8、Spring之基于注解的自动装配

8.1、场景模拟

8.1.1、UserDao接口及实现类

1. package org.rain.spring.dao;
3. /**
4. * @author liaojy
5. * @date 2023/8/5 - 18:32
6. */
7. public interface UserDao {
9.     void saveUser();
11. }

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1. package org.rain.spring.dao.impl;
3. import org.rain.spring.dao.UserDao;
4. import org.springframework.stereotype.Repository;
6. /**
7. * @author liaojy
8. * @date 2023/8/5 - 18:33
9. */
10. @Repository
11. public class UserDaoImpl implements UserDao {
12.     public void saveUser() {
13. <?xml version="1.0" encoding="UTF-8"?>
14. <beans xmlns="http://www.springframework.org/schema/beans"
15.        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
16.        xmlns:context="http://www.springframework.org/schema/context"
17.        xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context https://www.springframework.org/schema/context/spring-context.xsd">
19.    
20.     <context:component-scan base-package="org.rain.spring"></context:component-scan>
22. </beans>System.out.println("保存成功");
23.     }
24. }

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8.1.2、UserService接口及实现类

1. package org.rain.spring.service;
3. /**
4. * @author liaojy
5. * @date 2023/8/5 - 18:35
6. */
7. public interface UserService {
9.     void saveUser();
11. }

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1. package org.rain.spring.service.impl;
3. import org.rain.spring.dao.UserDao;
4. import org.rain.spring.service.UserService;
5. import org.springframework.stereotype.Service;
7. /**
8. * @author liaojy
9. * @date 2023/8/5 - 18:36
10. */
11. @Service
12. public class UserServiceImpl implements UserService {
14.     private UserDao userDao;
16.     public void saveUser() {
17. <?xml version="1.0" encoding="UTF-8"?>
18. <beans xmlns="http://www.springframework.org/schema/beans"
19.        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
20.        xmlns:context="http://www.springframework.org/schema/context"
21.        xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context https://www.springframework.org/schema/context/spring-context.xsd">
23.    
24.     <context:component-scan base-package="org.rain.spring"></context:component-scan>
26. </beans>userDao.saveUser();
27.     }
29. }

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8.1.3、UserController

1. package org.rain.spring.controller;
3. import org.rain.spring.service.UserService;
4. import org.springframework.stereotype.Controller;
6. /**
7. * @author liaojy
8. * @date 2023/8/5 - 18:38
9. */
10. @Controller("myUserController")
11. public class UserController {
13.     private UserService userService;
15.     public void saveUser(){
16. <?xml version="1.0" encoding="UTF-8"?>
17. <beans xmlns="http://www.springframework.org/schema/beans"
18.        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
19.        xmlns:context="http://www.springframework.org/schema/context"
20.        xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context https://www.springframework.org/schema/context/spring-context.xsd">
22.    
23.     <context:component-scan base-package="org.rain.spring"></context:component-scan>
25. </beans>userService.saveUser();
26.     }
28. }

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8.1.4、bean配置

1. <?xml version="1.0" encoding="UTF-8"?>
2. <beans xmlns="http://www.springframework.org/schema/beans"
3.        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
4.        xmlns:context="http://www.springframework.org/schema/context"
5.        xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context https://www.springframework.org/schema/context/spring-context.xsd">
7.    
8.     <context:component-scan base-package="org.rain.spring"></context:component-scan>
10. </beans>

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8.1.5、测试

由控制台日志可知，组件注解+组件扫描，存在尚未实现依赖注入的问题

1.     @Test    public void testAutowireByAnnotation(){<?xml version="1.0" encoding="UTF-8"?>
2. <beans xmlns="http://www.springframework.org/schema/beans"
3.        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
4.        xmlns:context="http://www.springframework.org/schema/context"
5.        xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context https://www.springframework.org/schema/context/spring-context.xsd">
7.    
8.     <context:component-scan base-package="org.rain.spring"></context:component-scan>
10. </beans>ApplicationContext ioc = new ClassPathXmlApplicationContext("spring-ioc-annotation.xml");<?xml version="1.0" encoding="UTF-8"?>
11. <beans xmlns="http://www.springframework.org/schema/beans"
12.        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
13.        xmlns:context="http://www.springframework.org/schema/context"
14.        xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context https://www.springframework.org/schema/context/spring-context.xsd">
16.    
17.     <context:component-scan base-package="org.rain.spring"></context:component-scan>
19. </beans>UserController userController = ioc.getBean(UserController.class);<?xml version="1.0" encoding="UTF-8"?>
20. <beans xmlns="http://www.springframework.org/schema/beans"
21.        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
22.        xmlns:context="http://www.springframework.org/schema/context"
23.        xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context https://www.springframework.org/schema/context/spring-context.xsd">
25.    
26.     <context:component-scan base-package="org.rain.spring"></context:component-scan>
28. </beans>userController.saveUser();    }

