如何通过JSP+Servlet实现生成登录验证码功能？

package servlet;

import java.awt.BasicStroke;
import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.geom.AffineTransform;
import java.awt.geom.Line2D;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.Random;

import javax.imageio.ImageIO;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

public class PictureCheckCode extends HttpServlet {

  private static final long serialVersionUID = 1L; 

  public PictureCheckCode() { 
   super(); 
  } 

  public void destroy() { 
   super.destroy(); 
  } 

  public void init() throws ServletException { 
   super.init(); 
  } 
  /*该方法主要作用是获得随机生成的颜色*/ 
  public Color getRandColor(int s,int e){ 
   Random random=new Random (); 
   if(s>255) s=255; 
   if(e>255) e=255; 
   int r,g,b; 
   r=s+random.nextInt(e-s); //随机生成RGB颜色中的r值 
   g=s+random.nextInt(e-s); //随机生成RGB颜色中的g值 
   b=s+random.nextInt(e-s); //随机生成RGB颜色中的b值 
   return new Color(r,g,b); 
  } 

 public void doGet(HttpServletRequest request, HttpServletResponse response)
   throws ServletException, IOException {
  System.out.println("this is doGet method");
  this.doPost(request, response);

 }

 public void doPost(HttpServletRequest request, HttpServletResponse response)
   throws ServletException, IOException {

  //设置不缓存图片 
  response.setHeader("Pragma", "No-cache"); 
  response.setHeader("Cache-Control", "No-cache"); 
  response.setDateHeader("Expires", 0); 
  //指定生成的响应图片,一定不能缺少这句话,否则错误. 
  response.setContentType("image/jpeg"); 
  int width=80,height=35;  //指定生成验证码的宽度和高度 
  BufferedImage image=new BufferedImage(width,height,BufferedImage.TYPE_INT_RGB); //创建BufferedImage对象,其作用相当于一图片 
  Graphics g=image.getGraphics();  //创建Graphics对象,其作用相当于画笔 
  Graphics2D g2d=(Graphics2D)g;  //创建Grapchics2D对象 
  Random random=new Random(); 
  Font mfont=new Font("楷体",Font.BOLD,16); //定义字体样式 
  g.setColor(getRandColor(200,250)); 
  g.fillRect(0, 0, width, height); //绘制背景 
  g.setFont(mfont);     //设置字体 
  g.setColor(getRandColor(180,200)); 

