java struts实现文件下载

public ActionForward download(ActionMapping mapping, ActionForm form,
   HttpServletRequest request, HttpServletResponse response)
   throws Exception {
  String filename = new String(request.getParameter("flag").getBytes(
    "iso-8859-1"), "gbk");
  String path=this.getServlet().getServletContext().getRealPath("");
  File file = new File(path+"//"+filename);
  // File file = new File(filename);
  // 设置response的编码方式
  response.setContentType("application/x-msdownload");
  // 写明要下载的文件的大小
  response.setContentLength((int) file.length());
  // 设置附加文件名
  response.setHeader("Content-Disposition", "attachment;filename="
    + filename);
  // 解决中文乱码
  response.setHeader("Content-Disposition", "attachment;filename="
    + new String(filename.getBytes("gbk"), "iso-8859-1"));
  // 读出文件到i/o流
  FileInputStream fis = new FileInputStream(file);
  BufferedInputStream buff = new BufferedInputStream(fis);
  byte[] b = new byte[1024];
  // 相当于我们的缓存
  long k = 0;
  // 该值用于计算当前实际下载了多少字节
  // 从response对象中得到输出流,准备下载
  OutputStream myout = response.getOutputStream();
  // 开始循环下载
  while (k < file.length()) {
   int j = buff.read(b, 0, 1024);
   k += j;
   // 将b中的数据写到客户端的内存
   myout.write(b, 0, j);
  }
  // 将写入到客户端的内存的数据,刷新到磁盘
  myout.flush();
 return null;
 }