初始化過程

    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();
            }
            // ...
    }

ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory()
-> AbstractRefreshableApplicationContext#refreshBeanFactory() get a beanFactory(DefaultListableBeanFactory)
-> loadBeanDefinitions(beanFactory)
-> AbstractXmlApplicationContext#loadBeanDefinitions, XmlBeanDefinitionReader, loadBeanDefinition...(not yet instantiate)

registerBeanPostProcessors(beanFactory)
-> 註冊BPP，其中就包含AOP的BPPAbstractAutoProxyCreator，註意BPP是not-lazy-init的

finishBeanFactoryInitialization(beanFactory); 實例化非lazy-init的bean
-> DefaultListableDeanFactory#preInstantiateSingletons() -> getBean(beanName)
-> AbstractBeanFactory#getBean(beanName) -> doGetBean(name, null, null, false) -> getSingleton -> createBean
-> AbstractAutowiredCapableBeanFactory#createBean -> doCreateBean -> populateBean -> initializeBean -> applyBeanPostProcessorsBeforeInitialization -> invokeInitMethods -> applyBeanPostProcessorsAfterInitialization -> getBeanPostProcessors() 然後對每個bean挨個調用bpp的postProcessAfterInitialization方法，對於AOP，在其中就會有判定是否需要包裝、是否需要生成代理等操作，最後生成一個代理（這部分就是AOP的代碼了）

AOP創建代理

Spring默認會加載AbstractAutoProxyCreator這個Bean（是一個BPP）
AbstractAutoProxyCreator#postProcessAfterInitialization -> wrapIfNecessary

    protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {
        if (beanName != null && this.targetSourcedBeans.contains(beanName)) {
            return bean;
        }
        if (Boolean.FALSE.equals(this.advisedBeans.get(cacheKey))) {
            return bean;
        }
        if (isInfrastructureClass(bean.getClass()) || shouldSkip(bean.getClass(), beanName)) {
            this.advisedBeans.put(cacheKey, Boolean.FALSE);
            return bean;
        }

        // Create proxy if we have advice.
        Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
        if (specificInterceptors != DO_NOT_PROXY) {
            this.advisedBeans.put(cacheKey, Boolean.TRUE);
            Object proxy = createProxy(
                    bean.getClass(), beanName, specificInterceptors, new SingletonTargetSource(bean));
            this.proxyTypes.put(cacheKey, proxy.getClass());
            return proxy;
        }

        this.advisedBeans.put(cacheKey, Boolean.FALSE);
        return bean;
    }

getAdvicesAndAdvisorsForBean方法找到所有的攔截器，調用createProxy方法生成代理。
生成Advisor[]設置到ProxyFactory中，然後調用ProxyFactory#getProxy方法獲取代理（JdkDynamicAopProxy和CglibAopProxy）。其中在生成Advisors是一個表復雜的過程，涉及匹配切點、包裝生成通知（AspectJPointcutAdvisor.）

ProxyFactor extends ProxyCreatorSupport extends AdvisedSupport

    protected final synchronized AopProxy createAopProxy() {
        if (!this.active) {
            activate();
        }
        return getAopProxyFactory().createAopProxy(this);
    }

ProxyFactory以自身為參數，因為把Advisors設置進去了，生成代理時就可以拿到了。生成AopProxy（CglibAopProxy或者JdkDynamicAopProxy的構造參數需要AdvisedSupport）。
對於不需要包裝（攔截）的方法，有一個攔截器是ExposeInvocationInterceptor。
在代理的invoke方法裏，會先從AdvisedSupport中獲取到攔截器鏈，最後包裝成一個MethodInvocation（ReflectiveMethodInvocation）調用其proceed方法。
關鍵是proceed這個方法，會遞歸調用其中每一個interceptor，這裏的this就是ReflectiveMethodInvocation，然後在interceptor的invoke方法裏，有前置/後置/環繞等邏輯，或者沒有邏輯（ExposeInvocationInterceptor是第一個攔截器），參數是MethodInvocation，然後再調用mi的proceed方法，回到ReflectiveMethodInvocation中繼續處理，攔截器鏈調完之後就會反射調用返回。此處有點繞，需要多看幾遍代碼。

return ((MethodInterceptor) interceptorOrInterceptionAdvice).invoke(this);

攔截器接口InvocationInterceptor，前置後置等邏輯都實現了該接口。
AspectJAfterAdvice
MethodBeforeAdviceInterceptor
AspectJAroundAdvice
等等

Spring初始化過程到AOP