如何校对前后台验证码

前一段代码是生成验证码的，后一段代码是用户登录时做判定用的，想在后面加上对验证码输入是否的判断。但是……
不知道哪位大大能帮帮我啊。import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;import java.util.Enumeration;
import java.util.Random;
import java.awt.*;
import java.awt.image.*;
import javax.imageio.*;
import javax.mail.Session;public class Img extends HttpServlet
{
public void doGet(HttpServletRequest request,
HttpServletResponse response) 
throws ServletException,IOException
{
//设置Content-Type响应头
response.setContentType("image/jpeg");
//下面三条语句用于设置页面不缓存
response.setHeader("Pragma","No-cache");
response.setHeader("Cache-Control","no-cache");
response.setDateHeader("Expires",0);

OutputStream out = response.getOutputStream();
int width=80, height=20;
//建立指定宽、高BufferedImage对象
BufferedImage image = new BufferedImage(width, height, 
BufferedImage.TYPE_INT_RGB);

Graphics g = image.getGraphics(); //生成随机类
Random random = new Random();

g.fillRect(0, 0, width, height);
//设置字体
g.setFont(new Font("Times New Roman",Font.ITALIC,18));
String sRand = "";
for (int i=0;i<4;i++){
    String rand = String.valueOf(random.nextInt(10));
    sRand += rand;
    // 设置随机颜色
    g.setColor(new Color(20 + random.nextInt(110),20 
     + random.nextInt(110),20 + random.nextInt(110)));
    //绘制
    g.drawString(rand,20*i + 6,16);
}
g.dispose();

// 输出图象到页面
ImageIO.write(image, "JPEG", out);


}
public void doPost(HttpServletRequest request,
HttpServletResponse response) 
throws ServletException,IOException
{
doGet(request,response);
}
}
<%@ page language="java" import="java.sql.*" pageEncoding="GB18030"%><%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
  <head>
    <base href="<%=basePath%>">
    
    <title>My JSP 'insert.jsp' starting page</title>
    
<meta http-equiv="pragma" content="no-cache">
<meta http-equiv="cache-control" content="no-cache">
<meta http-equiv="expires" content="0">    
<meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
<meta http-equiv="description" content="This is my page">
<!--
<link rel="stylesheet" type="text/css" href="styles.css">
-->  </head>
  
  <body>
    <%
    String username=request.getParameter("userName");
    String password=request.getParameter("psw");
    new oracle.jdbc.driver.OracleDriver();
    String url="jdbc:oracle:thin:@localhost:1521:orcl";
    Connection c=DriverManager.getConnection(url,"scott","tiger");
    Statement stmt=c.createStatement();
    Statement stmt1=c.createStatement();
    String sql1="select * from student where name='"+username+"'";
    String sql="insert into student values('"+username+"','"+password+"')";
    
    ResultSet rs=stmt1.executeQuery(sql1);    if(rs.next()){
    out.println("用户名已被占用");
    }else{
    stmt.executeUpdate(sql);
    response.sendRedirect("Login.html");
    }
    stmt1.close();
    stmt.close();
    c.close();
    }
    %>
  </body>
</html>