Servlet学习-登录验证码的实现

java的画图原理

//生成验证码的servlet
public class CreateCode extends HttpServlet {


    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        
        //禁止浏览器缓存随机图片
        response.setDateHeader("Expires", -1);
        response.setHeader("Cache-Control", "no-cache");
        response.setHeader("Pragma","no-cache");
        
        //通知客户机以图片的方式打开发送过去的数据
        response.setHeader("Control-Type", "image/jpeg");
        
        //在内存中创建一幅图片
        BufferedImage image= new BufferedImage(80,30,
                BufferedImage.TYPE_INT_RGB);
        
        //创建画笔
        Graphics g = image.getGraphics();
        
        //设置背景色
        g.setColor(Color.GRAY);
        g.fillRect(0, 0, 80, 30);
        
        //设置写入数据的颜色和字体
        g.setColor(Color.blue);
        g.setFont(new Font(null,Font.BOLD,20));
        
        //向图片上写数据
        String num = makeNum();
        //把随机生成的数据保存到session，用于验证
        request.getSession().setAttribute("checkcode", num);
        g.drawString(num, 0, 20);
        
        //把写好数据的图片输出到浏览器
        ImageIO.write(image, "jpg", response.getOutputStream());

    }
    
    //生成随机数
    public String makeNum()
    {
        Random r = new Random();
        String num = r.nextInt(9999)+"";
        StringBuffer str=new StringBuffer();
        for(int i=0;i<4-num.length();i++){
            str.append("0");
        }
        num = str.toString()+num;
        return num;
    }
    

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

}

调用的servlet：

//获取验证码
        String checkcode = (String) request.getSession().getAttribute("checkcode");//随机生成的
        String checkcode2 = request.getParameter("checkcode");//获取输入的
        
        if(checkcode.equals(checkcode2))//如果验证码正确
        {
            ;
        }else{
            request.setAttribute("error", "验证码错误！");
            //本地用getRequestDispatcher 效率高
            request.getRequestDispatcher("/LoginServlet").forward(request, response);
            return;
        }
//do something here

 主页显示调用画的jpg图片：

        out.println("<input type = 'checkbox' name='iskeep' value=keep>保存用户名和密码</br>");