<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");
       }
      }
     });
   }