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8.2、@Autowired注解

@Autowired：实现自动装配功能的注解，其能够标识的位置有三处

8.2.1、方式一：标识在成员变量上(常用)

1.     @Autowired
2.     private UserDao userDao;

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1.     @Autowired
2.     private UserService userService;

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8.2.2、方式二：标识在setter方法上

1.     private UserDao userDao;    @Autowired    public void setUserDao(UserDao userDao) {<?xml version="1.0" encoding="UTF-8"?>
2. <beans xmlns="http://www.springframework.org/schema/beans"
3.        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
4.        xmlns:context="http://www.springframework.org/schema/context"
5.        xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context https://www.springframework.org/schema/context/spring-context.xsd">
7.    
8.     <context:component-scan base-package="org.rain.spring"></context:component-scan>
10. </beans>this.userDao = userDao;    }

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1.     private UserService userService;    @Autowired    public void setUserService(UserService userService) {<?xml version="1.0" encoding="UTF-8"?>
2. <beans xmlns="http://www.springframework.org/schema/beans"
3.        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
4.        xmlns:context="http://www.springframework.org/schema/context"
5.        xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context https://www.springframework.org/schema/context/spring-context.xsd">
7.    
8.     <context:component-scan base-package="org.rain.spring"></context:component-scan>
10. </beans>this.userService = userService;    }

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8.2.3、方式三：标识在有参构造方法上

1.     private UserDao userDao;    @Autowired    public UserServiceImpl(UserDao userDao) {<?xml version="1.0" encoding="UTF-8"?>
2. <beans xmlns="http://www.springframework.org/schema/beans"
3.        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
4.        xmlns:context="http://www.springframework.org/schema/context"
5.        xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context https://www.springframework.org/schema/context/spring-context.xsd">
7.    
8.     <context:component-scan base-package="org.rain.spring"></context:component-scan>
10. </beans>this.userDao = userDao;    }

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1.     private UserService userService;    @Autowired    public UserController(UserService userService) {<?xml version="1.0" encoding="UTF-8"?>
2. <beans xmlns="http://www.springframework.org/schema/beans"
3.        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
4.        xmlns:context="http://www.springframework.org/schema/context"
5.        xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context https://www.springframework.org/schema/context/spring-context.xsd">
7.    
8.     <context:component-scan base-package="org.rain.spring"></context:component-scan>
10. </beans>this.userService = userService;    }

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8.3、@Autowired的工作流程

• 默认通过byType的方式，在IOC容器中匹配类型相同的bean，为属性赋值
  
• 如果有多个类型匹配的bean，则自动转换为通过byName的方式，在IOC容器中匹配id和属性名相同的bean，为属性赋值
  
• 如果有多个类型匹配的bean，且在IOC容器中没有找到id和属性名相同的bean；
  则可以使用@Qualifier注解指定某个bean的id，将这个bean赋值给属性
  

如果有多个类型匹配的bean，且在IOC容器中没有找到id和属性名相同的bean，也没有使用@Qualifier注解指定将某个bean赋值给属性；则会抛出异常NoUniqueBeanDefinitionException

1.     @Autowired
2.     @Qualifier("myUserService")
3.     private UserService userService;

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8.4、@Autowired的注意事项

在@Autowired注解中有一个属性required，默认值为true，意思是必要完成自动装配

如果在IOC容器中没有匹配的bean，就会抛出异常NoSuchBeanDefinitionException

为此可以将required属性的值设置为false，这样能装配就装配，不能装配就使用默认值

一般可以通过排查组件注解、组件扫描和@Qualifier注解，定位出为什么在IOC容器中没有匹配的bean

1.     @Autowired(required = false)
2.     private UserService userService;

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