protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { //设置ContentType字段值 response.setContentType("text/html;charset=utf-8"); //获取所要下载的文件名称 String filename = request.getParameter("filename"); //下载文件所在目录 // 通知浏览器以下载的方式打开 response.setContentType("application/octet-stream"); response.setHeader("Content-Disposition","attachment;filename="+filename); // 通过文件流读取文件 String path=request.getSession().getServletContext().getRealPath("/file/"+filename); System.out.println(path); InputStream in = new FileInputStream(path); String save="E:/download/" +filename; Path path1= Paths.get(save); Files.copy(in,path1); // 获取response对象的输出流 OutputStream out = response.getOutputStream(); Files.copy(path1,out); out.flush(); // byte[] buffer = new byte[1024]; // int len; // //循环取出流中的数据 // while ((len = in.read(buffer)) != -1) { // out.write(buffer, 0, len); // } // out.close(); // in.close(); }修改代码实现文件下载

以下是修改后的代码，实现了文件下载：

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    //设置ContentType字段值
    response.setContentType("text/html;charset=utf-8");
    //获取所要下载的文件名称
    String filename = request.getParameter("filename");
    //下载文件所在目录
    String filePath = request.getServletContext().getRealPath("/file/" + filename);
    File downloadFile = new File(filePath);

    //判断文件是否存在
    if (downloadFile.exists()) {
        //设置响应头，告诉浏览器是下载文件
        response.setContentType("application/octet-stream");
        response.setHeader("Content-Disposition", "attachment;filename=" + filename);

        //将文件从服务器读取到内存中，再写入到响应流中
        FileInputStream in = new FileInputStream(downloadFile);
        OutputStream out = response.getOutputStream();
        byte[] buffer = new byte[1024];
        int len;
        while ((len = in.read(buffer)) != -1) {
            out.write(buffer, 0, len);
        }
        in.close();
        out.close();
    } else {
        //文件不存在，返回404状态码
        response.sendError(HttpServletResponse.SC_NOT_FOUND);
    }
}

这里使用了`doGet`方法，因为文件下载一般是通过GET请求实现的。另外，代码中还做了文件存在性的判断，如果文件不存在，则返回404状态码。