Spring注解系列十九：自动装配-@Autowired

绝地灬酷狼 2022-01-27 04:57 304阅读 0赞

自动装配：Spring利用依赖注入（DI），完成对IOC容器中中各个组件的依赖关系赋值。  
**@Autowired：自动注入**  
**（1）、默认优先按照类型去容器中找对应的组件：applicationContext.getBean(BookDao.class)；找到就赋值**。  
1、BookDao

@Repository
    public class BookDao {
        
    }

2、BookService

@Service
    public class BookService {
    
        @Autowired
        private BookDao bookDao;
    
        @Override
        public String toString() {
            return "BookService{" +
                    "bookDao=" + bookDao +
                    '}';
        }
    
    }

3、MainConifgOfAutowired

@Configuration
    @ComponentScan({"com.atguigu.service","com.atguigu.dao"})
    public class MainConifgOfAutowired {
    
    }

测试：

@Test
    public void test01(){
    	AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext(MainConifgOfAutowired.class);
    	
    	BookService bookService = applicationContext.getBean(BookService.class);
    	System.out.println(bookService);
    
    	BookDao bean = applicationContext.getBean(BookDao.class);
    	System.out.println(bean);
    
    	applicationContext.close();
    }

![在这里插入图片描述][2019052511083069.png]  
**（2）、如果找到多个相同类型的组件，再将属性的名称作为组件的id去容器中查找。  
   如：applicationContext.getBean(“bookDao2”)**

1、修改BookDao，添加属性lable，用来区分bean

// 名字默认是类名首字母小写
    @Repository
    public class BookDao {
    
        private String lable = "1";
    
        public String getLable() {
            return lable;
        }
    
        public void setLable(String lable) {
            this.lable = lable;
        }
    
        @Override
        public String toString() {
            return "BookDao [lable=" + lable + "]";
        }
    
    }

2、修改MainConifgOfAutowired，再注入一个类型为BookDao的bean

@Configuration
    @ComponentScan({"com.atguigu.service","com.atguigu.dao"})
    public class MainConifgOfAutowired {
    
    	@Bean("bookDao2")
    	public BookDao bookDao(){
    		BookDao bookDao = new BookDao();
    		bookDao.setLable("2");
    		return bookDao;
    	}
    }

3、修改BookService，将属性的名称改为bookDao2

@Service
    public class BookService {
    
        @Autowired
        private BookDao bookDao2;
    
        @Override
        public String toString() {
            return "BookService{" +
                    "bookDao=" + bookDao2 +
                    '}';
        }
    }

再次运行测试方法test01  
![在这里插入图片描述][20190525112642859.png]  
**（3）、@Qualifier(“bookDao”)：使用@Qualifier指定需要装配的组件的id，而不是使用属性名。**  
1、修改BookService，使用@Qualifier指定需要装配的组件的id为bookDao

@Service
    public class BookService {
    
        @Qualifier("bookDao")
        @Autowired
        private BookDao bookDao2;
    
        @Override
        public String toString() {
            return "BookService{" +
                    "bookDao=" + bookDao2 +
                    '}';
        }
    }

再次运行测试方法test01  
![在这里插入图片描述][20190525113751900.png]  
**（4）、自动装配默认一定要将属性赋值好，没有就会报错；可以使用@Autowired(required=false)**  
1、修改BookDao ，注掉@Repository

// 名字默认是类名首字母小写
    //@Repository
    public class BookDao {
    
        private String lable = "1";
    
        public String getLable() {
            return lable;
        }
    
        public void setLable(String lable) {
            this.lable = lable;
        }
    
        @Override
        public String toString() {
            return "BookDao [lable=" + lable + "]";
        }
    
    }

2、修改MainConifgOfAutowired ，注掉@Bean(“bookDao2”)

@Configuration
    @ComponentScan({"com.atguigu.service","com.atguigu.dao"})
    public class MainConifgOfAutowired {
    
    	//@Bean("bookDao2")
    	public BookDao bookDao(){
    		BookDao bookDao = new BookDao();
    		bookDao.setLable("2");
    		return bookDao;
    	}
    }

再次运行测试方法test01，报错：

Caused by: org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'com.atguigu.dao.BookDao' available: expected at least 1 bean which qualifies as autowire candidate. Dependency annotations: {@org.springframework.beans.factory.annotation.Qualifier(value=bookDao), @org.springframework.beans.factory.annotation.Autowired(required=true)}

解决办法：  
 修改BookService，使用注解@Autowired(required=false)指定要自动注入的bean是否是必须的，默认是true。若有该bean就会被自动注入，没有就是null，不会报错。

@Service
    public class BookService {
    
        @Qualifier("bookDao")
        @Autowired(required=false)
        private BookDao bookDao2;
    
        @Override
        public String toString() {
            return "BookService{" +
                    "bookDao=" + bookDao2 +
                    '}';
        }
    }

再次运行测试方法test01  
![在这里插入图片描述][20190525115332438.png]  
**（5）、@Primary：让Spring进行自动装配的时候，默认使用首选的bean；也可以继续使用@Qualifier指定需要装配的bean的名字。**  
1、修改BookService，注掉@Qualifier(“bookDao”)

@Service
    public class BookService {
    
        //@Qualifier("bookDao")
        @Autowired(required=false)
        private BookDao bookDao;
    
        @Override
        public String toString() {
            return "BookService{" +
                    "bookDao=" + bookDao +
                    '}';
        }
    }

2、修改MainConifgOfAutowired。使用@Primary指定自动注入首选的bean

@Configuration
    @ComponentScan({"com.atguigu.service","com.atguigu.dao"})
    public class MainConifgOfAutowired {
    
    	@Primary
    	@Bean("bookDao2")
    	public BookDao bookDao(){
    		BookDao bookDao = new BookDao();
    		bookDao.setLable("2");
    		return bookDao;
    	}
    }

![在这里插入图片描述][20190525120034590.png]

[2019052511083069.png]: /images/20220127/203a1e1e3ee84ce98cfa2dbcdeffeb6e.png
[20190525112642859.png]: /images/20220127/fae4f30d6ac14ae0adf202215b6a62e0.png
[20190525113751900.png]: /images/20220127/7a5db893cc3b4c83bfba4d01b826751b.png
[20190525115332438.png]: /images/20220127/3d1b4e02f9b547d9beb32aad2da6c4a8.png
[20190525120034590.png]: /images/20220127/8550f8887c12418e8a0687674a8864f1.png