String id = request.getParameter("downLoadFileId");
  String fileName = request.getParameter("fileName");
  String path = "C:\\upload\\"+id+"\\"+fileName;(存放文件的路径)
  
  response.setHeader("Content-Disposition",  "attachment;filename="+fileName);
 

  response.setHeader("Connection",  "close"); 
  response.setHeader("Content-Type",  "application/octet-stream");
  try{
   OutputStream os = response.getOutputStream(); //不加此行将只能下载文本文件.下载jpg等就会出现打不开的现象.
   FileInputStream fis = new FileInputStream(path);
   byte[] b = new byte[1024];
   int i = 0;
 
   while ( (i = fis.read(b)) > 0 )
   {
    os.write(b, 0, i);
   }
   
   fis.close();
   os.flush();
   os.close();
   }
   catch ( Exception e )
   {
    System.out.println ( "IOException." + e );
   }