spring3 mvc注解校验@NotNull没有作用

在学习spring mvc注解校验的时候发现@NotNull没有起作用！如uname.而password的@Size能行.哪位达达指导下,告诉我问题出在哪里！
代码如下
 package com.zed.model;   import javax.validation.constraints.NotNull; import javax.validation.constraints.Size;   public class User {             @NotNull(message="用户名不能为空")     private String uname;           @Size(min=4,max=8, message="密码长度应为{min}-{max}位")     private String password;       public String getUname() {         return uname;     }       public void setUname(String uname) {         this.uname = uname;     }       public String getPassword() {         return password;     }       public void setPassword(String password) {         this.password = password;     }             }

控制器代码
package com.zed.web;   import javax.validation.Valid;   import org.springframework.stereotype.Controller; import org.springframework.validation.BindingResult; import org.springframework.web.bind.annotation.RequestMapping; import org.springframework.web.bind.annotation.RequestMethod;   import com.zed.model.User;   @Controllerpublic class UserController {       @RequestMapping(value="/userLogin", method=RequestMethod.POST)     public String userLogin(@Valid User user, BindingResult result) {                   if(result.hasErrors()) {             return "home";         } else {                         return "success";         }     } }

spring文件配置关键部分
 <context:component-scan base-package="com.zed" />   <mvc:annotation-driven />   <bean class="org.springframework.web.servlet.view.InternalResourceViewResolver">     <property name="prefix" value="/WEB-INF/views/" />     <property name="suffix" value=".jsp" /> </bean>

简单的表单
<form action="userLogin" method="post">     <label>user name</label>     <input name="uname" type="text">     <label>password</label>     <input name="password" type="text">     <input type="submit" value="submit"> </form>
知道null和""有区别不？ 用NotEmpty