首先看下@Import的javadoc文檔：

Provides functionality equivalent to the <import/> element in Spring XML. Allows for importing @Configuration classes, ImportSelector and ImportBeanDefinitionRegistrar implementations, as well as regular component classes (as of 4.2; analogous to AnnotationConfigApplicationContext.register). @Bean definitions declared in imported @Configuration classes should be accessed by using @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 @Configuration class methods. May be declared at the class level or as a meta-annotation.

簡單翻譯一下，這里面說了3個點：

（1）@Import就是用來向容器中導入bean的，導入的方式有三種：導入@Configuration、導入ImportSelector、導入ImportBeanDefinitionRegistrar。

（2）被@Import的類是被加載到了Spring容器當中，因此無論是類本身還是類里面用@Bean注解定義的bean都可以被@Autowired注入。

（3）@Import可以加在類上面，也可以作為元注解加在注解上面

下面就來分別舉例說明下。

1.導入@Configuration舉例

//這是個普通的類 public class Service1 {public Service1(){System.out.println("Service1");} } //這是個配置類 @Configuration public class DemoConfig {@Beanpublic Service2 service2(){return new Service2();} } @SpringBootApplication //這樣可以導入Service1和Service2這兩個Bean @Import({Service1.class, DemoConfig.class}) public class ImportDemoApplication {public static void main(String[] args) {SpringApplication.run(ImportDemoApplication.class, args);} }

2.導入ImportSelector舉例

public class DemoImportSelector implements ImportSelector {@Overridepublic String[] selectImports(AnnotationMetadata importingClassMetadata) {//這里返回bean的完整的名字return new String[]{"com.github.xjs.importdemo.service.Service3"};} } @SpringBootApplication //這樣就可以導入Service3這個bean @Import({DemoImportSelector.class}) public class ImportDemoApplication {public static void main(String[] args) {SpringApplication.run(ImportDemoApplication.class, args);} }

3.導入ImportBeanDefinitionRegistrar舉例

public class DemoRegistrar implements ImportBeanDefinitionRegistrar {@Overridepublic void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {//可以直接向容器注冊beanGenericBeanDefinition bd = new GenericBeanDefinition();bd.setBeanClass(Service4.class);registry.registerBeanDefinition("service4", bd);} } @SpringBootApplication //這樣就可以導入Service4這個bean @Import({DemoRegistrar.class}) public class ImportDemoApplication {public static void main(String[] args) {SpringApplication.run(ImportDemoApplication.class, args);} }

4.@Import作為元注解使用舉例

因為@Import可以作為元注解使用，因此可以用這個特性來做組件掃描，比如：

//首先定義一個用于掃描的注解，上面添加@Import元注解： @Retention(RetentionPolicy.RUNTIME) @Target({ElementType.TYPE}) @Import({DemoImportSelector.class}) public @interface DemoScan {/*** 要掃描的根路徑* */String basePackage() default ""; }

然后在啟動類或者配置類上添加這個注解：

@SpringBootApplication //首先添加這個注解 @DemoScan(basePackage="com.github.xjs") @Import({DemoImportSelector.class, DemoRegistrar.class}) public class ImportDemoApplication {public static void main(String[] args) {SpringApplication.run(ImportDemoApplication.class, args);} }

無論是ImportSelector還是ImportBeanDefinitionRegistrar在回調中都可以使用AnnotationMetadata獲取@DemoScan里面的屬性：

public class DemoImportSelector implements ImportSelector {@Overridepublic String[] selectImports(AnnotationMetadata importingClassMetadata) {//讀取DemoScan的屬性AnnotationAttributes annoAttrs = AnnotationAttributes.fromMap(importingClassMetadata.getAnnotationAttributes(DemoScan.class.getName()));String basePackage = annoAttrs.getString("basePackage");System.out.println("basePackage:"+basePackage);return new String[]{"com.github.xjs.importdemo.service.Service3"};} }

拿到了要掃描的路徑如何去掃描可以參考上一篇如何去掃描Mapper接口的，在Spring的源碼中有大量的這種用法，比如：ServletComponentScan與ServletComponentScanRegistrar、MapperScan與MapperScannerRegistrar等等。

5.@Import解析的源碼

// org.springframework.context.annotation.ConfigurationClassParser: private void processImports(ConfigurationClass configClass, SourceClass currentSourceClass,Collection<SourceClass> importCandidates, boolean checkForCircularImports) {...for (SourceClass candidate : importCandidates) {if (candidate.isAssignable(ImportSelector.class)) {// Candidate class is an ImportSelector -> delegate to it to determine importsClass<?> candidateClass = candidate.loadClass();ImportSelector selector = BeanUtils.instantiateClass(candidateClass, ImportSelector.class);ParserStrategyUtils.invokeAwareMethods(selector, this.environment, this.resourceLoader, this.registry);if (selector instanceof DeferredImportSelector) {this.deferredImportSelectorHandler.handle(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 definitionsClass<?> 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 classthis.importStack.registerImport(currentSourceClass.getMetadata(), candidate.getMetadata().getClassName());processConfigurationClass(candidate.asConfigClass(configClass));}}... } }

完整的源碼下載：

https://github.com/xjs1919/enumdemo 下的 import-demo

歡迎掃碼加關注：

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