annotation中的Autowired

<?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:context="http://www.springframework.org/schema/context"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
           http://www.springframework.org/schema/beans/spring-beans-2.5.xsd
           http://www.springframework.org/schema/context
           http://www.springframework.org/schema/context/spring-context-2.5.xsd">

   <context:annotation-config/>
  <bean id="u" class="com.bjsxt.dao.impl.UserDAOImpl">
  </bean>
  <bean id="u2" class="com.bjsxt.dao.impl.UserDAOImpl">
  </bean>
   <bean id="userService" class="com.bjsxt.service.UserService">    
  </bean>
</beans>

package com.bjsxt.service;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Qualifier;

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;
    }    
    //为什么Autowired必须放在set里面，其他地方无效
    @Autowired
    public void setUserDAO(@Qualifier("u")UserDAO userDAO) {
        this.userDAO = userDAO;
    }
    
    public void init(){
        System.out.println("init");
    }
    
    public void destroy(){
        System.out.println("destroy");
    }    
}

    @Test
    public void test() {
        ClassPathXmlApplicationContext ctx = new ClassPathXmlApplicationContext("beans.xml");
        
        UserService service = (UserService)ctx.getBean("userService");
        
        service.add(new User());
        
        ctx.destroy();

疑问：为什么autowired必须放在set方法里面在测试之中？

Annotation-based configuration

(The implicitly registered post-processors include AutowiredAnnotationBeanPostProcessor, CommonAnnotationBeanPostProcessor, PersistenceAnnotationBeanPostProcessor, as well as the aforementioned RequiredAnnotationBeanPostProcessor.)

Note that <context:annotation-config/> only looks for annotations on beans in the same application context it is defined in. This means that, if you put <context:annotation-config/> in a WebApplicationContext for a DispatcherServlet, it only checks for @Autowired beans in your controllers, and not your services. See Section 13.2, “The DispatcherServlet” for more information.

@Autowired

As expected, the @Autowired annotation may be applied to "traditional" setter methods:

public class SimpleMovieLister {

    private MovieFinder movieFinder;

    @Autowired
    public void setMovieFinder(MovieFinder movieFinder) {
        this.movieFinder = movieFinder;
    }

    // ...
}

The annotation may also be applied to methods with arbitrary names and/or multiple arguments:

public class MovieRecommender {

    private MovieCatalog movieCatalog;
    
    private CustomerPreferenceDao customerPreferenceDao;

    @Autowired
    public void prepare(MovieCatalog movieCatalog, CustomerPreferenceDao customerPreferenceDao) {
        this.movieCatalog = movieCatalog;
        this.customerPreferenceDao = customerPreferenceDao;
    }

    // ...
}

The @Autowired annotation may even be applied on constructors and fields:

public class MovieRecommender {

    @Autowired
    private MovieCatalog movieCatalog;
    
    private CustomerPreferenceDao customerPreferenceDao;

    @Autowired
    public MovieRecommender(CustomerPreferenceDao customerPreferenceDao) {
        this.customerPreferenceDao = customerPreferenceDao;
    }

    // ...
}

It is also possible to provide all beans of a particular type from the ApplicationContext by adding the annotation to a field or method that expects an array of that type:

public class MovieRecommender {

    @Autowired
    private MovieCatalog[] movieCatalogs;

    // ...
}

The same applies for typed collections:

public class MovieRecommender {

    private Set<MovieCatalog> movieCatalogs;
    
    @Autowired
    public void setMovieCatalogs(Set<MovieCatalog> movieCatalogs) {
        this.movieCatalogs = movieCatalogs;
    }

    // ...
}