很簡單的程式碼，不多說，直接上程式碼

這是後臺程式碼

@RequestMapping("/download.do")
    public String download(int id, HttpServletRequest request,
            HttpServletResponse response) {
		
		User user = userService.getById(id);
		
        response.setCharacterEncoding("utf-8");
        response.setContentType("multipart/form-data");
        response.setHeader("Content-Disposition", "attachment;fileName="
                +"ldk.png");
        
        String path = user.getQrPath();
        //獲取資料
		//########################下載檔案begin########################
        try {
            InputStream is = new FileInputStream(new File(path));
            OutputStream os = response.getOutputStream();
            byte[] b = new byte[2048];
            int length;
            while ((length = is.read(b)) > 0) {
                os.write(b, 0, length);
            }
 
             // 這裡主要關閉。
            os.close();
 
            is.close();
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
      //########################下載檔案end########################
            //  返回值要注意，要不然就出現下面這句錯誤！
            //java+getOutputStream() has already been called for this response
        return null;
    } 
 

<input type="button" onclick="window.location.href='${ctx}/users/download.do?id=${user.id}'" value="匯出"/>