Ajax struts2 验证用户名的问题？

首先我把代码亮出来
<script type="text/javascript">
var xmlHttp;
        var currentName;
 
        function createXMLHttpRequest() {
            if (window.ActiveXObject) {
                xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
            }
            else if (window.XMLHttpRequest) {
                xmlHttp = new XMLHttpRequest();
            }
        }
        
function checkUserName(field,url) {
            var name = field.value;
            if(name=="" || name.length<3) {
                document.getElementById("nameMessage").innerHTML="用户名应该不小于3位";
                return;
            }else {
                createXMLHttpRequest();
                url = url+"?username="+name;
                alert(url);
                currentName = name;
                xmlHttp.open("GET",url,true);
                xmlHttp.onreadystatechange = callback;
                xmlHttp.send(null);
            }
        }
        
        function callback() {
             alert("xmlHttp.readyState:"+xmlHttp.readyState);
             alert("xmlHttp.status:"+xmlHttp.status);
             if (xmlHttp.readyState == 4) {
                 if (xmlHttp.status == 200 || xmlHttp.status==0) {
                     var user = eval('(' + xmlHttp.responseText + ')');
                     if(user.registered) {
                         document.getElementById("nameMessage").innerHTML ="<font color='red'>"+currentName+", 用户名已经被注册 !</font>";
                     }else {
                         document.getElementById("nameMessage").innerHTML = "<font color='green'>"+currentName+", 用户名可以使用 !</font>";
                     }
                 }
             }
        }
        
</script>
==============================================
<td>用户名:</td>
        <td><input type="text" name="username" value="${username}" onblur="checkUserName(this,'register!check.action')"/></td>
        <td><span id="nameMessage"></span></td>
==============================================
public String check() throws Exception {
//        ActionContext ctx = ActionContext.getContext();
//        HttpServletRequest request = (HttpServletRequest)ctx.get(ServletActionContext.HTTP_REQUEST);       
//        String username = request.getParameter("inputName");
System.out.println("异步调用check方法");
        if(studentService.getByName(username) != null) {
            this.registered = true;
        }
        return "myJson";
    }
=============================================
<package name="ajax" extends="json-default">
      <action name="register" class="action.RegisterAction">
       <result name="success">/login.jsp</result>
       <result name="input">/register.jsp</result>
       <result name="myJson" type="json"></result>
       <!--<result name="myJson" type="json"></result>-->
      </action>
  </package>