Spring框架系列(9) - Spring AOP實現原理詳解之AOP切面的實現

pdai發表於2022-06-30

前文，我們分析了Spring IOC的初始化過程和Bean的生命週期等，而Spring AOP也是基於IOC的Bean載入來實現的。本文主要介紹Spring AOP原理解析的切面實現過程（將切面類的所有切面方法根據使用的註解生成對應Advice，並將Advice連同切入點匹配器和切面類等資訊一併封裝到Advisor，為後續交給代理增強實現做準備的過程）。@pdai

Spring框架系列(9) - Spring AOP實現原理詳解之AOP切面的實現

引入

我們應該從哪裡開始著手看Spring AOP的原始碼呢？和我們上文分析的IOC原始碼實現有什麼關係呢？

前文中我們寫了AOP的Demo，根據其XML配置我們不難發現AOP是基於IOC的Bean載入來實現的；這便使我們的主要入口

所以理解Spring AOP的初始化必須要先理解Spring IOC的初始化。

然後我們就能找到如下初始化的流程和aop對應的handler類

即parseCustomElement方法找到parse aop:aspectj-autoproxy的handler(org.springframework.aop.config.AopNamespaceHandler)

（PS：其實你會發現，最重要的是知識點的關聯關係，而不是知識點本身，就後續程式碼而言不就是打個斷點慢慢看的事了麼。）

aop配置標籤的解析

上文中，我們找到了AopNamespaceHandler，其實就是註冊BeanDefinition的解析器BeanDefinitionParser，將aop:xxxxxx配置標籤交給指定的parser來處理。

public class AopNamespaceHandler extends NamespaceHandlerSupport {

	/**
	 * Register the {@link BeanDefinitionParser BeanDefinitionParsers} for the
	 * '{@code config}', '{@code spring-configured}', '{@code aspectj-autoproxy}'
	 * and '{@code scoped-proxy}' tags.
	 */
	@Override
	public void init() {
		// In 2.0 XSD as well as in 2.5+ XSDs
        // 註冊解析<aop:config> 配置
		registerBeanDefinitionParser("config", new ConfigBeanDefinitionParser());
        // 註冊解析<aop:aspectj-autoproxy> 配置
		registerBeanDefinitionParser("aspectj-autoproxy", new AspectJAutoProxyBeanDefinitionParser());
		registerBeanDefinitionDecorator("scoped-proxy", new ScopedProxyBeanDefinitionDecorator());

		// Only in 2.0 XSD: moved to context namespace in 2.5+
		registerBeanDefinitionParser("spring-configured", new SpringConfiguredBeanDefinitionParser());
	}

}

config配置標籤的解析

<aop:config/>由ConfigBeanDefinitionParser這個類處理，作為parser類最重要的就是parse方法

@Override
@Nullable
public BeanDefinition parse(Element element, ParserContext parserContext) {
    CompositeComponentDefinition compositeDef =
            new CompositeComponentDefinition(element.getTagName(), parserContext.extractSource(element));
    parserContext.pushContainingComponent(compositeDef);

    configureAutoProxyCreator(parserContext, element);

    List<Element> childElts = DomUtils.getChildElements(element);
    for (Element elt: childElts) {
        String localName = parserContext.getDelegate().getLocalName(elt);
        if (POINTCUT.equals(localName)) {
            parsePointcut(elt, parserContext);
        }
        else if (ADVISOR.equals(localName)) {
            parseAdvisor(elt, parserContext);
        }
        else if (ASPECT.equals(localName)) {
            parseAspect(elt, parserContext);
        }
    }

    parserContext.popAndRegisterContainingComponent();
    return null;
}

打個斷點看下

parseAspect的方法如下, 處理方法不難，我這裡就不展開了

private void parseAspect(Element aspectElement, ParserContext parserContext) {
    String aspectId = aspectElement.getAttribute(ID);
    String aspectName = aspectElement.getAttribute(REF);

    try {
        this.parseState.push(new AspectEntry(aspectId, aspectName));
        List<BeanDefinition> beanDefinitions = new ArrayList<>();
        List<BeanReference> beanReferences = new ArrayList<>();

