初学spring，对spring内部中类转换的疑惑。

在学习spring的AOP时，对一个问题没搞清楚。

UserImpl.java
 
 public class UserDAOImpl implements UserDAO {
    public void saveUser(){
        System.out.println("user saved ...");
    }   
}

UserService.java
 
public class UserService {
    private UserDAO userDAO ;
     
    public void add(){
        userDAO.saveUser();
    }
     
    public void setUserDAO(UserDAO userDAO){
        this.userDAO = userDAO;
    }
    public UserDAO getUserDAO(){
        return this.userDAO;
    }
}

beans.xml
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:aop="http://www.springframework.org/schema/aop"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
           http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
           http://www.springframework.org/schema/aop 
           http://www.springframework.org/schema/aop/spring-aop-3.0.xsd">
 
  <bean id="user" class="org.ben.dao.impl.UserDAOImpl">
  </bean>
 
  <bean id="userService" class="org.ben.service.UserService">
    <property name="userDAO" ref="user"/>
  </bean>
 
    <bean id="log" class="org.ben.aop.LogInterceptor"></bean>
 
    <aop:config>
        <!-- <aop:pointcut expression="execution(public * org.ben..*.save*(..))" id="savePointcut"/> -->
        <aop:pointcut expression="execution(public * org.ben.dao.impl.UserDAOImpl.saveUser())" id="savePointcut"/>
        <aop:aspect id="logAspect" ref="log">
            <aop:before method="before" pointcut-ref="savePointcut"/>
            <aop:after method="after" pointcut-ref="savePointcut"/>
        </aop:aspect>
    </aop:config>
     
</beans>

UserServiceTest.java
 
public class UserServiceTest {
 
    <a href="http://my.oschina.net/azibug" class="referer" target="_blank">@Test</a>    public void testAdd() {
        ApplicationContext ctx = new ClassPathXmlApplicationContext("beans.xml");
        UserService service = (UserService) ctx.getBean("userService");
        service.add();
    }
     
    <a href="http://my.oschina.net/azibug" class="referer" target="_blank">@Test</a>    public void testSave(){
        ApplicationContext ctx = new ClassPathXmlApplicationContext("beans.xml");
        UserDAOImpl userDAO = (UserDAOImpl)ctx.getBean("user");//为什么在这里使用UserDAOImpl来获取bean的时候，会报java.lang.ClassCastException: $Proxy4 cannot be cast to org.ben.dao.impl.UserDAOImpl这个异常？按我理解，既然spring内部采用的是动态代理，而且我这个bean也实现了接口，为什么还是无法转换类型呢？
//翻了一下书，在里面对spring的讲解时都没有提到对bean有什么特别的要求。
        userDAO.saveUser();
    }
 
}