class AspectJAutoProxyRegistrar implements ImportBeanDefinitionRegistrar {

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

        // 注入一個bean名字為org.springframework.aop.config.internalAutoProxyCreator的AspectJAwareAdvisorAutoProxyCreator
        AopConfigUtils.registerAspectJAnnotationAutoProxyCreatorIfNecessary(registry);

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

}

@Nullable
public static BeanDefinition registerAspectJAnnotationAutoProxyCreatorIfNecessary(BeanDefinitionRegistry registry) {
    return registerAspectJAnnotationAutoProxyCreatorIfNecessary(registry, null);
}

@Nullable
public static BeanDefinition registerAspectJAnnotationAutoProxyCreatorIfNecessary(
        BeanDefinitionRegistry registry, @Nullable Object source) {
    // AnnotationAwareAspectJAutoProxyCreator
    return registerOrEscalateApcAsRequired(AnnotationAwareAspectJAutoProxyCreator.class, registry, source);
}

@Nullable
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;
    }

    // 注入AspectJAwareAdvisorAutoProxyCreator
    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;
}

protected List<Advisor> findEligibleAdvisors(Class<?> beanClass, String beanName) {
	/**
	 * @see AnnotationAwareAspectJAutoProxyCreator#findCandidateAdvisors()
	 */
	// 獲取容器中所有的切面Advisor
	// 這里返回的切面中的方法已經(jīng)是有序的了，先按注解順序（Around.class, Before.class, After.class, AfterReturning.class, AfterThrowing.class），再按方法名稱
	List<Advisor> candidateAdvisors = findCandidateAdvisors();
	// 獲取所有能夠作用于當(dāng)前Bean上的Advisor
	List<Advisor> eligibleAdvisors = findAdvisorsThatCanApply(candidateAdvisors, beanClass, beanName);
	/**
	 * @see AspectJAwareAdvisorAutoProxyCreator#extendAdvisors(java.util.List)
	 */
	// 往集合第一個位置加入了一個DefaultPointcutAdvisor
	extendAdvisors(eligibleAdvisors);
	if (!eligibleAdvisors.isEmpty()) {
		/**
		 * @see AspectJAwareAdvisorAutoProxyCreator#sortAdvisors(java.util.List)
		 */
		// 這里是對切面進(jìn)行排序，例如有@Order注解或者實(shí)現(xiàn)了Ordered接口
		eligibleAdvisors = sortAdvisors(eligibleAdvisors);
	}
	return eligibleAdvisors;
}

protected List<Advisor> findCandidateAdvisors() {
	// Add all the Spring advisors found according to superclass rules.
	// 獲取容器中所有的切面Advisor
	List<Advisor> advisors = super.findCandidateAdvisors();
	// Build Advisors for all AspectJ aspects in the bean factory.
	if (this.aspectJAdvisorsBuilder != null) {
		// 這里還需要解析@Aspect注解，生成Advisor
		advisors.addAll(this.aspectJAdvisorsBuilder.buildAspectJAdvisors());
	}
	return advisors;
}

public static List<Advisor> findAdvisorsThatCanApply(List<Advisor> candidateAdvisors, Class<?> clazz) {
	if (candidateAdvisors.isEmpty()) {
		return candidateAdvisors;
	}
	List<Advisor> eligibleAdvisors = new ArrayList<>();
	// InstantiationModelAwarePointcutAdvisorImpl
	for (Advisor candidate : candidateAdvisors) {
		if (candidate instanceof IntroductionAdvisor && canApply(candidate, clazz)) {
			// IntroductionAdvisor類型為引入切面，具體類型為DeclareParentsAdvisor
			eligibleAdvisors.add(candidate);
		}
	}
	boolean hasIntroductions = !eligibleAdvisors.isEmpty();
	for (Advisor candidate : candidateAdvisors) {
		if (candidate instanceof IntroductionAdvisor) {
			// already processed
			continue;
		}
		// PointCut中的ClassFilter.match 匹配類
		// PointCut中的MethodMatcher.match 匹配方法
		if (canApply(candidate, clazz, hasIntroductions)) {
			// @Aspect，類型為InstantiationModelAwarePointcutAdvisorImpl
			eligibleAdvisors.add(candidate);
		}
	}
	return eligibleAdvisors;
}

public Object postProcessAfterInitialization(@Nullable Object bean, String beanName) {
	if (bean != null) {
		// 先從緩存中獲取代理對象
		Object cacheKey = getCacheKey(bean.getClass(), beanName);
		if (this.earlyProxyReferences.remove(cacheKey) != bean) {
			// 按需生成代理對象
			return wrapIfNecessary(bean, beanName, cacheKey);
		}
	}
	return bean;
}

protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {
	if (StringUtils.hasLength(beanName) && 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.
	/**
	 * @see AbstractAdvisorAutoProxyCreator#getAdvicesAndAdvisorsForBean(java.lang.Class, java.lang.String, org.springframework.aop.TargetSource)
	 */
	// 獲取與當(dāng)前Bean匹配的切面
	Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
	if (specificInterceptors != DO_NOT_PROXY) {
		this.advisedBeans.put(cacheKey, Boolean.TRUE);
		// 創(chuàng)建代理
		Object proxy = createProxy(
				bean.getClass(), beanName, specificInterceptors, new SingletonTargetSource(bean));
		this.proxyTypes.put(cacheKey, proxy.getClass());
		return proxy;
	}

