文件下載的五大步驟

1. 通過請求參數，獲取要下載的文件名

2. 通過響應頭，告訴客戶端返回的數據類型

3. 通過響應頭，告訴客戶端收到的數據是用於下載的

4. 通過文件名，讀取要下載的文件內容

5. 通過響應輸出流，回傳要下載的文件到客戶端

 

    @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);
    }