Spring Boot自动注入原理

启用自动注入

使用注解@EnableAutoConfiguration开启自动注入功能。

@EnableAutoConfiguration

@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {
	// ignore...
}

@AutoConfigurationPackage

@Import(AutoConfigurationPackages.Registrar.class)
public @interface AutoConfigurationPackage {

}

Spring Boot通过AutoConfigurationImportSelector.class和AutoConfigurationPackages.Registrar.class这两个类配合使用，实现动态注入。
其中，类AutoConfigurationImportSelector.class会加载spring-autoconfigure-metadata.properties，并且进行过滤筛选，将需要动态注入的类全路径交给AutoConfigurationPackages.Registrar.class进行初始化注入。

动态注入SpingBean

实现bean动态注入可以通过两种方式。

方案一：实现ImportSelector接口

/* @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);

}

方案二：实现ImportSelector接口

/* @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.
    * @param importingClassMetadata annotation metadata of the importing class
    * @param registry current bean definition registry
    */
   void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry);

}