ajax异步回调函数无法调用

ajax.js代码：

url正确，已经进入到对应的aciton并执行有结果，但是 xmlHttp.onreadyStatechange=function() 这句一直未执行，我在函数里写了alert并未跳出，请大神看看娜丽有问题~~

action的代码：

public ActionForward execute(ActionMapping mapping, ActionForm form,

HttpServletRequest request, HttpServletResponse response) throws JSONException {

ManagerForm managerForm = (ManagerForm) form;// TODO Auto-generated method stub

List signList=null;

SignInService signService=(SignInService)getBean("signInService");

signList=signService.findAll(signIn);

JSONObject result = new JSONObject();

result.put("signlist", signList);

request.setAttribute("ajaxResult", result);

return null;

}

jsp的代码：

<body>

</body>

xmlHttp.onreadyStatechange 改为
xmlHttp.onreadystatechange 试试