@EnableXXX  在工作中经常使用到，意味着开启某功能， 使用简单，那么@EnableXXX  到底干了那些事情呢。

@EnableXXX        ------->   @Import(A.class)

要实现某功能，则需要导入指定的class(es) 。

A为Configuration类   

或者   A implements   ImportBeanDefinitionRegistrar     

或者   A  extends SpringFactoryImportSelector       

或者   A  extends AdviceModeImportSelector

场景依次编号为1 2 3  4.

场景1 ：   直接指定Configuration 类      @EnableZuulProxy    则意味着会自动注册 ZuulProxyConfiguration 定义的bean 。

@EnableCircuitBreaker @EnableDiscoveryClient @Target(ElementType.TYPE) @Retention(RetentionPolicy.RUNTIME) @Import(ZuulProxyConfiguration.class) public @interface EnableZuulProxy { }

场景2：   找到指定package内有特定声明的class，实例化然后注册到 BeanDefinitionRegistry。     

@Retention(RetentionPolicy.RUNTIME) @Target(ElementType.TYPE) @Documented @Import(FeignClientsRegistrar.class) public @interface EnableFeignClients {

FeignClientsRegistrar 会登记所有 @FeignClient 声明的客户端。 Registrar    登记员。。。

场景3：   场景3比场景1稍微含蓄一点，复杂一点。

根据  spring.factories  的配置找到相应的 Configuration 类 ， 然后注册相应的bean 。

@Target(ElementType.TYPE) @Retention(RetentionPolicy.RUNTIME) @Documented @Inherited @Import(EnableDiscoveryClientImportSelector.class) public @interface EnableDiscoveryClient {

@EnableHystrix和@EnableCircuitBreaker是等价的  （当你的pom文件中只引入了一种CircuitBreake实现时）

@EnableEurekaClient和EnableDiscoveryClient是等价的   （当你的pom文件中只引入了一种DiscoveryClient实现时）

具体实现：    根据key 查找相应的Configuration 类。   key 为最终的 annotationClass。

@Override public String[] selectImports(AnnotationMetadata metadata) { if (!isEnabled()) { return new String[0]; } AnnotationAttributes attributes = AnnotationAttributes.fromMap( metadata.getAnnotationAttributes(this.annotationClass.getName(), true)); Assert.notNull(attributes, "No " + getSimpleName() + " attributes found. Is " + metadata.getClassName() + " annotated with @" + getSimpleName() + "?"); // Find all possible auto configuration classes, filtering duplicates List factories = new ArrayList<>(new LinkedHashSet<>(SpringFactoriesLoader .loadFactoryNames(this.annotationClass, this.beanClassLoader))); if (factories.isEmpty() && !hasDefaultFactory()) { throw new IllegalStateException("Annotation @" + getSimpleName() + " found, but there are no implementations. Did you forget to include a starter?"); }

if (factories.size() > 1) { // there should only ever be one DiscoveryClient, but there might be more than // one factory log.warn("More than one implementation " + "of @" + getSimpleName() + " (now relying on @Conditionals to pick one): " + factories); }

return factories.toArray(new String[factories.size()]); }

public static List loadFactoryNames(Class<?> factoryClass, ClassLoader classLoader) { String factoryClassName = factoryClass.getName(); try { Enumeration urls = (classLoader != null ? classLoader.getResources(FACTORIES_RESOURCE_LOCATION) : ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION)); List result = new ArrayList(); while (urls.hasMoreElements()) { URL url = urls.nextElement(); Properties properties = PropertiesLoaderUtils.loadProperties(new UrlResource(url)); String factoryClassNames = properties.getProperty(factoryClassName); result.addAll(Arrays.asList(StringUtils.commaDelimitedListToStringArray(factoryClassNames))); } return result; } catch (IOException ex) { throw new IllegalArgumentException("Unable to load [" + factoryClass.getName() + "] factories from location [" + FACTORIES_RESOURCE_LOCATION + "]", ex); } }

这里出现了熟悉的ClassLoader 。

原来 ClassLoader 除了能 loadClass 之外，还能 getResources 。

场景4 ：   实现了aop 功能。 。。

@Target(ElementType.TYPE) @Retention(RetentionPolicy.RUNTIME) @Documented @Import(CachingConfigurationSelector.class) public @interface EnableCaching {

相关联的 Configuration

场景3和场景4 总归都是实现了   ImportSelector  接口。 

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); }

大概的意思翻译一下：  要实现XXX功能， 则需要加载那些_Configuration 类 。_

Configuration   的前生就是Spring项目中的 xml 文件 。