//用于获取四位随机数
     private char mapTable[] = {'0','1','2','3','4','5','6','7','8','9'};

     //生成验证码,并返回随机生成的数字
     public String getEnsure(int width, int height, OutputStream os){
         if (width <= 0)
             width = 60;
         if (height <= 0)
             height = 20;

         BufferedImage image = new BufferedImage(width, height,BufferedImage.TYPE_INT_RGB);

         // 获取图形上下文
         Graphics g = image.getGraphics();

         // 设定背景色
         g.setColor(new Color(0xDCCCCC));
         g.fillRect(0, 0, width, height);

         // 画边框
         g.setColor(Color.black);
         g.drawRect(0, 0, width - 1, height - 1);

         // 取随机产生的认证码
         String strEnsure = "";

         // 4代表4位验证码
         for (int i = 0; i < 4; ++i){
             strEnsure += mapTable[(int) (mapTable.length * Math.random())];
         }

         // 将认证码显示到图象中
         g.setColor(Color.red);
         g.setFont(new Font("Atlantic Inline", Font.PLAIN, 14));

         // 画的具体坐标
         String str = strEnsure.substring(0, 1);
         g.drawString(str, 8, 14);
         str = strEnsure.substring(1, 2);
         g.drawString(str, 20, 15);
         str = strEnsure.substring(2, 3);
         g.drawString(str, 35, 18);
         str = strEnsure.substring(3, 4);
         g.drawString(str, 45, 15);

         // 释放图形上下文
         g.dispose();

         try{
             // 输出图象到页面
             ImageIO.write(image, "JPEG", os);
         } catch (IOException e){
             return "";
         }
         
         return strEnsure;
     }

public class CheckServlet extends HttpServlet {

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

    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        //禁用缓存，每次访问此页面，都重新生成
        response.setHeader("Pragma","No-cache"); 
        response.setHeader("Cache-Control","no-cache"); 
        response.setDateHeader("Expires", 0); 

        //生成验证码的实例对象
        CheckCode ie = new CheckCode();

        //调用里面的方法，返回的是生成的验证码中的字符串
        String str = ie.getEnsure(0,0,response.getOutputStream());

        //获得session，并把字符串保存在session中，为后面的对比做基础
        HttpSession session = request.getSession();
        session.setAttribute("strEnsure", str);      
        
    }

}