        List<Element> declareParents = DomUtils.getChildElementsByTagName(aspectElement, DECLARE_PARENTS);
        for (int i = METHOD_INDEX; i < declareParents.size(); i++) {
            Element declareParentsElement = declareParents.get(i);
            beanDefinitions.add(parseDeclareParents(declareParentsElement, parserContext));
        }

        // We have to parse "advice" and all the advice kinds in one loop, to get the
        // ordering semantics right.
        NodeList nodeList = aspectElement.getChildNodes();
        boolean adviceFoundAlready = false;
        for (int i = 0; i < nodeList.getLength(); i++) {
            Node node = nodeList.item(i);
            if (isAdviceNode(node, parserContext)) {
                if (!adviceFoundAlready) {
                    adviceFoundAlready = true;
                    if (!StringUtils.hasText(aspectName)) {
                        parserContext.getReaderContext().error(
                                "<aspect> tag needs aspect bean reference via 'ref' attribute when declaring advices.",
                                aspectElement, this.parseState.snapshot());
                        return;
                    }
                    beanReferences.add(new RuntimeBeanReference(aspectName));
                }
                AbstractBeanDefinition advisorDefinition = parseAdvice(
                        aspectName, i, aspectElement, (Element) node, parserContext, beanDefinitions, beanReferences);
                beanDefinitions.add(advisorDefinition);
            }
        }

        AspectComponentDefinition aspectComponentDefinition = createAspectComponentDefinition(
                aspectElement, aspectId, beanDefinitions, beanReferences, parserContext);
        parserContext.pushContainingComponent(aspectComponentDefinition);

        List<Element> pointcuts = DomUtils.getChildElementsByTagName(aspectElement, POINTCUT);
        for (Element pointcutElement : pointcuts) {
            parsePointcut(pointcutElement, parserContext);
        }

        parserContext.popAndRegisterContainingComponent();
    }
    finally {
        this.parseState.pop();
    }
}

aspectj-autoproxy配置標籤的解析

<aop:aspectj-autoproxy/>則由AspectJAutoProxyBeanDefinitionParser這個類處理的，我們看下parse 方法

@Override
@Nullable
public BeanDefinition parse(Element element, ParserContext parserContext) {
    // 註冊AspectJAnnotationAutoProxyCreator
    AopNamespaceUtils.registerAspectJAnnotationAutoProxyCreatorIfNecessary(parserContext, element);
    // 擴充BeanDefinition
    extendBeanDefinition(element, parserContext);
    return null;
}

AopNamespaceUtils.registerAspectJAnnotationAutoProxyCreatorIfNecessary方法對應如下

public static void registerAspectJAnnotationAutoProxyCreatorIfNecessary(
        ParserContext parserContext, Element sourceElement) {

    BeanDefinition beanDefinition = AopConfigUtils.registerAspectJAnnotationAutoProxyCreatorIfNecessary(
            parserContext.getRegistry(), parserContext.extractSource(sourceElement));
    useClassProxyingIfNecessary(parserContext.getRegistry(), sourceElement);
    registerComponentIfNecessary(beanDefinition, parserContext);
}

AopConfigUtils.registerAspectJAnnotationAutoProxyCreatorIfNecessary對應如下

@Nullable
public static BeanDefinition registerAspectJAnnotationAutoProxyCreatorIfNecessary(
        BeanDefinitionRegistry registry, @Nullable Object source) {

    return registerOrEscalateApcAsRequired(AnnotationAwareAspectJAutoProxyCreator.class, registry, source);
}

