Spring再接触 注入类型

package com.bjsxt.service;
import com.bjsxt.dao.UserDAO;
import com.bjsxt.model.User;



public class UserService {
    
    private UserDAO userDAO;  
    public void add(User user) {
        userDAO.save(user);
    }
    public UserDAO getUserDAO() {
        return userDAO;
    }
    public void setUserDAO(UserDAO userDAO) {
        this.userDAO = userDAO;
    }
    
    public UserService(UserDAO userDAO) {
        super();
        this.userDAO = userDAO;
    }
}

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
           http://www.springframework.org/schema/beans/spring-beans-2.5.xsd">

  <bean id="u" class="com.bjsxt.dao.impl.UserDAOImpl">
  </bean>
    
  <bean id="userService" class="com.bjsxt.service.UserService">
      <!-- 
      <property name="userDAO" ref="u" />
       -->
       <constructor-arg>
           <ref bean="u"/>
       </constructor-arg>
  </bean>
  

</beans>

    @Test
    public void testAdd() throws Exception {
        ApplicationContext ctx = new ClassPathXmlApplicationContext("beans.xml");
        
        
        UserService service = (UserService)ctx.getBean("userService");
        
        
        User u = new User();
        u.setUsername("zhangsan");
        u.setPassword("zhangsan");
        service.add(u);
    }