使用 Get方法，通过设置session和request设置页面状态
<body>
<div>
<form method=get>
<tr>
<td align=left>请输入认证码：<input type=text name=rand maxlength=4 value=""></td>
<td><img border=0 src="checkcode" onclick="this.src = ‘checkcode?a=‘ + Math.random() + 100"></td>
<td colspan=2 align=center><input type=submit value="check"></td>
</tr>
</form><%
String rand = (String) session.getAttribute("rand");
String input = request.getParameter("rand");
if (input != null) {
if (rand.equals(input)) {
%>
<font color=green>验证码正确！</font>
<%
} else {
%>
<font color=red>验证码错误！</font>
<%
}
}
%>
</div>
</body>
上面是login.jsp下面是serverlet的
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
//设置页面不缓存
response.setHeader("Pragma","No-cache");
response.setHeader("Cache-Control","no-cache");
response.setDateHeader("Expires", 0);// 在内存中创建图象
int width=60, height=20;
BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);// 获取图形上下文
Graphics g = image.getGraphics();//生成随机类
Random random = new Random();// 设定背景色
g.setColor(getRandColor(200,250));
g.fillRect(0, 0, width, height);//设定字体
g.setFont(new Font("Times New Roman",Font.PLAIN,18));//画边框
g.setColor(new Color(height));
g.drawRect(0,0,width-1,height-1);// 随机产生155条干扰线，使图象中的认证码不易被其它程序探测到
g.setColor(getRandColor(160,200));
for (int i=0;i<150;i++)
{
int x = random.nextInt(width);
int y = random.nextInt(height);
int xl = random.nextInt(12);
int yl = random.nextInt(12);
g.drawLine(x,y,x+xl,y+yl);
}// 取随机产生的认证码(4位数字)
String sRand="";
for (int i=0;i<4;i++){
String rand=String.valueOf(random.nextInt(36));
if(Integer.parseInt(rand)>9){
int a = Integer.parseInt(rand)-9+64;
rand = String.valueOf((char)a);
//生成随机大写字母
}
sRand+=rand;
// 将认证码显示到图象中
g.setColor(new Color(20+random.nextInt(110),20+random.nextInt(110),20+random.nextInt(110)));
g.drawString(rand,13*i+6,16);
}// 将认证码存入SESSION
HttpSession session = request.getSession();
session.setAttribute("rand",sRand);// 图象生效
g.dispose();// 输出图象到页面
ImageIO.write(image, "JPEG", response.getOutputStream());
}@Override
public String getServletInfo() {
return "Short description";
}// </editor-fold>Color getRandColor(int fc,int bc){//给定范围获得随机颜色
Random random = new Random();
if(fc>255) fc=255;
if(bc>255) bc=255;
int r=fc+random.nextInt(bc-fc);
int g=fc+random.nextInt(bc-fc);
int b=fc+random.nextInt(bc-fc);
return new Color(r,g,b);
}
JSP验证码页面-Serverlet