Spring注解驱动开发——AOP的原理

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AOP的原理：@EnableAspectJAutoProxy注解会启用Spring的AOP，那么@EnableAspectJAutoProxy是什么呢？

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Import(AspectJAutoProxyRegistrar.class)
public @interface EnableAspectJAutoProxy {
	boolean proxyTargetClass() default false;
	boolean exposeProxy() default false;
}

可以看到@EnableAspectJAutoProxy导入了一个AspectJAutoProxyRegistrar，而AspectJAutoProxyRegistrar实现了ImportBeanDefinitionRegistrar接口，通过该接口的registerBeanDefinitions方法在IOC容器中自定义注册组件AnnotationAwareAspectJAutoProxyCreator(AspectJ的自动代理创建器)，这里判断了一下EnableAspectJAutoProxy注解的一些属性值

class AspectJAutoProxyRegistrar implements ImportBeanDefinitionRegistrar {

	@Override
	public void registerBeanDefinitions(
			AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {

		AopConfigUtils.registerAspectJAnnotationAutoProxyCreatorIfNecessary(registry);

		AnnotationAttributes enableAspectJAutoProxy =
				AnnotationConfigUtils.attributesFor(importingClassMetadata, EnableAspectJAutoProxy.class);
		if (enableAspectJAutoProxy != null) {
			if (enableAspectJAutoProxy.getBoolean("proxyTargetClass")) {
				AopConfigUtils.forceAutoProxyCreatorToUseClassProxying(registry);
			}
			if (enableAspectJAutoProxy.getBoolean("exposeProxy")) {
				AopConfigUtils.forceAutoProxyCreatorToExposeProxy(registry);
			}
		}
	}

}

private static BeanDefinition registerOrEscalateApcAsRequired(Class<?> cls, BeanDefinitionRegistry registry,
		@Nullable Object source) {

	Assert.notNull(registry, "BeanDefinitionRegistry must not be null");

	if (registry.containsBeanDefinition(AUTO_PROXY_CREATOR_BEAN_NAME)) {
		BeanDefinition apcDefinition = registry.getBeanDefinition(AUTO_PROXY_CREATOR_BEAN_NAME);
		if (!cls.getName().equals(apcDefinition.getBeanClassName())) {
			int currentPriority = findPriorityForClass(apcDefinition.getBeanClassName());
			int requiredPriority = findPriorityForClass(cls);
			if (currentPriority < requiredPriority) {
				apcDefinition.setBeanClassName(cls.getName());
			}
		}
		return null;
	}

	RootBeanDefinition beanDefinition = new RootBeanDefinition(cls);
	beanDefinition.setSource(source);
	beanDefinition.getPropertyValues().add("order", Ordered.HIGHEST_PRECEDENCE);
	beanDefinition.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
	registry.registerBeanDefinition(AUTO_PROXY_CREATOR_BEAN_NAME, beanDefinition);
	return beanDefinition;
}

AnnotationAwareAspectJAutoProxyCreator是什么呢？看其继承关系：

    ->AspectJAwareAdvisorAutoProxyCreator

        ->AbstractAdvisorAutoProxyCreator

            ->AbstractAutoProxyCreator

                ->ProxyProcessorSupport

                -->implements SmartInstantiationAwareBeanPostProcessor,BeanFactoryAware

可以看到它其实也是个后置处理器，而且装配了BeanFactory。