servlet:
=============
import java.io.IOException;
import javax.servlet.ServletException;

import javax.servlet.http.*;
import javax.servlet.ServletContext;
import java.io.OutputStream;
import java.util.Random;
import java.awt.*;
import java.awt.image.*;
import com.sun.image.codec.jpeg.*;
import webutil.*;
/**
 * @version  1.0
 * @author dennis
 */
public class Getshowimg extends HttpServlet {
 private String code;
 
 public void service(HttpServletRequest req, HttpServletResponse resp)
  throws ServletException, IOException {
   
   resp.setContentType("image/jpeg;charset=gb2312");
   int width=80, height=20;
   BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_RG;
   Graphics g = image.getGraphics();
   //以下填充背景颜色
   g.setColor(Color.decode("#66CCCF"));
   g.fillRect(0, 0, width, height);
   g.setColor(Color.decode("#01556B"));
   g.setFont(new Font("Times New Roman",Font.PLAIN,18));
// 生成随机类
   Random random = new Random();   
// 随机产生155条干扰线，使图象中的认证码不易被其它程序探测到
   g.setColor(getRandColor(160,200));   
   for (int i=0;i<155;i++)
   {
     int x = random.nextInt(width);
     int y = random.nextInt(height);
           int xl = random.nextInt(12);
           int yl = random.nextInt(12);
     g.drawLine(x,y,x+xl,y+yl);
   }
   
   String rstr = "";
   RandomStrg rst = new RandomStrg();
   rst.setCharset("a-zA-Z0-9");
   rst.setLength("6");
   
   
   try{
     rst.generateRandomObject();
     rstr= rst.getRandom();
    
   }catch(Exception ex){
     ex.printStackTrace();
     
   }
   
   String sRand="";
   for (int i=0;i<rstr.length();i++){
       String rand=String.valueOf(rstr.charAt);
//       sRand+=rand;
       // 将认证码显示到图象中
       g.setColor(new Color(20+random.nextInt(110),20+random.nextInt(110),20+random.nextInt(110)));//调用函数出来的颜色相同，可能是因为种子太接近，所以只能直接生成
       g.drawString(rand,13*i+5,15);
   }
//   g.drawString(req.getParameter("yzm"),5,15);

   
   //g.drawString(rstr,5,15);
   g.dispose();    
   OutputStream toClient = resp.getOutputStream();
   JPEGImageEncoder encoder =JPEGCodec.createJPEGEncoder(toClient);
   encoder.encode(image);
            toClient.close();
 
   
            req.getSession().setAttribute("rstr",rstr);
            this.code = rstr;
            System.out.println(req.getSession().getAttribute("rstr"));
   }
 
   public String getCode(){
     return this.code;
   }
   
   Color getRandColor(int fc,int bc){//给定范围获得随机颜色
       Random random = new Random();
       if(fc>255) fc=255;
       if(bc>255) bc=255;
       int r=fc+random.nextInt(bc-fc);
       int g=fc+random.nextInt(bc-fc);
       int b=fc+random.nextInt(bc-fc);
       return new Color(r,g,b);
   }
 
}

jsp:
=====
<form name = form1 method="post">

验证码：<input name="yzm" type="text" size="12"  id="yzm0"></td>
  <div id="yzm1" style="width:10" style="display:none"></div><IMG  id="img0" src="Getshowimg" vspace=3>
<input type=submit name=submit value="submit" onclick="javascript:return checkcode();" >
</form>
<%

out.println("session::"+session.getAttribute("rstr") );
%>

为何第一次打印的session值为空？？再刷新一次页面显示的session是刷新之前servlet里面设置的值？？请问怎样才能第一次就显示和servlet里面的值一样啊？？