SpringMVC使用ResponseEntity實現文件下載，及圖片base64的字節數組上傳於下載

本文主要通過ResponseEntity<byte[]>實現文件下
該類實現響應頭、文件數據（以字節存儲）、狀態封裝在一起交給瀏覽器處理以實現瀏覽器的文件下載。
ResponseEntity參數解釋：ResponseEntity(T body, MultiValueMap<String, String> headers, HttpStatus statusCode)
其中ResponseEntity<T> extends HttpEntity<T>，很明顯的繼承關系，HttpEntity是一個實體類，在new ResponseEntity<byte[]>(b, headers, statusCode);這句初始化的時候，會將T body, MultiValueMap<String, String> headers兩個參數傳給父類，本類中存放狀態碼，在HttpEntity類的源碼中可以看到：

public HttpEntity(T body, MultiValueMap<String, String> headers) {
this.body = body;
HttpHeaders tempHeaders = new HttpHeaders();
if (headers != null) {
tempHeaders.putAll(headers);
}
//將header頭轉變成只能讀取的對象，而不是寫入的對象。
this.headers = HttpHeaders.readOnlyHttpHeaders(tempHeaders);
}

HttpHeaders類說明：表示HTTP請求和響應頭，將字符串頭名映射到字符串值的列表。
在這里為什么要用HttpHeaders類，是因為MultiValueMap接口的實現類是：HttpHeaders、LinkedMultiValueMap以及靜態類MultiValueMapAdapter
話不多說直接上代碼：

//下載練習

@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("文件名處理出錯", e);
    }
    headers.setContentDispositionFormData("attachment", fileName);
    headers.setContentType(MediaType.APPLICATION_OCTET_STREAM);  //二進制流（或者字節數組）的形式返回

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

或者根據臨時文件保存的路徑獲取輸入流，然后copy給輸出流進行下載

InputStream inputStream = null;
OutputStream outputStream = null;
try {
    inputStream = new FileInputStream(filePath);
    outputStream = response.getOutputStream();
    response.setContentType("application/x-download");
    response.addHeader("Content-Disposition", "attachment;filename=" + info.getFileName());
    IOUtils.copy(inputStream, outputStream);
    outputStream.flush();
}catch (Exception e){
    e.printStackTrace();
}finally {
    IOUtils.closeQuietly(inputStream);
    IOUtils.closeQuietly(outputStream);
}

//上傳文件並保存

@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("保存文件失敗", e);
        ResultDto dto = ResultDtoFactory.toNack("文件上傳失敗");
        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("讀取模版內容出錯", e);
        return ResultDtoFactory.toNack("讀取模板內容出錯，請重新上傳");
    }
    getBaseService().save(templateDto);
    return ResultDtoFactory.toAck();
}

//另外圖片的上傳於下載

@PostMapping("/upload")
public void upload(@RequestParam String configKey, MultipartFile file, HttpServletResponse response) throws IOException {
    ConfigDto configDto = new ConfigDto();
    configDto.setConfigKey(configKey);
    String base64Content = Base64Utils.encodeToString(IOUtils.toByteArray(file.getInputStream()));
    configDto.setConfigValue(base64Content);
    configService.save(configDto);
    ResultDto resultDto = ResultDtoFactory.toAck();
    response.setContentType("text/html; charset=utf-8");
    response.getWriter().write(JSON.toJSONString(resultDto));
}

@GetMapping("/download")
public void download(@RequestParam(required = true) String configKey, HttpServletResponse response) throws IOException {
    List<ConfigDto> byKey = configService.findByKey(configKey);
    if (CollectionUtils.isNotEmpty(byKey)) {
        ConfigDto configDto = byKey.get(0);
        if(configDto!=null){
            String base64Content = configDto.getConfigValue();
            byte[] bytes = Base64Utils.decodeFromString(base64Content);
            IOUtils.copy(new ByteArrayInputStream(bytes), response.getOutputStream());
            response.getOutputStream().flush();
        }
    }
}