@Import注入之ImportSelector和ImportBeanDefinitionRegistrar

- 日理万妓 2022-02-17 15:59 196阅读 0赞

在spring-boot中随处可以见的就是@Import， 它的作用就是通过导入的方式把实例注入到IOC中，本文主要介绍下@Import的用法。

知识点回顾：

给IOC容器中注册Bean的方式：

1.  包扫描+组件标注注解（@Controller/@Service/@Repository/@Component）
2.  @Bean
3.  @Import

@Import源码可以发现@Import注解**只能注解在类**上

/**
 * Indicates one or more {@link Configuration @Configuration} classes to import.
 *
 * Provides functionality equivalent to the {@code <import/>} element in Spring XML.
 * Allows for importing {@code @Configuration} classes, {@link ImportSelector} and
 * {@link ImportBeanDefinitionRegistrar} implementations, as well as regular component
 * classes (as of 4.2; analogous to {@link AnnotationConfigApplicationContext#register}).
 *
 * {@code @Bean} definitions declared in imported {@code @Configuration} classes should be
 * accessed by using {@link org.springframework.beans.factory.annotation.Autowired @Autowired}
 * injection. Either the bean itself can be autowired, or the configuration class instance
 * declaring the bean can be autowired. The latter approach allows for explicit, IDE-friendly
 * navigation between {@code @Configuration} class methods.
 *
 * May be declared at the class level or as a meta-annotation.
 *
 * If XML or other non-{@code @Configuration} bean definition resources need to be
 * imported, use the {@link ImportResource @ImportResource} annotation instead.
 *
 * @author Chris Beams
 * @author Juergen Hoeller
 * @since 3.0
 * @see Configuration
 * @see ImportSelector
 * @see ImportResource
 */
 @Target({ElementType.TYPE})
 @Retention(RetentionPolicy.RUNTIME)
 @Documented
 public @interface Import {
 Class<?>[] value();
 }

从上面的英文注释可得知，参数value上可以配置3种类型的值：

### 普通的class ###

普通的class，容器中就会自动注册这个组件，id默认是全类名

public class Blue {}
    public class Red {}
    @Configuration
    @Import({ Blue.class, Red.class })
    public class ColorConfig {
    }

### ImportSelector ###

它需要返回需要导入的组件的全类名数组，它可以实现一些Aware接口：

*  EnvironmentAware
 *  BeanFactoryAware
 *  BeanClassLoaderAware
 *  ResourceLoaderAware

/**
 * Interface to be implemented by types that determine which @{@link Configuration}
 * class(es) should be imported based on a given selection criteria, usually one or more
 * annotation attributes.
 *
 * An {@link ImportSelector} may implement any of the following
 * {@link org.springframework.beans.factory.Aware Aware} interfaces, and their respective
 * methods will be called prior to {@link #selectImports}:
 * <ul>
 * <li>{@link org.springframework.context.EnvironmentAware EnvironmentAware}</li>
 * <li>{@link org.springframework.beans.factory.BeanFactoryAware BeanFactoryAware}</li>
 * <li>{@link org.springframework.beans.factory.BeanClassLoaderAware BeanClassLoaderAware}</li>
 * <li>{@link org.springframework.context.ResourceLoaderAware ResourceLoaderAware}</li>
 * </ul>
 *
 * ImportSelectors are usually processed in the same way as regular {@code @Import}
 * annotations, however, it is also possible to defer selection of imports until all
 * {@code @Configuration} classes have been processed (see {@link DeferredImportSelector}
 * for details).
 *
 * @author Chris Beams
 * @since 3.1
 * @see DeferredImportSelector
 * @see Import
 * @see ImportBeanDefinitionRegistrar
 * @see Configuration
 */
 public interface ImportSelector {
 
 	/**
 	 * Select and return the names of which class(es) should be imported based on
 	 * the {@link AnnotationMetadata} of the importing @{@link Configuration} class.
 	 */
 	String[] selectImports(AnnotationMetadata importingClassMetadata);
 }

