简单验证码的实现

下载CheckCodeServlet
jsp页面调用

 <script type="text/javascript">
        function changeImg(){
        var imgObj=document.getElementById("img");
        imgObj.src="CheckCodeService?ran="+Math.random();
                }
  </script>

显示

<img src="CheckCodeService" id="img"/>
 <a href="javascript:changeImg()">看不清换一张</a>