到這裡，我們發現AOP的建立工作是交給AnnotationAwareAspectJAutoProxyCreator來完成的。

註解切面代理建立類(AnnotationAwareAspectJAutoProxyCreator)

AnnotationAwareAspectJAutoProxyCreator是如何工作的呢？這時候我們就要看AnnotationAwareAspectJAutoProxyCreator類結構關係了。

如下是類結構關係

它實現了兩類介面：

BeanFactoryAware屬於Bean級生命週期介面方法
InstantiationAwareBeanPostProcessor 和 BeanPostProcessor 這兩個介面實現，一般稱它們的實現類為“後處理器”，是容器級生命週期介面方法；

結合前文Spring Bean生命週期的流程

我們就可以定位到核心的初始化方法肯定在postProcessBeforeInstantiation和postProcessAfterInitialization中。

postProcessBeforeInstantiation

如下是上述類結構中postProcessBeforeInstantiation的方法，讀者在自己看程式碼的時候建議打個斷點看，可以方便理解

@Override
public Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) {
    Object cacheKey = getCacheKey(beanClass, beanName);

    if (!StringUtils.hasLength(beanName) || !this.targetSourcedBeans.contains(beanName)) {
        // 如果已經在快取中，則忽略
        if (this.advisedBeans.containsKey(cacheKey)) {
            return null;
        }
        // 是否是aop基礎類？是否跳過？
        if (isInfrastructureClass(beanClass) || shouldSkip(beanClass, beanName)) {
            this.advisedBeans.put(cacheKey, Boolean.FALSE);
            return null;
        }
    }

    // Create proxy here if we have a custom TargetSource.
    // Suppresses unnecessary default instantiation of the target bean:
    // The TargetSource will handle target instances in a custom fashion.
    TargetSource targetSource = getCustomTargetSource(beanClass, beanName);
    if (targetSource != null) {
        if (StringUtils.hasLength(beanName)) {
            this.targetSourcedBeans.add(beanName);
        }
        Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(beanClass, beanName, targetSource);
        Object proxy = createProxy(beanClass, beanName, specificInterceptors, targetSource);
        this.proxyTypes.put(cacheKey, proxy.getClass());
        return proxy;
    }

    return null;
}

判斷是否是aop基礎類

是否是aop基礎類的判斷方法 isInfrastructureClass 如下

@Override
protected boolean isInfrastructureClass(Class<?> beanClass) {
    // Previously we setProxyTargetClass(true) in the constructor, but that has too
    // broad an impact. Instead we now override isInfrastructureClass to avoid proxying
    // aspects. I'm not entirely happy with that as there is no good reason not
    // to advise aspects, except that it causes advice invocation to go through a
    // proxy, and if the aspect implements e.g the Ordered interface it will be
    // proxied by that interface and fail at runtime as the advice method is not
    // defined on the interface. We could potentially relax the restriction about
    // not advising aspects in the future.
    // 父類判斷它是aop基礎類 or 使用@Aspect註解
    return (super.isInfrastructureClass(beanClass) ||
            (this.aspectJAdvisorFactory != null && this.aspectJAdvisorFactory.isAspect(beanClass)));
}

父類判斷它是否是aop基礎類的方法 super.isInfrastructureClass(beanClass), 本質上就是判斷該類是否實現了Advice, Pointcut, Advisor或者AopInfrastructureBean介面。

protected boolean isInfrastructureClass(Class<?> beanClass) {
    // 該類是否實現了Advice, Pointcut, Advisor或者AopInfrastructureBean介面
    boolean retVal = Advice.class.isAssignableFrom(beanClass) ||
            Pointcut.class.isAssignableFrom(beanClass) ||
            Advisor.class.isAssignableFrom(beanClass) ||
            AopInfrastructureBean.class.isAssignableFrom(beanClass);
    if (retVal && logger.isTraceEnabled()) {
        logger.trace("Did not attempt to auto-proxy infrastructure class [" + beanClass.getName() + "]");
    }
    return retVal;
}

