SpringBoot 的 @Import 用于将指定的类实例注入之Spring IOC Container中。
今天抽空在仔细看了下Springboot 关于 @Import 的处理过程， 记下来以后看。
1. @Import
先看Spring对它的注释 (文档贴过来的）， 总结下来作用就是和xml配置的 <import />标签作用一样，允许通过它引入 @Configuration 注解的类 (java config)， 引入ImportSelector接口(这个比较重要， 因为要通过它去判定要引入哪些@Configuration) 和 ImportBeanDefinitionRegistrar 接口的实现， 也包括 @Component注解的普通类。
但是如果要引入另一个xml 文件形式配置的 bean, 则需要通过 @ImportResource 注解。
/** * Indicates one or more {@link Configuration @Configuration} classes to import. * * <p>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}). * * <p>{@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. * * <p>May be declared at the class level or as a meta-annotation. * * <p>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)@Documentedpublic @interface Import { /** * {@link Configuration}, {@link ImportSelector}, {@link ImportBeanDefinitionRegistrar} * or regular component classes to import. */ Class<?>[] value();} 2. ImportSelector
因为 @Import 的实现有很多时候需要借助 ImportSelector 接口， 所以我们再看下这个接口的描述, 总结下来就是需要通过这个接口的实现去决定要引入哪些 @Configuration。 它如果实现了以下四个Aware 接口， 那么spring保证会在调用它之前先调用Aware接口的方法。
至于为什么要保证调用Aware， 我个人觉得应该是你可以通过这些Aware去感知系统里边所有的环境变量， 从而决定你具体的选择逻辑。
/** * 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. * * <p>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> * * <p>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);} 3. Springboot 对@Import注解的处理过程
Springboot对注解的处理都发生在AbstractApplicationContext -> refresh() -> invokeBeanFactoryPostProcessors(beanFactory) -> ConfigurationClassPostProcessor -> postProcessBeanDefinitionRegistry()方法中。
(稍微说下也免得我自己忘了， springboot初始化的普通context(非web) 是AnnotationConfigApplicationContext, 在初始化的时候会初始化两个工具类， AnnotatedBeanDefinitionReader 和 ClassPathBeanDefinitionScanner 分别用来从 annotation driven 的配置和xml的配置中读取beanDefinition并向context注册, 那么在初始化 AnnotatedBeanDefinitionReader 的时候， 会向BeanFactory注册一个ConfigurationClassPostProcessor 用来处理所有的基于annotation的bean, 这个ConfigurationClassPostProcessor 是 BeanFactoryPostProcessor 的一个实现，springboot会保证在 invokeBeanFactoryPostProcessors(beanFactory) 方法中调用注册到它上边的所有的BeanFactoryPostProcessor)
如下代码显示是通过 ConfigurationClassParser 类来转换的
// Parse each @Configuration class ConfigurationClassParser parser = new ConfigurationClassParser( this.metadataReaderFactory, this.problemReporter, this.environment, this.resourceLoader, this.componentScanBeanNameGenerator, registry);
那么在 ConfigurationClassParser -> processConfigurationClass() -> doProcessConfigurationClass() 方法中我们找到了 (这里边的流程还是很清楚的， 分别按次序处理了@PropertySource， @ComponentScan, @Import, @ImportResource, 在处理这些注解的时候是通过递归处理来保证所有的都被处理了)
// Process any @Import annotations processImports(configClass, sourceClass, getImports(sourceClass), true);
那接下来就看它到底是怎么做的 . 流程依然清晰 ：
首先， 判断如果被import的是 ImportSelector.class 接口的实现， 那么初始化这个被Import的类， 然后调用它的selectImports方法去获得所需要的引入的configuration， 然后递归处理
其次， 判断如果被import的是 ImportBeanDefinitionRegistrar 接口的实现， 那么初始化后将对当前对象的处理委托给这个ImportBeanDefinitionRegistrar (不是特别明白， 只是我的猜测）
最后， 将import引入的类作为一个正常的类来处理 ( 调用最外层的doProcessConfigurationClass())
所以， 从这里我们知道， 如果你引入的是一个正常的component， 那么会作为@Compoent或者@Configuration来处理， 这样在BeanFactory里边可以通过getBean拿到， 但如果你是 ImportSelector 或者 ImportBeanDefinitionRegistrar 接口的实现， 那么spring并不会将他们注册到beanFactory中，而只是调用他们的方法。
private void processImports(ConfigurationClass configClass, SourceClass currentSourceClass, Collection<SourceClass> importCandidates, boolean checkForCircularImports) { if (importCandidates.isEmpty()) { return; } if (checkForCircularImports && isChainedImportOnStack(configClass)) { this.problemReporter.error(new CircularImportProblem(configClass, this.importStack)); } else { this.importStack.push(configClass); try { for (SourceClass candidate : importCandidates) { if (candidate.isAssignable(ImportSelector.class)) { // Candidate class is an ImportSelector -> delegate to it to determine imports Class<?> candidateClass = candidate.loadClass(); ImportSelector selector = BeanUtils.instantiateClass(candidateClass, ImportSelector.class); ParserStrategyUtils.invokeAwareMethods( selector, this.environment, this.resourceLoader, this.registry); if (this.deferredImportSelectors != null && selector instanceof DeferredImportSelector) { this.deferredImportSelectors.add( new DeferredImportSelectorHolder(configClass, (DeferredImportSelector) selector)); } else { String[] importClassNames = selector.selectImports(currentSourceClass.getMetadata()); Collection<SourceClass> importSourceClasses = asSourceClasses(importClassNames); processImports(configClass, currentSourceClass, importSourceClasses, false); } } else if (candidate.isAssignable(ImportBeanDefinitionRegistrar.class)) { // Candidate class is an ImportBeanDefinitionRegistrar -> // delegate to it to register additional bean definitions Class<?> candidateClass = candidate.loadClass(); ImportBeanDefinitionRegistrar registrar = BeanUtils.instantiateClass(candidateClass, ImportBeanDefinitionRegistrar.class); ParserStrategyUtils.invokeAwareMethods( registrar, this.environment, this.resourceLoader, this.registry); configClass.addImportBeanDefinitionRegistrar(registrar, currentSourceClass.getMetadata()); } else { // Candidate class not an ImportSelector or ImportBeanDefinitionRegistrar -> // process it as an @Configuration class this.importStack.registerImport( currentSourceClass.getMetadata(), candidate.getMetadata().getClassName()); processConfigurationClass(candidate.asConfigClass(configClass)); } } } catch (BeanDefinitionStoreException ex) { throw ex; } catch (Throwable ex) { throw new BeanDefinitionStoreException( "Failed to process import candidates for configuration class [" + configClass.getMetadata().getClassName() + "]", ex); } finally { this.importStack.pop(); } } } 以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持VeVb武林网。
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