response.addHeader("Content-Disposition", "attachment;filename=" + name);

action例子：

//下載檔案
 public void downfile(ActionMapping actionMapping, ActionForm actionForm,
   HttpServletRequest request, HttpServletResponse response) throws Exception {
  String name=request.getParameter("filename");
  if(null!=name&&!"".equals(name)){
   response.addHeader("Content-Disposition", "attachment;filename=" + name);