急，如果jsp页面拿到的和Session里面的不一样该怎么办

package validate;

import java.awt.*;
import java.awt.image.BufferedImage;
import java.io.IOException;

import java.util.*;

import javax.imageio.ImageIO;
import javax.servlet.ServletException;
import javax.servlet.http.*;

/**
* 验证码生成类
* @author Administrator
*
*/
public class ValidateCode extends HttpServlet{

private static final long serialVersionUID = 1L;
HttpSession session;
String rand=null;
public void doGet(HttpServletRequest request,HttpServletResponse response)throws ServletException,IOException{
//设置页面不缓存
session=request.getSession();
response.setHeader("Pragma", "No-cache");
response.setHeader("Cache-Control", "no-cache");
response.setDateHeader("Expires", 0);
//输出图像到页面
ImageIO.write(this.creatImage(), "JPEG", response.getOutputStream());