本文主要通过ResponseEntity<byte[]>实现文件下
该类实现响应头、文件数据（以字节存储）、状态封装在一起交给浏览器处理以实现浏览器的文件下载。
ResponseEntity参数解释：ResponseEntity(T body, MultiValueMap<String, String> headers, HttpStatus statusCode)
其中ResponseEntity<T> extends HttpEntity<T>，很明显的继承关系，HttpEntity是一个实体类，在new ResponseEntity<byte[]>(b, headers, statusCode);这句初始化的时候，会将T body, MultiValueMap<String, String> headers两个参数传给父类，本类中存放状态码，在HttpEntity类的源码中可以看到：

public HttpEntity(T body, MultiValueMap<String, String> headers) {
this.body = body;
HttpHeaders tempHeaders = new HttpHeaders();
if (headers != null) {
tempHeaders.putAll(headers);
}
//将header头转变成只能读取的对象，而不是写入的对象。
this.headers = HttpHeaders.readOnlyHttpHeaders(tempHeaders);
}

HttpHeaders类说明：表示HTTP请求和响应头，将字符串头名映射到字符串值的列表。
在这里为什么要用HttpHeaders类，是因为MultiValueMap接口的实现类是：HttpHeaders、LinkedMultiValueMap以及静态类MultiValueMapAdapter
话不多说直接上代码：

//下载练习

@GetMapping("/download")
public ResponseEntity<byte[]> download(@RequestParam(name = "id") Long id, HttpServletRequest request) {
    TemplateDto byId = getBaseService().findById(id);
    HttpHeaders headers = new HttpHeaders();
    String fileName = null;
    try {
        fileName = FileExtraUtils.handleFileName(request, "download.pdf");
    } catch (UnsupportedEncodingException e) {
        //ignore
        log.warn("文件名处理出错", e);
    }
    headers.setContentDispositionFormData("attachment", fileName);
    headers.setContentType(MediaType.APPLICATION_OCTET_STREAM);

    return new ResponseEntity<>(byId.getTemplaeContent(), headers, HttpStatus.OK);
}

//上传文件并保存

@PostMapping("/upload")
public void upload(@Param("file") MultipartFile file, HttpServletResponse response) throws IOException {
    File tempFile;
    try {
        tempFile = File.createTempFile(file.getOriginalFilename(), ".pdf");
        file.transferTo(tempFile);
        HashMap data = new HashMap();
        data.put("fileName", tempFile.getCanonicalPath());
        data.put("displayName", file.getOriginalFilename());
        ResultDto dto =  ResultDtoFactory.toAckData(data);
        response.setContentType("text/html; charset=utf-8");
        response.getWriter().write(JSON.toJSONString(dto));
    } catch (IOException e) {
        log.error("保存文件失败", e);
        ResultDto dto = ResultDtoFactory.toNack("文件上传失败");
        response.setContentType("text/html; charset=utf-8");
        response.getWriter().write(JSON.toJSONString(dto));
    }
}

@Override
public ResultDto create(TemplateDto templateDto) {
    String fileName = templateDto.getFileName();
    try (FileInputStream fis = new FileInputStream(new File(fileName))) {
        templateDto.setTemplaeContent(IOUtils.toByteArray(fis));
    } catch (IOException e) {
        log.error("读取模版内容出错", e);
        return ResultDtoFactory.toNack("读取模板内容出错，请重新上传");
    }
    getBaseService().save(templateDto);
    return ResultDtoFactory.toAck();
}