用IDEA学习getRealPath遇到的问题

 1 protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
 2         //1.1获取文件名
 3         String filename = request.getParameter("filename");
 4 
 5         //1.2获得文件路径
 6         ServletContext servletContext = this.getServletContext();
 7         String realPath = servletContext.getRealPath("/img/" + filename);
 8 
 9         System.out.println(realPath);
10         //1.3将文件读入内存中:输入流
11         FileInputStream fis = new FileInputStream(realPath);
12 
13         //2.1设置response 的响应头类型
14         String mimeType = servletContext.getMimeType("filename");
15         response.setHeader("content-type","mimeType");
16         //2.2设置response的响应头的打开方式
17         response.setHeader("content-disposition","attachment;filename="+filename);
18 
19         //3.将文件输出：输出流
20         ServletOutputStream sos = response.getOutputStream();
21         byte[] buff = new byte[1024 * 5];
22         int len = 0;
23         while((len = fis.read(buff)) != -1){
24             sos.write(buff, 0, len);
25         }
26         fis.close();