Servlet学习-登录验证码的实现

//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);
}

}

//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

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