是否應該跳過shouldSkip

通過斷點輔助，candidateAdvisors是就是xml配置的通知是對應的

@Override
protected boolean shouldSkip(Class<?> beanClass, String beanName) {
    // TODO: Consider optimization by caching the list of the aspect names
    List<Advisor> candidateAdvisors = findCandidateAdvisors();
    for (Advisor advisor : candidateAdvisors) {
        if (advisor instanceof AspectJPointcutAdvisor &&
                ((AspectJPointcutAdvisor) advisor).getAspectName().equals(beanName)) {
            return true;
        }
    }
    return super.shouldSkip(beanClass, beanName);
}

切面方法轉成Advisor

findCandidateAdvisors方法如下：

@Override
protected List<Advisor> findCandidateAdvisors() {
    // 在父類中找到所有的advisor：基於xml配置的<aop:before/>生成的
    List<Advisor> advisors = super.findCandidateAdvisors();
    // 為bean Factory中AspectJ切面構建advistor：通過AspectJ註解的方式生成Advisor類
    if (this.aspectJAdvisorsBuilder != null) {
        advisors.addAll(this.aspectJAdvisorsBuilder.buildAspectJAdvisors());
    }
    return advisors;
}

在當前的bean Factory中通過AspectJ註解的方式生成Advisor類，buildAspectJAdvisors方法如下

/**
    * Look for AspectJ-annotated aspect beans in the current bean factory,
    * and return to a list of Spring AOP Advisors representing them.
    * <p>Creates a Spring Advisor for each AspectJ advice method.
    * @return the list of {@link org.springframework.aop.Advisor} beans
    * @see #isEligibleBean
    */
public List<Advisor> buildAspectJAdvisors() {
    List<String> aspectNames = this.aspectBeanNames;

    if (aspectNames == null) {
        synchronized (this) {
            aspectNames = this.aspectBeanNames;
            if (aspectNames == null) {
                List<Advisor> advisors = new ArrayList<>();
                aspectNames = new ArrayList<>();
                String[] beanNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(
                        this.beanFactory, Object.class, true, false);
                for (String beanName : beanNames) {
                    if (!isEligibleBean(beanName)) {
                        continue;
                    }
                    // We must be careful not to instantiate beans eagerly as in this case they
                    // would be cached by the Spring container but would not have been weaved.
                    Class<?> beanType = this.beanFactory.getType(beanName, false);
                    if (beanType == null) {
                        continue;
                    }
                    if (this.advisorFactory.isAspect(beanType)) {
                        aspectNames.add(beanName);
                        AspectMetadata amd = new AspectMetadata(beanType, beanName);
                        if (amd.getAjType().getPerClause().getKind() == PerClauseKind.SINGLETON) {
                            MetadataAwareAspectInstanceFactory factory =
                                    new BeanFactoryAspectInstanceFactory(this.beanFactory, beanName);
                            List<Advisor> classAdvisors = this.advisorFactory.getAdvisors(factory);
                            // 單例加到advisorsCache, 非單例加到aspectFactoryCache
                            if (this.beanFactory.isSingleton(beanName)) {
                                this.advisorsCache.put(beanName, classAdvisors);
                            }
                            else {
                                this.aspectFactoryCache.put(beanName, factory);
                            }
                            advisors.addAll(classAdvisors);
                        }
                        else {
                            // Per target or per this.
                            if (this.beanFactory.isSingleton(beanName)) {
                                throw new IllegalArgumentException("Bean with name '" + beanName +
                                        "' is a singleton, but aspect instantiation model is not singleton");
                            }
                            MetadataAwareAspectInstanceFactory factory =
                                    new PrototypeAspectInstanceFactory(this.beanFactory, beanName);
                            this.aspectFactoryCache.put(beanName, factory);
                            // advisorFactory工廠獲取advisors
                            advisors.addAll(this.advisorFactory.getAdvisors(factory));
                        }
                    }
                }
                this.aspectBeanNames = aspectNames;
                return advisors;
            }
        }
    }

