你用struts2如何实现验证码的

<script type="text/javascript">           /*            注册dojo事件用来显示验证码        */                      dojo.addOnLoad(function() {          dojo.event.topic.subscribe('/retrieveUser', this, function(data, type, e){          if(type == 'load') {                    showUser(data);                } else if(type == 'error') {                    alert('Can not retrieve the Vocde');                }            });        });        function showUser(strUser) {                  var UserHolder = document.getElementById('im');            UserHolder.src ='getVocde.action';                    }        </script><s:url id="userUrl" value="getVocde.action" /><td colspan="2">                                    验证码                                    <input maxlength="4" size="2" name="logon.vcode">                                    <img border="0" src="getVocde.action" width="57" height="22" id="im"                                         align="middle">                                    <a target="main" href="MyCountManager/BSP_User_getpass.jsp">&nbsp;找回密码</a>&nbsp;&nbsp;                                    <sx:a  href="%{userUrl}"  notifyTopics="/retrieveUser">看不清,换一张</sx:a>

@Action(value ="getVocde", results = {@Result(name = "input",  type = "stream",params={"contentType", "image/jpeg", "contentDisposition", "filename=document.jpeg","bufferSize", "1024"})})    public String execute()throws IOException {        VImage v=new VImage();          //这是生成验证码的servlet        try {            v.doGet(ServletActionContext.getRequest(), ServletActionContext.getResponse());        } catch (ServletException e) {            e.printStackTrace();        } catch (IOException e) {            e.printStackTrace();        }        return "input";    }