  //绘制100条颜色和位置全部为随机产生的线条,该线条为2f 
  for(int i=0;i<100;i++){ 
   int x=random.nextInt(width-1); 
   int y=random.nextInt(height-1); 
   int x1=random.nextInt(6)+1; 
   int y1=random.nextInt(12)+1; 
   BasicStroke bs=new BasicStroke(2f,BasicStroke.CAP_BUTT,BasicStroke.JOIN_BEVEL); //定制线条样式 
   Line2D line=new Line2D.Double(x,y,x+x1,y+y1); 
   g2d.setStroke(bs); 
   g2d.draw(line);  //绘制直线 
  } 
  //输出由英文，数字，和中文随机组成的验证文字，具体的组合方式根据生成随机数确定。 
  String sRand=""; 
  String ctmp=""; 
  int itmp=0; 
  //制定输出的验证码为四位 
  for(int i=0;i<4;i++){ 
   switch(random.nextInt(3)){ 
    case 1:  //生成A-Z的字母 
      itmp=random.nextInt(26)+65; 
      ctmp=String.valueOf((char)itmp); 
      break; 
    case 2:  //生成汉字 
      String[] rBase={"0","1","2","3","4","5","6","7","8","9","a","b","c","d","e","f"}; 
      //生成第一位区码 
      int r1=random.nextInt(3)+11; 
      String str_r1=rBase[r1]; 
      //生成第二位区码 
      int r2; 
      if(r1==13){ 
       r2=random.nextInt(7);  
      }else{ 
       r2=random.nextInt(16); 
      } 
      String str_r2=rBase[r2]; 
      //生成第一位位码 
      int r3=random.nextInt(6)+10; 
      String str_r3=rBase[r3]; 
      //生成第二位位码 
      int r4; 
      if(r3==10){ 
       r4=random.nextInt(15)+1; 
      }else if(r3==15){ 
       r4=random.nextInt(15); 
      }else{ 
       r4=random.nextInt(16); 
      } 
      String str_r4=rBase[r4]; 
      //将生成的机内码转换为汉字 
      byte[] bytes=new byte[2]; 
      //将生成的区码保存到字节数组的第一个元素中 
      String str_12=str_r1+str_r2; 
      int tempLow=Integer.parseInt(str_12, 16); 
      bytes[0]=(byte) tempLow; 
      //将生成的位码保存到字节数组的第二个元素中 
      String str_34=str_r3+str_r4; 
      int tempHigh=Integer.parseInt(str_34, 16); 
      bytes[1]=(byte)tempHigh; 
      ctmp=new String(bytes); 
      break; 
    default: 
      itmp=random.nextInt(10)+48; 
      ctmp=String.valueOf((char)itmp); 
      break; 
   } 
   sRand+=ctmp; 
   Color color=new Color(20+random.nextInt(110),20+random.nextInt(110),random.nextInt(110)); 
   g.setColor(color); 
   //将生成的随机数进行随机缩放并旋转制定角度 PS.建议不要对文字进行缩放与旋转,因为这样图片可能不正常显示 
   /*将文字旋转制定角度*/ 
   Graphics2D g2d_word=(Graphics2D)g; 
   AffineTransform trans=new AffineTransform(); 
   trans.rotate((45)*3.14/180,15*i+8,7); 
   /*缩放文字*/ 
   float scaleSize=random.nextFloat()+0.8f; 
   if(scaleSize>1f) scaleSize=1f; 
   trans.scale(scaleSize, scaleSize); 
   g2d_word.setTransform(trans); 
   g.drawString(ctmp, 15*i+18, 14); 
  } 
  HttpSession session=request.getSession(true); 
  session.setAttribute("randCheckCode", sRand); 
  System.out.println(sRand);
  g.dispose(); //释放g所占用的系统资源 
  ImageIO.write(image,"JPEG",response.getOutputStream()); //输出图片 
 }

}

<servlet>
 <description>This is the description of my J2EE component</description>
 <display-name>This is the display name of my J2EE component</display-name>
 <servlet-name>PictureCheckCode</servlet-name>
 <servlet-class>servlet.PictureCheckCode</servlet-class>
 </servlet>

<servlet-mapping>
 <servlet-name>PictureCheckCode</servlet-name>
 <url-pattern>/pictureCheckCode</url-pattern>
 </servlet-mapping>

<%@ page language="java" import="java.util.*" pageEncoding="utf-8"%>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>

<title>登录页面</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">
<script language="javascript">
 function myReload() {
  document.getElementById("CreateCheckCode").src = document
    .getElementById("CreateCheckCode").src
    + "?nocache=" + new Date().getTime();
 }
</script>

</head>

<body>
 <form action="loginServlet" method="get">
   <p class="font tdheight">
    验证码：<input type="text" name="checkCode"
     style="width:70px;height:35px;"> <img
     src="pictureCheckCode" id="CreateCheckCode" align="middle"
     style="margin-top:-10px;"> <br> <a href=""
     onclick="myReload()"
     style="font-size:12px;color:red;margin-left:156px;"> 看不清,换一个</a>
   </p>
   <br><input type="submit" value="提交"
      class="font" style="width:240px;height:35px;background:#f19e28">

  </form>
</body>
</html>

public void doGet(HttpServletRequest request, HttpServletResponse response)
   throws ServletException, IOException {

  String code = request.getParameter("checkCode");
  HttpSession session = request.getSession();
  if (!code.equals(session.getAttribute("randCheckCode"))) {
   request.setAttribute("errormsg", "验证码不正确");
  } 
  System.out.println(request.getAttribute("errormsg"));
 }