    if (aspectNames.isEmpty()) {
        return Collections.emptyList();
    }
    List<Advisor> advisors = new ArrayList<>();
    for (String aspectName : aspectNames) {
        List<Advisor> cachedAdvisors = this.advisorsCache.get(aspectName);
        if (cachedAdvisors != null) {
            advisors.addAll(cachedAdvisors);
        }
        else {
            MetadataAwareAspectInstanceFactory factory = this.aspectFactoryCache.get(aspectName);
            advisors.addAll(this.advisorFactory.getAdvisors(factory));
        }
    }
    return advisors;
}

上述方法本質上的思路是：用DCL雙重鎖的單例實現方式，拿到切面類裡的切面方法，將其轉換成advisor（並放入快取中）。

轉換的成advisor的方法是：this.advisorFactory.getAdvisors

@Override
public List<Advisor> getAdvisors(MetadataAwareAspectInstanceFactory aspectInstanceFactory) {
    Class<?> aspectClass = aspectInstanceFactory.getAspectMetadata().getAspectClass();
    String aspectName = aspectInstanceFactory.getAspectMetadata().getAspectName();
    validate(aspectClass);

    // We need to wrap the MetadataAwareAspectInstanceFactory with a decorator
    // so that it will only instantiate once.
    MetadataAwareAspectInstanceFactory lazySingletonAspectInstanceFactory =
            new LazySingletonAspectInstanceFactoryDecorator(aspectInstanceFactory);

    List<Advisor> advisors = new ArrayList<>();
    for (Method method : getAdvisorMethods(aspectClass)) {
        // Prior to Spring Framework 5.2.7, advisors.size() was supplied as the declarationOrderInAspect
        // to getAdvisor(...) to represent the "current position" in the declared methods list.
        // However, since Java 7 the "current position" is not valid since the JDK no longer
        // returns declared methods in the order in which they are declared in the source code.
        // Thus, we now hard code the declarationOrderInAspect to 0 for all advice methods
        // discovered via reflection in order to support reliable advice ordering across JVM launches.
        // Specifically, a value of 0 aligns with the default value used in
        // AspectJPrecedenceComparator.getAspectDeclarationOrder(Advisor).
        Advisor advisor = getAdvisor(method, lazySingletonAspectInstanceFactory, 0, aspectName);
        if (advisor != null) {
            advisors.add(advisor);
        }
    }

    // If it's a per target aspect, emit the dummy instantiating aspect.
    if (!advisors.isEmpty() && lazySingletonAspectInstanceFactory.getAspectMetadata().isLazilyInstantiated()) {
        Advisor instantiationAdvisor = new SyntheticInstantiationAdvisor(lazySingletonAspectInstanceFactory);
        advisors.add(0, instantiationAdvisor);
    }

    // Find introduction fields.
    for (Field field : aspectClass.getDeclaredFields()) {
        Advisor advisor = getDeclareParentsAdvisor(field);
        if (advisor != null) {
            advisors.add(advisor);
        }
    }

    return advisors;
}

getAdvisor方法如下

@Override
@Nullable
public Advisor getAdvisor(Method candidateAdviceMethod, MetadataAwareAspectInstanceFactory aspectInstanceFactory,
        int declarationOrderInAspect, String aspectName) {

    validate(aspectInstanceFactory.getAspectMetadata().getAspectClass());

    AspectJExpressionPointcut expressionPointcut = getPointcut(
            candidateAdviceMethod, aspectInstanceFactory.getAspectMetadata().getAspectClass());
    if (expressionPointcut == null) {
        return null;
    }

    // 封裝成advisor
    return new InstantiationModelAwarePointcutAdvisorImpl(expressionPointcut, candidateAdviceMethod,
            this, aspectInstanceFactory, declarationOrderInAspect, aspectName);
}

