一 .文件下载

ResponseEntity用于控制器方法的返回值类型，该控制器方法的返回值就是响应到浏览器的响应报文

1. 搞一张图片

 2. index.html

 <a th:href="@{/test/down}">下载图片</a>

3 .创建控制器FileUpAndDownController

 @RequestMapping("/test/down")
    public ResponseEntity<byte[]> testResponseEntity(HttpSession session) throws IOException {
        //获取ServletContext对象
        ServletContext servletContext = session.getServletContext();
        //获取服务器中文件的真实路径
        String realPath = servletContext.getRealPath("img");
        realPath = realPath + File.separator + "1.jpg";
        //创建输入流
        InputStream is = new FileInputStream(realPath);
        //创建字节数组，is.available()获取输入流所对应文件的字节数
        byte[] bytes = new byte[is.available()];
        //将流读到字节数组中
        is.read(bytes);
        //创建HttpHeaders对象设置响应头信息
        MultiValueMap<String, String> headers = new HttpHeaders();
        //设置要下载方式以及下载文件的名字
        headers.add("Content-Disposition", "attachment;filename=1.jpg");
        //设置响应状态码
        HttpStatus statusCode = HttpStatus.OK;
        //创建ResponseEntity对象
        ResponseEntity<byte[]> responseEntity = new ResponseEntity<>(bytes, headers, statusCode);
        //关闭输入流
        is.close();
        return responseEntity;
    }

二 .文件上传

1. index.html

    <form th:action="@{/test/up}" method="post" enctype="multipart/form-data">
        头像：<input type="file" name="photo"><br>
        <input type="submit" value="上传">
    </form>

2 .创建控制器FileUpAndDownController

 @RequestMapping("/test/up")
    public String testUp(MultipartFile photo, HttpSession session) throws IOException {
        //获取上传的文件的文件名
        String fileName = photo.getOriginalFilename();
        //获取上传的文件的后缀名
        String hzName = fileName.substring(fileName.lastIndexOf("."));
        //获取uuid
        String uuid = UUID.randomUUID().toString();
        //拼接一个新的文件名
        fileName = uuid + hzName;
        //获取ServletContext对象
        ServletContext servletContext = session.getServletContext();
        //获取当前工程下photo目录的真实路径
        String photoPath = servletContext.getRealPath("photo");
        //创建photoPath所对应的File对象
        File file = new File(photoPath);
        //判断file所对应目录是否存在
        if(!file.exists()){
            file.mkdir();
        }
        String finalPath = photoPath + File.separator + fileName;
        //上传文件
        photo.transferTo(new File(finalPath));
        return "success";
    }

3. spring.xml 里面配置文件上传解析器

 <!--配置文件上传解析器-->
    <bean id="multipartResolver" class="org.springframework.web.multipart.commons.CommonsMultipartResolver">

4 .添加依赖

<!-- https://mvnrepository.com/artifact/commons-fileupload/commons-fileupload -->
<dependency>
    <groupId>commons-fileupload</groupId>
    <artifactId>commons-fileupload</artifactId>
    <version>1.3.1</version>
</dependency>

总结：

* 文件上传的要求：
* 1、form表单的请求方式必须为post
* 2、form表单必须设置属性enctype="multipart/form-data"