spring AOP何时添加AnnotationAwareAspectJAutoProxyCreator

1.spring AOP的原理都知道是从解析自定义标签时注册一个beanDefiniton到beanFactory，然后在bean加载过程中做动态代理(initializeBean时会做一些最后的初始化，包括aware接口的调用,applyBeanPostProcessorsBefore/AfterXXX,initMethod，InitializingBean接口的调用)

2.但是真正添加AnnotationAwareAspectJAutoProxyCreator是在 ClassPathXmlApplicationContext("..")初始化时refresh（）方法中的registerBeanPostProcessors()添加.

3.创建AOP代理时Spring会判断当前bean是否适合做代理：（if(!earlyProxyReferences.contains(cacheKey))）,也就是不能和earlyProxyReferences冲突，确保代理只有一个。

4.我是学物理的，为了方便理解，我习惯把AOP叫成“代码短路”设计，spring中有很多针对特殊灵活的配置和场景设计了“短路”的代码。

5.spring aop的增强器采用的是链式设计:具体可以看JdkDynamicAopProxy中invoke方法

关键代码：
registerBeanPostProcessors(beanFactory);

public void refresh() throws BeansException, IllegalStateException {
		synchronized (this.startupShutdownMonitor) {
			// Prepare this context for refreshing.
			prepareRefresh();

			// Tell the subclass to refresh the internal bean factory.
			ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

			// Prepare the bean factory for use in this context.
			prepareBeanFactory(beanFactory);

			try {
				// Allows post-processing of the bean factory in context subclasses.
				postProcessBeanFactory(beanFactory);

				// Invoke factory processors registered as beans in the context.
				invokeBeanFactoryPostProcessors(beanFactory);

				// Register bean processors that intercept bean creation.
				registerBeanPostProcessors(beanFactory);

				// Initialize message source for this context.
				initMessageSource();

				// Initialize event multicaster for this context.
				initApplicationEventMulticaster();

				// Initialize other special beans in specific context subclasses.
				onRefresh();

				// Check for listener beans and register them.
				registerListeners();

				// Instantiate all remaining (non-lazy-init) singletons.
				finishBeanFactoryInitialization(beanFactory);

				// Last step: publish corresponding event.
				finishRefresh();
			}

			catch (BeansException ex) {
				logger.warn("Exception encountered during context initialization - cancelling refresh attempt", ex);

				// Destroy already created singletons to avoid dangling resources.
				destroyBeans();

				// Reset 'active' flag.
				cancelRefresh(ex);

				// Propagate exception to caller.
				throw ex;
			}
		}
	}