獲取表示式的切點

獲取表示式的切點的方法getPointcut如下：

@Nullable
private AspectJExpressionPointcut getPointcut(Method candidateAdviceMethod, Class<?> candidateAspectClass) {
    AspectJAnnotation<?> aspectJAnnotation =
            AbstractAspectJAdvisorFactory.findAspectJAnnotationOnMethod(candidateAdviceMethod);
    if (aspectJAnnotation == null) {
        return null;
    }

    AspectJExpressionPointcut ajexp =
            new AspectJExpressionPointcut(candidateAspectClass, new String[0], new Class<?>[0]);
    ajexp.setExpression(aspectJAnnotation.getPointcutExpression());
    if (this.beanFactory != null) {
        ajexp.setBeanFactory(this.beanFactory);
    }
    return ajexp;
}

AbstractAspectJAdvisorFactory.findAspectJAnnotationOnMethod的方法如下

private static final Class<?>[] ASPECTJ_ANNOTATION_CLASSES = new Class<?>[] {
        Pointcut.class, Around.class, Before.class, After.class, AfterReturning.class, AfterThrowing.class};

/**
    * Find and return the first AspectJ annotation on the given method
    * (there <i>should</i> only be one anyway...).
    */
@SuppressWarnings("unchecked")
@Nullable
protected static AspectJAnnotation<?> findAspectJAnnotationOnMethod(Method method) {
    for (Class<?> clazz : ASPECTJ_ANNOTATION_CLASSES) {
        AspectJAnnotation<?> foundAnnotation = findAnnotation(method, (Class<Annotation>) clazz);
        if (foundAnnotation != null) {
            return foundAnnotation;
        }
    }
    return null;
}

findAnnotation方法如下

@Nullable
private static <A extends Annotation> AspectJAnnotation<A> findAnnotation(Method method, Class<A> toLookFor) {
    A result = AnnotationUtils.findAnnotation(method, toLookFor);
    if (result != null) {
        return new AspectJAnnotation<>(result);
    }
    else {
        return null;
    }
}

AnnotationUtils.findAnnotation 獲取註解方法如下

/**
    * Find a single {@link Annotation} of {@code annotationType} on the supplied
    * {@link Method}, traversing its super methods (i.e. from superclasses and
    * interfaces) if the annotation is not <em>directly present</em> on the given
    * method itself.
    * <p>Correctly handles bridge {@link Method Methods} generated by the compiler.
    * <p>Meta-annotations will be searched if the annotation is not
    * <em>directly present</em> on the method.
    * <p>Annotations on methods are not inherited by default, so we need to handle
    * this explicitly.
    * @param method the method to look for annotations on
    * @param annotationType the annotation type to look for
    * @return the first matching annotation, or {@code null} if not found
    * @see #getAnnotation(Method, Class)
    */
@Nullable
public static <A extends Annotation> A findAnnotation(Method method, @Nullable Class<A> annotationType) {
    if (annotationType == null) {
        return null;
    }

    // Shortcut: directly present on the element, with no merging needed?
    if (AnnotationFilter.PLAIN.matches(annotationType) ||
            AnnotationsScanner.hasPlainJavaAnnotationsOnly(method)) {
        return method.getDeclaredAnnotation(annotationType);
    }

    // Exhaustive retrieval of merged annotations...
    return MergedAnnotations.from(method, SearchStrategy.TYPE_HIERARCHY, RepeatableContainers.none())
            .get(annotationType).withNonMergedAttributes()
            .synthesize(MergedAnnotation::isPresent).orElse(null);
}