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

protected Object createProxy(Class<?> beanClass, @Nullable String beanName,
		@Nullable Object[] specificInterceptors, TargetSource targetSource) {

	if (this.beanFactory instanceof ConfigurableListableBeanFactory) {
		AutoProxyUtils.exposeTargetClass((ConfigurableListableBeanFactory) this.beanFactory, beanName, beanClass);
	}

	// 創(chuàng)建代理工廠
	ProxyFactory proxyFactory = new ProxyFactory();
	proxyFactory.copyFrom(this);

	if (!proxyFactory.isProxyTargetClass()) {
		// 進(jìn)來說明proxyTargetClass=false，指定JDK代理
		if (shouldProxyTargetClass(beanClass, beanName)) {
			// 進(jìn)來這里說明BD中有個屬性preserveTargetClass=true，可以BD中屬性設(shè)置的優(yōu)先級最高
			proxyFactory.setProxyTargetClass(true);
		}
		else {
			// 這里會判斷bean有沒有實(shí)現(xiàn)接口，沒有就只能使用CGlib
			evaluateProxyInterfaces(beanClass, proxyFactory);
		}
	}

	Advisor[] advisors = buildAdvisors(beanName, specificInterceptors);
	proxyFactory.addAdvisors(advisors); // 切面
	proxyFactory.setTargetSource(targetSource); // 目標(biāo)對象
	customizeProxyFactory(proxyFactory);

	proxyFactory.setFrozen(this.freezeProxy);
	if (advisorsPreFiltered()) {
		proxyFactory.setPreFiltered(true);
	}

	// 使用JDK或者CGlib創(chuàng)建代理對象
	return proxyFactory.getProxy(getProxyClassLoader());
}

public Object getProxy(@Nullable ClassLoader classLoader) {
	if (logger.isTraceEnabled()) {
		logger.trace("Creating JDK dynamic proxy: " + this.advised.getTargetSource());
	}
	Class<?>[] proxiedInterfaces = AopProxyUtils.completeProxiedInterfaces(this.advised, true);
	findDefinedEqualsAndHashCodeMethods(proxiedInterfaces);
	return Proxy.newProxyInstance(classLoader, proxiedInterfaces, this);
}

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
	Object oldProxy = null;
	boolean setProxyContext = false;

	TargetSource targetSource = this.advised.targetSource;
	Object target = null;

	try {
		if (!this.equalsDefined && AopUtils.isEqualsMethod(method)) {
			// The target does not implement the equals(Object) method itself.
			return equals(args[0]);
		}
		else if (!this.hashCodeDefined && AopUtils.isHashCodeMethod(method)) {
			// The target does not implement the hashCode() method itself.
			return hashCode();
		}
		else if (method.getDeclaringClass() == DecoratingProxy.class) {
			// There is only getDecoratedClass() declared -> dispatch to proxy config.
			return AopProxyUtils.ultimateTargetClass(this.advised);
		}
		else if (!this.advised.opaque && method.getDeclaringClass().isInterface() &&
				method.getDeclaringClass().isAssignableFrom(Advised.class)) {
			// Service invocations on ProxyConfig with the proxy config...
			return AopUtils.invokeJoinpointUsingReflection(this.advised, method, args);
		}

		Object retVal;

		if (this.advised.exposeProxy) {
			// Make invocation available if necessary.
			oldProxy = AopContext.setCurrentProxy(proxy);
			setProxyContext = true;
		}

		// Get as late as possible to minimize the time we "own" the target,
		// in case it comes from a pool.
		target = targetSource.getTarget(); // 目標(biāo)對象
		Class<?> targetClass = (target != null ? target.getClass() : null); // 目標(biāo)對象的類型

		// Get the interception chain for this method.
		// 這里會對方法進(jìn)行匹配，因?yàn)椴皇悄繕?biāo)對象中的所有方法都需要增強(qiáng)
		List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass);

		// Check whether we have any advice. If we don't, we can fallback on direct
		// reflective invocation of the target, and avoid creating a MethodInvocation.
		if (chain.isEmpty()) {
			// We can skip creating a MethodInvocation: just invoke the target directly
			// Note that the final invoker must be an InvokerInterceptor so we know it does
			// nothing but a reflective operation on the target, and no hot swapping or fancy proxying.
			// 沒有匹配的切面，直接通過反射調(diào)用目標(biāo)對象的目標(biāo)方法
			Object[] argsToUse = AopProxyUtils.adaptArgumentsIfNecessary(method, args);
			retVal = AopUtils.invokeJoinpointUsingReflection(target, method, argsToUse);
		}
		else {
			// We need to create a method invocation...
			MethodInvocation invocation =
					new ReflectiveMethodInvocation(proxy, target, method, args, targetClass, chain);
			// Proceed to the joinpoint through the interceptor chain.
			/**
			 * @see ReflectiveMethodInvocation#proceed()
			 */
			// 這里才是增強(qiáng)的調(diào)用，重點(diǎn)，火炬的傳遞
			retVal = invocation.proceed();
		}

