转载自 Spring Enable*高级应用及原理

 

Enable*

之前的文章用到了一些Enable*开头的注解，比如EnableAsync、EnableScheduling、EnableAspectJAutoProxy、EnableCaching等，Enable表示开启/允许一项功能。

Enable*工作原理

我们只需要几个很简单的注解就能开启一个复杂的功能，这是多么简易的用法，这是怎么办到的？

首先来看看计划任务@EnableScheduling的源代码

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Import(SchedulingConfiguration.class)
@Documented
public @interface EnableScheduling {}

主要核心的配置就是导入了一个配置文件，所以谜底也就接开了。

@Import(SchedulingConfiguration.class)

@Import用法

来看看Import的源码

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Import {/*** {@link Configuration}, {@link ImportSelector}, {@link ImportBeanDefinitionRegistrar}* or regular component classes to import.*/Class<?>[] value();}

1、Configuration

即上面的用法，直接导入Configuration配置类。

2、ImportSelector

根据条件选择导入不同的配置类，参考@EnableAsync

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

public class AsyncConfigurationSelector extends AdviceModeImportSelector<EnableAsync> {private static final String ASYNC_EXECUTION_ASPECT_CONFIGURATION_CLASS_NAME ="org.springframework.scheduling.aspectj.AspectJAsyncConfiguration";/*** {@inheritDoc}* @return {@link ProxyAsyncConfiguration} or {@code AspectJAsyncConfiguration} for* {@code PROXY} and {@code ASPECTJ} values of {@link EnableAsync#mode()}, respectively*/@Overridepublic String[] selectImports(AdviceMode adviceMode) {switch (adviceMode) {case PROXY:return new String[] { ProxyAsyncConfiguration.class.getName() };case ASPECTJ:return new String[] { ASYNC_EXECUTION_ASPECT_CONFIGURATION_CLASS_NAME };default:return null;}}}

3、ImportBeanDefinitionRegistrar

动态注册Bean，参考@EnableAspectJAutoProxy

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

class AspectJAutoProxyRegistrar implements ImportBeanDefinitionRegistrar {/*** Register, escalate, and configure the AspectJ auto proxy creator based on the value* of the @{@link EnableAspectJAutoProxy#proxyTargetClass()} attribute on the importing* {@code @Configuration} class.*/@Overridepublic void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {AopConfigUtils.registerAspectJAnnotationAutoProxyCreatorIfNecessary(registry);AnnotationAttributes enableAspectJAutoProxy =AnnotationConfigUtils.attributesFor(importingClassMetadata, EnableAspectJAutoProxy.class);if (enableAspectJAutoProxy.getBoolean("proxyTargetClass")) {AopConfigUtils.forceAutoProxyCreatorToUseClassProxying(registry);}if (enableAspectJAutoProxy.getBoolean("exposeProxy")) {AopConfigUtils.forceAutoProxyCreatorToExposeProxy(registry);}}}