SpringBoot前置知识02-spring注解发展史-易微帮

springboot前置知识01-spring注解发展史

spring1.x

spring配置只能通过xml配置文件的方式注入bean,需要根据业务分配配置文件，通过import标签关联。

spring1.2版本出现@Transactional注解

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
       http://www.springframework.org/schema/beans/spring-beans.xsd">

    <bean id="userService" class="com.shaoby.service.UserServiceImpl"></bean>
</beans>

public class UserServiceImpl {
}

public class StartApp {
    public static void main(String[] args) {
        ClassPathXmlApplicationContext cp = new ClassPathXmlApplicationContext("applicationContext.xml");
        Object userService = cp.getBean("userService");
        System.out.println(userService);
    }
}

spring2.X

简化配置，通过compoment-scan和@Component（2.0）、 @Controller（2.5）、@Service（2.5）、 @Repository（2.5）注解实现bean管理

context:annotation-config标签向spring容器中注册AutowiredAnnotationBeanPostProcessor、CommonAnnotationBeanPostProcessor、PersistenceAnnotationBeanPostProcessor、RequiredAnnotationBeanPostProcessor，可以使用@ Resource 、@ PostConstruct、@ PreDestroy等注解

使用compoment-scan后可以将context:annotation-config移除

2.X版本没有脱离配置文件

<?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.xsd http://www.springframework.org/schema/context https://www.springframework.org/schema/context/spring-context.xsd">
<!--    <context:annotation-config/>-->
    <context:component-scan base-package="com.shaoby.*" />
</beans>

@Component
public class OtherServiceImpl {
}

@Component
public class UserServiceImpl {
    @Autowired
    private OtherServiceImpl otherService;
}

public class AppStart {
    public static void main(String[] args) {
        ClassPathXmlApplicationContext cp = new ClassPathXmlApplicationContext("applicationContext.xml");
        for (String beanDefinitionName : cp.getBeanDefinitionNames()) {
            System.out.println(beanDefinitionName);
        }
    }
}

spring3.X

spring3.0

新增@Configuration标记类为配置类，代替applicationContext.xml文件；@Bean注解注入对象，相当于xml中的bean标签；

无法脱离配置文件，需要借助context即xml中的compoment-scan标签，否则无法识别@Component、@Controller、@Service、@Repository注解；

提供@ImportResource注解，关联applicationContext配置文件；

所以就升级成@Configuration注解+applicationContext.xml配合Bean注解使用或者@Configuration注解+applicationContext.xml配合@Component等四个注解使用

<?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.xsd http://www.springframework.org/schema/context https://www.springframework.org/schema/context/spring-context.xsd">
    <context:component-scan base-package="com.shaoby.*" />
</beans>

@Configuration
@ImportResource("classpath:applicationContext.xml")
public class AppConfig {
    @Bean
    public OtherServiceImpl otherService(){
        return new OtherServiceImpl();
    }
}

@Service
public class UserServiceImpl {
}

public class OtherServiceImpl {
}

public class AppStart {
    public static void main(String[] args) {
        ApplicationContext ap = new AnnotationConfigApplicationContext(AppConfig.class);
        for (String beanDefinitionName : ap.getBeanDefinitionNames()) {
            System.out.println(beanDefinitionName);
        }
    }
}

spring3.1

1. 新增@ComportScan注解，默认扫描当前包及其子包下是@Component等四个注解修饰的所有类，脱离配置文件；

@Configuration
@ComponentScan("com.shaoby.*")
public class AppConfig {
}

@Component
public class OtherServiceImpl {
}
@Service
public class UserServiceImpl {
}

2. 新增@Import注解

代替import标签，在配置类中导入其他配置类

@Configuration
public class OtherConfig {
    @Bean
    public OtherServiceImpl otherService(){
        return new OtherServiceImpl();
    }
}

@Configuration
@ComponentScan("com.shaoby.*")
@Import({OtherConfig.class})
public class AppConfig {
}

可以在配置类中直接导入bean

@Configuration
@Import(StudentServiceImpl.class)
public class OtherConfig {
    @Bean
    public OtherServiceImpl otherService(){
        return new OtherServiceImpl();
    }
}

public class StudentServiceImpl {
}

高级用法-动态注入

@Import注解如果引入了实现ImportSelector注解的类，不会将该类型注入到容器中，而是将selectImports方法返回类型的全类路径字符串的数据注入到容器中

public class MyImportSelector implements ImportSelector {
    @Override
    public String[] selectImports(AnnotationMetadata importingClassMetadata) {
        return new String[]{Logger.class.getName(),Redis.class.getName()};
    }
}

@Configuration
@Import(MyImportSelector.class)
public class JavaConfig {
    public static void main(String[] args) {
        ApplicationContext ap = new AnnotationConfigApplicationContext(JavaConfig.class);
        for (String beanDefinitionName : ap.getBeanDefinitionNames()) {
            System.out.println(beanDefinitionName);
        }
    }
}

org.springframework.context.annotation.internalConfigurationAnnotationProcessor
org.springframework.context.annotation.internalAutowiredAnnotationProcessor
org.springframework.context.annotation.internalCommonAnnotationProcessor
org.springframework.context.event.internalEventListenerProcessor
org.springframework.context.event.internalEventListenerFactory
javaConfig
com.shaoby.importtest.Logger
com.shaoby.importtest.Redis

