@Principle of Autowired annotation

One 、@Autowired

Concept ：

@Autowired notes , It can be to Members of the class Variable 、 Method and constructor for annotation , Complete the work of automatic assembly . adopt @Autowired To eliminate set ,get Method .

In the use of @Autowired Before , We have a bean When configuring properties , It's using xml file , such as ：

<property name=" Property name " value="  Property value "/>    

But in use @Autowired after , We only need to use one where we need it @Autowired That's all right. . For example, we want to inject an attribute  

public interface UserService {
    public void getUser();
}

@Service
public class UserServiceImpl implements UserService{

    @Autowired
    private UserDao dao;

    public void getUser() {
        System.out.println(" obtain 2");
        dao.getUser();
    }
}

public interface UserController {
    public void getUser();
}

@Controller
public class UserControllerImpl implements UserController{

    @Autowired
    private UserService service;

    public void getUser() {
        System.out.println(" pick up information 3");
        service.getUser();
    }
}

public interface UserDao {
    public void getUser();
}

@Repository
public class UserDaoImpl implements UserDao{

    public void getUser() {
        System.out.println(" Get user data ");
    }
}

  test ：

 @Test
    public void getUser(){
        ApplicationContext applicationContext = new ClassPathXmlApplicationContext("application.xml");
        UserController user = applicationContext.getBean("UserController", USerController.class);
        User.getUser();
    }

  We are using @Autowired Don't go again xml Continue to configure in the file .

Attention to detail ：

1、 Use @Autowired The current class of must also be spring Container hosting （ hit @Coponent、@Controller、@Service 、@repository）

2、 Whether it's public and private Decorated fields can be automatically injected

3、 By default , Use @Autowired The attributes of annotations must be assembled , If this type of bean Inject , You're going to report a mistake . If it is allowed not to be assembled @Autowired Of required The attribute is false

4、@Autowired Is type based injection , If the current type attribute has only one in the container Bean, Then the attribute name is not limited , However, it is generally recommended to follow the rule of lowercase class names ‘

5、 If the current attribute type has more than one in the container Bean, Then you must pass the attribute name perhaps @Qualifier Appoint Bean name

6、@Autowired Can play in XXX[] 、List On , All... In the container will be XXX Type of bean Inject it all 、 And the attribute name has no constraint , But note that you can pass @Qualifier Specify injection specify beanName Of bean, Attribute names have no binding effect

7、@Autowired Can play in Map<String,XXX> On , All XXX Type of bean Will be injected ,beanName by key , The object is value, But note that you can pass @Qualifier Specify injection specify beanName Of bean, Attribute names have no binding effect