封裝成Advisor

注：Advisor 是 advice的包裝器，包含了advice及其它資訊

由InstantiationModelAwarePointcutAdvisorImpl構造完成

public InstantiationModelAwarePointcutAdvisorImpl(AspectJExpressionPointcut declaredPointcut,
        Method aspectJAdviceMethod, AspectJAdvisorFactory aspectJAdvisorFactory,
        MetadataAwareAspectInstanceFactory aspectInstanceFactory, int declarationOrder, String aspectName) {

    this.declaredPointcut = declaredPointcut;
    this.declaringClass = aspectJAdviceMethod.getDeclaringClass();
    this.methodName = aspectJAdviceMethod.getName();
    this.parameterTypes = aspectJAdviceMethod.getParameterTypes();
    this.aspectJAdviceMethod = aspectJAdviceMethod;
    this.aspectJAdvisorFactory = aspectJAdvisorFactory;
    this.aspectInstanceFactory = aspectInstanceFactory;
    this.declarationOrder = declarationOrder;
    this.aspectName = aspectName;

    if (aspectInstanceFactory.getAspectMetadata().isLazilyInstantiated()) {
        // Static part of the pointcut is a lazy type.
        Pointcut preInstantiationPointcut = Pointcuts.union(
                aspectInstanceFactory.getAspectMetadata().getPerClausePointcut(), this.declaredPointcut);

        // Make it dynamic: must mutate from pre-instantiation to post-instantiation state.
        // If it's not a dynamic pointcut, it may be optimized out
        // by the Spring AOP infrastructure after the first evaluation.
        this.pointcut = new PerTargetInstantiationModelPointcut(
                this.declaredPointcut, preInstantiationPointcut, aspectInstanceFactory);
        this.lazy = true;
    }
    else {
        // A singleton aspect.
        this.pointcut = this.declaredPointcut;
        this.lazy = false;
        this.instantiatedAdvice = instantiateAdvice(this.declaredPointcut);
    }
}

通過pointcut獲取advice

private Advice instantiateAdvice(AspectJExpressionPointcut pointcut) {
    Advice advice = this.aspectJAdvisorFactory.getAdvice(this.aspectJAdviceMethod, pointcut,
            this.aspectInstanceFactory, this.declarationOrder, this.aspectName);
    return (advice != null ? advice : EMPTY_ADVICE);
}

交給aspectJAdvisorFactory獲取

@Override
@Nullable
public Advice getAdvice(Method candidateAdviceMethod, AspectJExpressionPointcut expressionPointcut,
        MetadataAwareAspectInstanceFactory aspectInstanceFactory, int declarationOrder, String aspectName) {

    // 獲取切面類
    Class<?> candidateAspectClass = aspectInstanceFactory.getAspectMetadata().getAspectClass();
    validate(candidateAspectClass);

    // 獲取切面註解
    AspectJAnnotation<?> aspectJAnnotation =
            AbstractAspectJAdvisorFactory.findAspectJAnnotationOnMethod(candidateAdviceMethod);
    if (aspectJAnnotation == null) {
        return null;
    }

    // If we get here, we know we have an AspectJ method.
    // Check that it's an AspectJ-annotated class
    if (!isAspect(candidateAspectClass)) {
        throw new AopConfigException("Advice must be declared inside an aspect type: " +
                "Offending method '" + candidateAdviceMethod + "' in class [" +
                candidateAspectClass.getName() + "]");
    }

    if (logger.isDebugEnabled()) {
        logger.debug("Found AspectJ method: " + candidateAdviceMethod);
    }

