new RuntimeException("文件类型不合法,只能是 jpg、gif、png、jpeg 类型！");
                }
            }
        } else if(type.equals("add")) {
            GoodsImg goodsImg = new GoodsImg();
            goodsImg.setId(UUID.randomUUID().toString());
            goodsImg.setGoodsId(goodsId);
            goodsImg.setUrl(null);
            goodsImg.setSort(1);
            goodsImgDao.insert(goodsImg);
        }
    }