jsp页面如何调用servlet中生成的验证码

 //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);                  }}