    // 切面註解轉換成advice
    AbstractAspectJAdvice springAdvice;

    switch (aspectJAnnotation.getAnnotationType()) {
        case AtPointcut: // AtPointcut忽略
            if (logger.isDebugEnabled()) {
                logger.debug("Processing pointcut '" + candidateAdviceMethod.getName() + "'");
            }
            return null;
        case AtAround:
            springAdvice = new AspectJAroundAdvice(
                    candidateAdviceMethod, expressionPointcut, aspectInstanceFactory);
            break;
        case AtBefore:
            springAdvice = new AspectJMethodBeforeAdvice(
                    candidateAdviceMethod, expressionPointcut, aspectInstanceFactory);
            break;
        case AtAfter:
            springAdvice = new AspectJAfterAdvice(
                    candidateAdviceMethod, expressionPointcut, aspectInstanceFactory);
            break;
        case AtAfterReturning:
            springAdvice = new AspectJAfterReturningAdvice(
                    candidateAdviceMethod, expressionPointcut, aspectInstanceFactory);
            AfterReturning afterReturningAnnotation = (AfterReturning) aspectJAnnotation.getAnnotation();
            if (StringUtils.hasText(afterReturningAnnotation.returning())) {
                springAdvice.setReturningName(afterReturningAnnotation.returning());
            }
            break;
        case AtAfterThrowing:
            springAdvice = new AspectJAfterThrowingAdvice(
                    candidateAdviceMethod, expressionPointcut, aspectInstanceFactory);
            AfterThrowing afterThrowingAnnotation = (AfterThrowing) aspectJAnnotation.getAnnotation();
            if (StringUtils.hasText(afterThrowingAnnotation.throwing())) {
                springAdvice.setThrowingName(afterThrowingAnnotation.throwing());
            }
            break;
        default:
            throw new UnsupportedOperationException(
                    "Unsupported advice type on method: " + candidateAdviceMethod);
    }

    // 最後將其它切面資訊配置到advice
    springAdvice.setAspectName(aspectName);
    springAdvice.setDeclarationOrder(declarationOrder);
    String[] argNames = this.parameterNameDiscoverer.getParameterNames(candidateAdviceMethod);
    if (argNames != null) {
        springAdvice.setArgumentNamesFromStringArray(argNames);
    }
    springAdvice.calculateArgumentBindings();

    return springAdvice;
}

小結

回頭看，主要是處理使用了@Aspect註解的切面類，然後將切面類的所有切面方法根據使用的註解生成對應Advice，並將Advice連同切入點匹配器和切面類等資訊一併封裝到Advisor的過程。

postProcessAfterInitialization

有了Adisor, 注入到合適的位置並交給代理（cglib和jdk)實現了。

/**
* Create a proxy with the configured interceptors if the bean is
* identified as one to proxy by the subclass.
* @see #getAdvicesAndAdvisorsForBean
*/
@Override
public Object postProcessAfterInitialization(@Nullable Object bean, String beanName) {
    if (bean != null) {
        Object cacheKey = getCacheKey(bean.getClass(), beanName);
        if (this.earlyProxyReferences.remove(cacheKey) != bean) {
            return wrapIfNecessary(bean, beanName, cacheKey);
        }
    }
    return bean;
}

後文中將分別介紹代理的建立和實現：

總結

通過上文的分析，我們做下小結：

由IOC Bean載入方法棧中找到parseCustomElement方法，找到parse aop:aspectj-autoproxy的handler(org.springframework.aop.config.AopNamespaceHandler)
AopNamespaceHandler註冊了<aop:aspectj-autoproxy/>的解析類是AspectJAutoProxyBeanDefinitionParser
AspectJAutoProxyBeanDefinitionParser的parse 方法通過AspectJAwareAdvisorAutoProxyCreator類去建立
AspectJAwareAdvisorAutoProxyCreator實現了兩類介面，BeanFactoryAware和BeanPostProcessor；根據Bean生命週期方法找到兩個核心方法：postProcessBeforeInstantiation和postProcessAfterInitialization
1. postProcessBeforeInstantiation：主要是處理使用了@Aspect註解的切面類，然後將切面類的所有切面方法根據使用的註解生成對應Advice，並將Advice連同切入點匹配器和切面類等資訊一併封裝到Advisor
2. postProcessAfterInitialization：主要負責將Advisor注入到合適的位置，建立代理（cglib或jdk)，為後面給代理進行增強實現做準備。