@Import注解如果引入了实现ImportBeanDefinitionRegistrar的类也可以

public class MyImportBeanDefinitionRegistrar implements ImportBeanDefinitionRegistrar {
    @Override
    public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
        registry.registerBeanDefinition("logger", new RootBeanDefinition(Logger.class));
        registry.registerBeanDefinition("redis", new RootBeanDefinition(Redis.class));
    }
}

@Configuration
@Import(MyImportBeanDefinitionRegistrar.class)
public class JavaConfig {
    public static void main(String[] args) {
        ApplicationContext ap = new AnnotationConfigApplicationContext(JavaConfig.class);
        for (String beanDefinitionName : ap.getBeanDefinitionNames()) {
            System.out.println(beanDefinitionName);
        }
    }
}

org.springframework.context.annotation.internalConfigurationAnnotationProcessor
org.springframework.context.annotation.internalAutowiredAnnotationProcessor
org.springframework.context.annotation.internalCommonAnnotationProcessor
org.springframework.context.event.internalEventListenerProcessor
org.springframework.context.event.internalEventListenerFactory
javaConfig
logger
redis

3. 新增Enabled模块，即EnabledXXX注解，它是结合@Import注解使用的

@Configuration
public class RedisAutoConfiguration {
    @Bean
    public RedisTemplate redisTemplate(){
        return new RedisTemplate();
    }
}

public class RedisTemplate {
}

定义开关注解

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Import(RedisAutoConfiguration.class)
public @interface EnabledAutoRedisConfiguration {
}

@Configuration
@EnabledAutoRedisConfiguration
public class AppConfig {
}

org.springframework.context.annotation.internalConfigurationAnnotationProcessor
org.springframework.context.annotation.internalAutowiredAnnotationProcessor
org.springframework.context.annotation.internalCommonAnnotationProcessor
org.springframework.context.event.internalEventListenerProcessor
org.springframework.context.event.internalEventListenerFactory
appConfig
com.shaoby.ebabledtest.RedisAutoConfiguration
redisTemplate

注掉@EnabledAutoRedisConfiguration

org.springframework.context.annotation.internalConfigurationAnnotationProcessor
org.springframework.context.annotation.internalAutowiredAnnotationProcessor
org.springframework.context.annotation.internalCommonAnnotationProcessor
org.springframework.context.event.internalEventListenerProcessor
org.springframework.context.event.internalEventListenerFactory
appConfig

spring4.X

新增注解：@Conditional(条件注解)，@EventListListener（事件监听），@AliasFor（别名），@CrossOrigin（解决跨域问题）

@Conditional注解

这个注解控制在什么条件下注入bean,这个注解传入一个实现Condition接口的类

@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Conditional {

	/**
	 * All {@link Condition} classes that must {@linkplain Condition#matches match}
	 * in order for the component to be registered.
	 */
	Class<? extends Condition>[] value();

}

在Condition的实现类中可以根据需求判断是否需要注入容器中

@FunctionalInterface
public interface Condition {

	/**
	 * Determine if the condition matches.
	 * @param context the condition context
	 * @param metadata the metadata of the {@link org.springframework.core.type.AnnotationMetadata class}
	 * or {@link org.springframework.core.type.MethodMetadata method} being checked
	 * @return {@code true} if the condition matches and the component can be registered,
	 * or {@code false} to veto the annotated component's registration
	 */
	boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata);

}

实战

public class JavaConfig {
    
    /** 根据MyConditionalOnClass中matches方法返回值判断是否将UsersService注入到容器中*/
    @Conditional(MyConditionalOnClass.class)
    @Bean
    public UserService userService(){
        return new UserService();
    }

    public static void main(String[] args) {
        ApplicationContext ap = new AnnotationConfigApplicationContext(JavaConfig.class);
        for (String beanDefinitionName : ap.getBeanDefinitionNames()) {
            System.out.println(beanDefinitionName);
        }

    }
}

public class MyConditionalOnClass implements Condition {
   @Override
   public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
      /** 根据需求判断是否需要将bean注入到容器中*/
      boolean userServiceFlag = context.getRegistry().containsBeanDefinition("userService");
      return userServiceFlag;
   }
}

public class UserService {
}

spring5.X

新增@Indexed（类索引），需要引入依赖。它被标识在@Component注解上，会在编译时会在MATE-INF文件夹下生成spring.components文件夹存储要DI的所有类的全类路径。这样会提高代码启动速度。

<dependency>
   <groupId>org.springframework</groupId>
   <artifactId>spring-context-indexer</artifactId>
</dependency>

总结

在spring的发展中注解的开发已经为SpringBoot的诞生做好了铺垫，其中@EnabledAutoXXXConfiguration、@ConditionalOnXXX、@Import等注解是SpringBoot自动装配原理的核心注解。

SpringBoot前置知识02-spring注解发展史