		// Massage return value if necessary.
		Class<?> returnType = method.getReturnType();
		if (retVal != null && retVal == target &&
				returnType != Object.class && returnType.isInstance(proxy) &&
				!RawTargetAccess.class.isAssignableFrom(method.getDeclaringClass())) {
			// Special case: it returned "this" and the return type of the method
			// is type-compatible. Note that we can't help if the target sets
			// a reference to itself in another returned object.
			retVal = proxy;
		}
		else if (retVal == null && returnType != Void.TYPE && returnType.isPrimitive()) {
			throw new AopInvocationException(
					"Null return value from advice does not match primitive return type for: " + method);
		}
		return retVal;
	}
	finally {
		if (target != null && !targetSource.isStatic()) {
			// Must have come from TargetSource.
			targetSource.releaseTarget(target);
		}
		if (setProxyContext) {
			// Restore old proxy.
			AopContext.setCurrentProxy(oldProxy);
		}
	}
}

public Object proceed() throws Throwable {
	// We start with an index of -1 and increment early.
	if (this.currentInterceptorIndex == this.interceptorsAndDynamicMethodMatchers.size() - 1) {
		// 執(zhí)行到最后一個Advice，才會到這里執(zhí)行目標(biāo)方法
		return invokeJoinpoint();
	}

	Object interceptorOrInterceptionAdvice =
			this.interceptorsAndDynamicMethodMatchers.get(++this.currentInterceptorIndex);
	if (interceptorOrInterceptionAdvice instanceof InterceptorAndDynamicMethodMatcher) {
		// Evaluate dynamic method matcher here: static part will already have
		// been evaluated and found to match.
		// dm.isRuntime()=true的走這
		InterceptorAndDynamicMethodMatcher dm =
				(InterceptorAndDynamicMethodMatcher) interceptorOrInterceptionAdvice;
		Class<?> targetClass = (this.targetClass != null ? this.targetClass : this.method.getDeclaringClass());
		if (dm.methodMatcher.matches(this.method, targetClass, this.arguments)) {
			return dm.interceptor.invoke(this);
		}
		else {
			// Dynamic matching failed.
			// Skip this interceptor and invoke the next in the chain.
			return proceed();
		}
	}
	else {
		// It's an interceptor, so we just invoke it: The pointcut will have
		// been evaluated statically before this object was constructed.
		// 走這
		return ((MethodInterceptor) interceptorOrInterceptionAdvice).invoke(this);
	}
}

private static final ThreadLocal<MethodInvocation> invocation =
 new NamedThreadLocal<>("Current AOP method invocation");

public Object invoke(MethodInvocation mi) throws Throwable {
	MethodInvocation oldInvocation = invocation.get();
	invocation.set(mi);
	try {
		return mi.proceed();
	}
	finally {
		invocation.set(oldInvocation);
	}
}

public Object invoke(MethodInvocation mi) throws Throwable {
	if (!(mi instanceof ProxyMethodInvocation)) {
		throw new IllegalStateException("MethodInvocation is not a Spring ProxyMethodInvocation: " + mi);
	}
	ProxyMethodInvocation pmi = (ProxyMethodInvocation) mi;
	ProceedingJoinPoint pjp = lazyGetProceedingJoinPoint(pmi);
	JoinPointMatch jpm = getJoinPointMatch(pmi);
	return invokeAdviceMethod(pjp, jpm, null, null);
}

public Object invoke(MethodInvocation mi) throws Throwable {
	this.advice.before(mi.getMethod(), mi.getArguments(), mi.getThis());
	return mi.proceed();
}

public Object invoke(MethodInvocation mi) throws Throwable {
	try {
		return mi.proceed();
	}
	finally {
		invokeAdviceMethod(getJoinPointMatch(), null, null);
	}
}

public Object invoke(MethodInvocation mi) throws Throwable {
	Object retVal = mi.proceed();
	this.advice.afterReturning(retVal, mi.getMethod(), mi.getArguments(), mi.getThis());
	return retVal;
}

public Object invoke(MethodInvocation mi) throws Throwable {
	try {
		return mi.proceed();
	}
	catch (Throwable ex) {
		if (shouldInvokeOnThrowing(ex)) {
			invokeAdviceMethod(getJoinPointMatch(), null, ex);
		}
		throw ex;
	}
}

Around begin // 環(huán)繞通知開始
Before // 前置通知
UserServiceImpl
 // 目標(biāo)方法的執(zhí)行
AfterReturning
 // 返回后通知
After
 // 后置通知
Around end // 環(huán)繞通知結(jié)束