文件下载的五大步骤

1. 通过请求参数，获取要下载的文件名

2. 通过响应头，告诉客户端返回的数据类型

3. 通过响应头，告诉客户端收到的数据是用于下载的

4. 通过文件名，读取要下载的文件内容

5. 通过响应输出流，回传要下载的文件到客户端

 

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String downloadFileName = req.getParameter("filename");
        ServletContext servletContext = getServletContext();
        String mimeType = servletContext.getMimeType("/file/" + downloadFileName);
        resp.setContentType(mimeType);
        if (req.getHeader("User-Agent").contains("Firefox")) {
            resp.setHeader("Content-Disposition", "attachment; filename==?UTF-8?B?" + new BASE64Encoder().encode(downloadFileName.getBytes("UTF-8")) + "?=");
        } else {
            resp.setHeader("Content-Disposition", "attachment; filename=" + URLEncoder.encode(downloadFileName, "UTF-8"));
        }

        InputStream resourceAsStream = servletContext.getResourceAsStream("/file/" + downloadFileName);
        OutputStream outputStream = resp.getOutputStream();
        IOUtils.copy(resourceAsStream, outputStream);
    }