下面演示下它的用法：

public class MyImportSelector implements ImportSelector {
    	@Override
    	public String[] selectImports(AnnotationMetadata importingClassMetadata) {
    		// 方法不要返回null值，就是到导入到容器中的组件全类名
    		//return new String[]{"com.github.bean.Blue"};
    		return new String[] { Blue.class.getName(), Yellow.class.getName() };
    	}
    }
    @Configuration
    @Import({ MyImportSelector.class })
    public class ColorConfig {
    }

### ImportBeanDefinitionRegistrar ###

它提供了手动注册bean到容器中，它可以实现一些Aware接口：

*  EnvironmentAware
 *  BeanFactoryAware
 *  BeanClassLoaderAware
 *  ResourceLoaderAware

/**
 * Interface to be implemented by types that register additional bean definitions when
 * processing @{@link Configuration} classes. Useful when operating at the bean definition
 * level (as opposed to {@code @Bean} method/instance level) is desired or necessary.
 *
 * Along with {@code @Configuration} and {@link ImportSelector}, classes of this type
 * may be provided to the @{@link Import} annotation (or may also be returned from an
 * {@code ImportSelector}).
 *
 * An {@link ImportBeanDefinitionRegistrar} may implement any of the following
 * {@link org.springframework.beans.factory.Aware Aware} interfaces, and their respective
 * methods will be called prior to {@link #registerBeanDefinitions}:
 * <ul>
 * <li>{@link org.springframework.context.EnvironmentAware EnvironmentAware}</li>
 * <li>{@link org.springframework.beans.factory.BeanFactoryAware BeanFactoryAware}
 * <li>{@link org.springframework.beans.factory.BeanClassLoaderAware BeanClassLoaderAware}
 * <li>{@link org.springframework.context.ResourceLoaderAware ResourceLoaderAware}
 * </ul>
 *
 * See implementations and associated unit tests for usage examples.
 *
 * @author Chris Beams
 * @since 3.1
 * @see Import
 * @see ImportSelector
 * @see Configuration
 */
 public interface ImportBeanDefinitionRegistrar {
 	/**
 	 * Register bean definitions as necessary based on the given annotation metadata of
 	 * the importing {@code @Configuration} class.
 	 * Note that {@link BeanDefinitionRegistryPostProcessor} types may not be
 	 * registered here, due to lifecycle constraints related to {@code @Configuration}
 	 * class processing.
 	 * @param importingClassMetadata annotation metadata of the importing class
 	 * @param registry current bean definition registry
 	 */
 	public void registerBeanDefinitions(
 			AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry);
 }

下面演示下它的用法：

public class MyImportBeanDefinitionRegistrar implements ImportBeanDefinitionRegistrar {
    	/**
    	 * AnnotationMetadata：当前类的注解信息
    	 * BeanDefinitionRegistry:BeanDefinition注册类；
    	 */
    	@Override
    	public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) { 
    		boolean definition = registry.containsBeanDefinition(Red.class.getName());
    		boolean definition2 = registry.containsBeanDefinition(Blue.class.getName());
    		if (definition && definition2) {
    			// 指定Bean定义信息
    			RootBeanDefinition beanDefinition = new RootBeanDefinition(Yellow.class);
    			// 注册一个Bean，指定bean名
    			registry.registerBeanDefinition(Yellow.class.getName(), beanDefinition);
    		}
    	}
    }
    @Configuration
    @Import({ MyImportSelector.class, MyImportBeanDefinitionRegistrar.class })
    public class ColorConfig {
    }

这里重点下：

**AnnotationMetadata**，它可以获取当前类（被@Import标记）的注解信息，它使用标准的反射来获取制定类的内部注解信息。

public interface AnnotationMetadata extends ClassMetadata, AnnotatedTypeMetadata {
 Set<String> getAnnotationTypes();
 
 Set<String> getMetaAnnotationTypes(String var1);
 
 boolean hasAnnotation(String var1);
 
 boolean hasMetaAnnotation(String var1);
 
 boolean hasAnnotatedMethods(String var1);
 
 Set<MethodMetadata> getAnnotatedMethods(String var1);
 }

**AnnotationConfigApplicationContext**，@Import内部制定的类作为regular component，而AnnotationConfigApplicationContext才能识别。![76685476][]

[76685476]: /images/20220217/cc09af3049b94c9f8cabdd344fe78b1a.png