Spring注解系列十九：自动装配-@Autowired

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(); }

（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 （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 （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 （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; } }