ajax的校验功能怎样用SSM整合框架实现

<bean class="org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter"> 
 <property name="messageConverters"> 
 <list> 
 <ref bean="mappingJackson2HttpMessageConverter" /> 
 </list> 
 </property> 
 </bean> 
 <bean id="mappingJackson2HttpMessageConverter" class="org.springframework.http.converter.json.MappingJackson2HttpMessageConverter"> 
 <property name="supportedMediaTypes"> 
 <list> 
 <value>text/html;charset=UTF-8</value> 
 <value>text/json;charset=UTF-8</value> 
 <value>application/json;charset=UTF-8</value> </list> 
 </property> 
 </bean>

@ResponseBody
 @RequestMapping(value="queryByUser", method=RequestMethod.POST,produces="application/json;charset=UTF-8") 
 public User queryByName(User user,HttpServletRequest request){
 User u = userBiz.queryByName(user);
 return u;
 }

function checkUser(){
 var username=$("#username").val();
 
 $.ajax({
 url:"queryByUser",
 type:"post",
 data:{"username":username},
 dataType:"json",
 success:function(data){
 if (data!=null ) {
 $("#userSpan").text("用户名已存在");
 $("#username").val('');
 }else if(data==null && username !=''){
 $("#userSpan").text("用户名可用");
 }
 }
 });
 }