参照Spring IOC 源码，BeanDefinition 加载以及注册的流程
解析标签阶段<aop:config>标签为例子（注册BeanPostProcessor）
org.springframework.context.support.AbstractApplicationContext#refresh
org.springframework.context.support.AbstractApplicationContext#obtainFreshBeanFactory
org.springframework.context.support.AbstractRefreshableApplicationContext#refreshBeanFactory
org.springframework.context.support.AbstractXmlApplicationContext#loadBeanDefinitions(org.springframework.beans.factory.support.DefaultListableBeanFactory)
org.springframework.context.support.AbstractXmlApplicationContext#loadBeanDefinitions(org.springframework.beans.factory.xml.XmlBeanDefinitionReader)
org.springframework.beans.factory.support.AbstractBeanDefinitionReader#loadBeanDefinitions(org.springframework.core.io.Resource...)
org.springframework.beans.factory.xml.XmlBeanDefinitionReader#loadBeanDefinitions(org.springframework.core.io.Resource)
org.springframework.beans.factory.xml.XmlBeanDefinitionReader#loadBeanDefinitions(org.springframework.core.io.support.EncodedResource)
org.springframework.beans.factory.xml.XmlBeanDefinitionReader#doLoadBeanDefinitions
org.springframework.beans.factory.xml.XmlBeanDefinitionReader#registerBeanDefinitions
org.springframework.beans.factory.xml.DefaultBeanDefinitionDocumentReader#registerBeanDefinitions
org.springframework.beans.factory.xml.DefaultBeanDefinitionDocumentReader#doRegisterBeanDefinitions
org.springframework.beans.factory.xml.DefaultBeanDefinitionDocumentReader#parseBeanDefinitions
org.springframework.beans.factory.xml.BeanDefinitionParserDelegate#parseCustomElement(org.w3c.dom.Element)
org.springframework.beans.factory.xml.BeanDefinitionParserDelegate#parseCustomElement(org.w3c.dom.Element, org.springframework.beans.factory.config.BeanDefinition)
org.springframework.beans.factory.xml.NamespaceHandlerSupport#parse
org.springframework.aop.config.ConfigBeanDefinitionParser#parse
org.springframework.aop.config.ConfigBeanDefinitionParser#configureAutoProxyCreator
org.springframework.aop.config.AopNamespaceUtils#registerAspectJAutoProxyCreatorIfNecessary
org.springframework.aop.config.AopConfigUtils#registerAspectJAutoProxyCreatorIfNecessary(org.springframework.beans.factory.support.BeanDefinitionRegistry, java.lang.Object)
org.springframework.beans.factory.support.DefaultListableBeanFactory#registerBeanDefinition
实例化Bean阶段 (执行BeanPostProcessors接口的postProcessAfterInitialization方法)
org.springframework.context.support.AbstractApplicationContext#refresh
org.springframework.context.support.AbstractApplicationContext#finishBeanFactoryInitialization
org.springframework.beans.factory.support.AbstractBeanFactory#getBean(java.lang.String, java.lang.Class<T>)
org.springframework.beans.factory.support.AbstractBeanFactory#doGetBean
org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#createBean(java.lang.String, org.springframework.beans.factory.support.RootBeanDefinition, java.lang.Object[])
org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#doCreateBean
org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#initializeBean(java.lang.String, java.lang.Object, org.springframework.beans.factory.support.RootBeanDefinition)
org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#applyBeanPostProcessorsAfterInitialization
总结
解析标签阶段
如图在解析标签阶段，注册了一个AspectJAwareAdvisorAutoProxyCreator， AspectJAwareAdvisorAutoProxyCreator是一个实现了BeanPostProcessor的类。
它在IOC执行总体的位置如下。
实例化Bean阶段
在实例化Bean的阶段，获取BeanPostProcessor，并且执行。所以执行了AbstractAutowireCapableBeanFactory#applyBeanPostProcessorsAfterInitialization。所以Spring AOP的入口是在AspectJAwareAdvisorAutoProxyCreator.postProcessAfterInitialization()方法(父类)
它在IOC执行总体的位置如下。
