为方便查看，只写关键部分代码

1.在jsp页面中 ,添加链接servlet

<tr>
<td style="text-align:right;width:20%;"> </td>
<td colspan="2" style="width:50%">
<!--k1 图片由CheckImgServlet生成 -->
<img  src="${pageContext.request.contextPath}/servlet/CheckImgServlet" width="180"
height="30" class="textinput"  style="height:30px;" id="img" />  
<!--k1 点一下文字链接切换激活图片   -->
<a href="javascript:void(0);" onclick="changeImage()">看不清换一张</a>
</td>
</tr>

2.前端JS脚本执行切换图片

<script type="text/javascript">
//前端点击切换图片,跳转到CheckImgServlet并把当前时间传过去
function changeImage() 
{
     document.getElementById("img").src ="${pageContext.request.contextPath}/servlet/CheckImgServlet?time="+ new Date().getTime();
}
</script>

3.验证码生成工具，将验证码文件new_words.txt放入/WEB-INF/目录下，随机读取保存到session中

public class CheckImgServlet extends HttpServlet
{

    // 集合中保存所有成语
    private List<String> words = new ArrayList<String>();

    @Override
    public void init() throws ServletException
    {
// web工程中读取 文件，必须使用绝对磁盘路径
        String path = getServletContext().getRealPath("/WEB-INF/new_words.txt");
//创建一个使用默认大小输入缓冲区的BufferedReader
            BufferedReader reader = new BufferedReader(newInputStreamReader(new FileInputStream(path), "UTF-8"));
            String line;
            //读取一个文本行,如果已到达流末尾，则返回 null 
            while ((line = reader.readLine()) != null)
            {
                //每一行放进集合
                words.add(line);
            }
            reader.close();
    }

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

        // 禁止缓存3种方式
        // response.setHeader("Cache-Control", "no-cache");
        // response.setHeader("Pragma", "no-cache");
        response.setDateHeader("Expires", -1);

        int width = 120;
        int height = 30;

        // 步骤一 绘制一张内存中图片
        BufferedImage bufferedImage = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);

        // 步骤二 图片绘制背景颜色 ---通过绘图对象
        Graphics graphics = bufferedImage.getGraphics();// 得到画图对象 --- 画笔
        // 绘制任何图形之前 都必须指定一个颜色
        graphics.setColor(getRandColor(200, 250));
        graphics.fillRect(0, 0, width, height);

        // 步骤三 绘制边框
        graphics.setColor(Color.WHITE);
        graphics.drawRect(0, 0, width - 1, height - 1);

        // 步骤四 四个随机数字
        Graphics2D graphics2d = (Graphics2D) graphics;
        // 设置输出字体
        graphics2d.setFont(new Font("宋体", Font.BOLD, 18));

        Random random = new Random();// 生成随机数
        int index = random.nextInt(words.size());
        String word = words.get(index);// 获得成语

        System.out.println("session word=" + word);

        // 定义x坐标
        int x = 10;
        for (int i = 0; i < word.length(); i++)
        {
            // 随机颜色
            graphics2d.setColor(new Color(20 + random.nextInt(110), 20 + random
                    .nextInt(110), 20 + random.nextInt(110)));
            // 旋转 -30 --- 30度
            int jiaodu = random.nextInt(60) - 30;
            // 换算弧度
            double theta = jiaodu * Math.PI / 180;

            // 获得字母数字
            char c = word.charAt(i);

            // 将c 输出到图片
            graphics2d.rotate(theta, x, 20);
            graphics2d.drawString(String.valueOf(c), x, 20);
            graphics2d.rotate(-theta, x, 20);
            x += 30;
        }

//k4 将验证码内容保存session
        request.getSession().setAttribute("checkcode_session", word);

        // 步骤五 绘制干扰线
        graphics.setColor(getRandColor(160, 200));
        int x1;
        int x2;
        int y1;
        int y2;
        for (int i = 0; i < 30; i++)
        {
            x1 = random.nextInt(width);
            x2 = random.nextInt(12);
            y1 = random.nextInt(height);
            y2 = random.nextInt(12);
            graphics.drawLine(x1, y1, x1 + x2, x2 + y2);
        }

        // 将上面图片输出到浏览器 ImageIO
        graphics.dispose();// 释放资源
        ImageIO.write(bufferedImage, "jpg", response.getOutputStream());

    }

    private Color getRandColor(int fc, int bc)
    {
        // 取其随机颜色
        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);
    }

}

4.用户提交数据到servlet处理

public class UserRegisterServlet extends HttpServlet
{

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

//用户输入的验证码
        String ckcode = (String) request.getParameter("ckcode");

//session里面保存的验证码
        String checkcode_session = (String) request.getSession().getAttribute("checkcode_session");

        if (checkcode_session != null)
        {
            if (checkcode_session.equals(ckcode))
            {//验证码符合条件
                    //用dbutils工具封装数据
                    Map<String, String[]> parameterMap = request.getParameterMap();
                    User user = new User();
                    BeanUtils.populate(user, parameterMap);

                    System.out.println(user);
// 插入用户数据,跳转到登陆成功界面
                    UserService service = new UserServiceImpl();
                    service.register(user);

                    request.getRequestDispatcher("/registersuccess.jsp").forward(request, response);
            }
            else
            {
                //验证码不正确
            request.setAttribute("ckcode_msg", "验证码有误!");
            request.getRequestDispatcher("/register.jsp").forward(request,response);

            }

        }

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

}