<form action="loginServlet" method="post">

  请输入验证码：<input type="text" name="code" />

  <img src="getCodeServlet" /><br />

  <button type="submit">提交</button>

</form>

 @WebServlet("/getCodeServlet")

 public class GetCodeServlet extends HttpServlet {

     //验证码的宽、高

     private static int WIDTH=80;

     private static int HEIGHT=25;

     //绘制背景

     private void drawBg(Graphics g){

         //rgb

         g.setColor(new Color(128, 128, 128));

         //绘制矩形。x,y,wigth,height

         g.fillRect(0,0,WIDTH,HEIGHT);

         //随机绘制100个干扰点

         Random random=new Random();

         for (int i=0;i<100;i++){

             //产生(0,1）上的小数，*WIDTH|HEIGHT，再取整也行

             int x=random.nextInt(WIDTH);

             int y=random.nextInt(HEIGHT);

             g.drawOval(x,y,1,1);

             //干扰点的颜色也可以随机，随机产生red，green，blue即可

             //g.setColor(new Color(red,green,blue));

         }

     }

     //绘制验证码

     private void drawCode(Graphics g,char[] code){

         g.setColor(Color.BLACK);

         //字体、样式（多个时竖线分隔）、字号

         g.setFont(new Font("serif",Font.ITALIC|Font.BOLD,18));

         //在不同位置绘制验证码字符，参数：要绘制的String、横、纵坐标。+""是为了char转String。

         g.drawString(code[0]+"",1,17);

         g.drawString(code[1]+"",16,15);

         g.drawString(code[2]+"",31,18);

         g.drawString(code[3]+"",46,16);

     }

     //随机产生4位验证码

     private char[] getCode(){

         String chars="0123456789QWERTYUIOPASDFGHJKLZXCVBNMqwertyuiopasdfghjklzxcvbnm";

         char[] code=new char[4];

         Random random=new Random();

         for (int i=0;i<4;i++){

             //[0,62)

             int index= random.nextInt(62);

             code[i]=chars.charAt(index);

         }

         return code;

     }

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

         HttpSession session = request.getSession();

         ServletOutputStream sos = response.getOutputStream();

         response.setContentType("image/jpeg");

         //设置浏览器不缓存此图片

         response.setHeader("Pragma","No-cache");

         response.setHeader("Cache-Control","no-cache");

         response.setDateHeader("Expires",0);

         //创建内存图片

         BufferedImage bufferedImage = new BufferedImage(WIDTH, HEIGHT, TYPE_INT_RGB);

         Graphics g= bufferedImage.getGraphics();

         char[] code=getCode();

         //将验证码放到session域中。session对象要在提交响应之前获得

         session.setAttribute("code",new String(code));

         drawBg(g);

         drawCode(g,code);

         g.dispose();

         //将图片输出到浏览器

         ByteArrayOutputStream baos = new ByteArrayOutputStream();

         ImageIO.write(bufferedImage,"JPEG",baos);

         baos.writeTo(sos);

         baos.close();

         sos.close();

     }

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

         doPost(request,response);

     }

 }

 @WebServlet("/loginServlet")

 public class LoginServlet extends HttpServlet {

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

         response.setContentType("text/html;charset=utf-8");

         HttpSession session = request.getSession();

         String trueCode= (String) session.getAttribute("code");

         String code=request.getParameter("code");

         if (code.equals(trueCode)){

             response.getWriter().write("验证码正确");

         }

         else {

             response.getWriter().write("验证码错误");

         }

     }

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

         doPost(request,response);

     }

 }

//先转换为全大写|全小写，再判断

        trueCode=trueCode.toLowerCase();

        code=code.toLowerCase();

        //trueCode=trueCode.toUpperCase();

        //code=trueCode.toUpperCase();