简单的jsp验证码

<%@ page language="java" contentType="text/html;"
import="java.util.*"
import="java.awt.*,java.awt.image.*,java.io.*,com.sun.image.codec.jpeg.*"
%><%
response.setHeader("Pragma","No-cache");
response.setHeader("Cache-Control","no-cache");
response.setDateHeader("Expires", 0);
int tmpNum = Math.abs(new Random().nextInt() % 1000000) + 1;
  String code =  tmpNum+ "";
   int StrLen =code.length();
   int j = 4 - StrLen;
   for(int i=0;i<4 - StrLen; i++)
     code= "0" + code;
  // session.setMaxInactiveInterval(10);
     session.putValue("CheckCode",code);
//out.print("document.write('"+code+"');");

        int width = 35,height = 12;
    BufferedImage bi = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
    Graphics g = bi.getGraphics();

    g.fillRect(0,0,width,height);
    g.setColor(Color.black);
    g.drawString(code, 1, height-1);

    response.setContentType("image/jpeg");
  ServletOutputStream fos=response.getOutputStream();
  JPEGImageEncoder jpegie = JPEGCodec.createJPEGEncoder(fos);
  jpegie.encode(bi);
  fos.close();
  if ( 1==1) return ;
%>