response.setContentType("text/html;charset=utf-8");
  response.setCharacterEncoding("utf-8");
  //获取绝对路径.通过ervletContext()
  String path= this.getServletContext().getRealPath("/img/1.jpg");
  //设置请求的头信息，第一参数：头的名称,第二参数：头值，URLEncoder.encode()转编码值
  response.setHeader("content-disposition", "attachment;filename="+URLEncoder.encode(path.substring(path.lastIndexOf("\\")+1),"utf-8") );
  FileInputStream in=new FileInputStream(path);
  byte[] img=new byte[1024];
  int length=0;
  try {
   while ((length=in.read(img, 0, img.length))!=-1) {
    response.getOutputStream().write(img, 0, length);
   }
  } catch (Exception e) {
   e.printStackTrace();
  }finally{
   in.close();
  }