1.位置

在spring-context包中

2.作用

ImportSelector 开放接口实现，提供SPI机制，@Import注解来导入。
DeferredImportSelector 延迟导入。

3.原理

3.1加载import类

ImportSelector 和DeferredImportSelector 导入的类，全部放到ConfigurationClassParser内部类ImportStack的属性：
private final MultiValueMap<String, AnnotationMetadata> imports

3.2存放延迟加载类

但是放入时机不一样，DeferredImportSelector 首次导入时，存放到ConfigurationClassParser内部类DeferredImportSelectorHandler中的属性：
private List<ConfigurationClassParser.DeferredImportSelectorHolder> deferredImportSelectors;

3.3加载延迟加载类

执行
this.deferredImportSelectorHandler.process();
将deferredImportSelector导入的类放入ConfigurationClassParser内部类ImportStack的属性：
private final MultiValueMap<String, AnnotationMetadata> imports

4.时序

SpringApplication.run(Class<?> primarySource,String... args)
->SpringApplication.run(Class<?>[] primarySources,String[] args)
  ->SpringApplication.run(String... args)
    ->SpringApplication.refreshContext(ConfigurableApplicationContext context) 
       ->AbstractApplicationContext.refresh()
          ->AbstractApplicationContext.invokeBeanFactoryPostProcessors
            ->PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors
               ->ConfigurationClassPostProcessor.invokeBeanDefinitionRegistryPostProcessors
                 ->ConfigurationClassPostProcessor.processConfigBeanDefinitions
                    ->ConfigurationClassParser.parse(Set<BeanDefinitionHolder> configCandidates)
                       ->ConfigurationClassParser.parse(AnnotationMetadata metadata, String beanName) 
                         ->ConfigurationClassParser.processConfigurationClass
                           ->ConfigurationClassParser.doProcessConfigurationClass
                              ->this.processImports
                              ->ConfigurationClassParser.processImports递归调用
                                 ->ImportSelector.selectImports
                                   ->this.deferredImportSelectorHandler.handle(configClass, (DeferredImportSelector)selector);
                                   ->this.importStack.registerImport
                       ->this.deferredImportSelectorHandler.process();
                          ->DeferredImportSelectorGroupingHandler.register
                          ->DeferredImportSelectorGroupingHandler.processGroupImports()
                             ->ConfigurationClassParser.this.processImports
                               ->this.importStack.registerImport(currentSourceClass.getMetadata(), candidate.getMetadata().getClassName());

5.实例