关于java:Spring框架系列9-Spring-AOP实现原理详解之AOP切面的实现

前文，咱们剖析了 Spring IOC 的初始化过程和 Bean 的生命周期等，而 Spring AOP 也是基于 IOC 的 Bean 加载来实现的。本文次要介绍 Spring AOP 原理解析的切面实现过程（将切面类的所有切面办法依据应用的注解生成对应 Advice，并将 Advice 连同切入点匹配器和切面类等信息一并封装到 Advisor，为后续交给代理加强实现做筹备的过程）。@pdai

• Spring 框架系列(9) – Spring AOP 实现原理详解之 AOP 切面的实现

  • 引入

  • aop 配置标签的解析

    • config 配置标签的解析
    • aspectj-autoproxy 配置标签的解析

  • 注解切面代理创立类(AnnotationAwareAspectJAutoProxyCreator)

    • postProcessBeforeInstantiation

      • 判断是否是 aop 根底类
      • 是否应该跳过 shouldSkip
      • 切面办法转成 Advisor
      • 获取表达式的切点
      • 封装成 Advisor
      • 小结
    • postProcessAfterInitialization
  • 总结
  • 更多文章

引入

咱们应该从哪里开始着手看 Spring AOP 的源码呢？和咱们上文剖析的 IOC 源码实现有什么关系呢？

1. 前文中咱们写了 AOP 的 Demo，依据其 XML 配置咱们不难发现AOP 是基于 IOC 的 Bean 加载来实现的；这便使咱们的次要入口

所以了解 Spring AOP 的初始化必须要先了解 Spring IOC 的初始化。

2. 而后咱们就能找到如下 初始化的流程和 aop 对应的 handler类

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

（PS：其实你会发现，最重要的是知识点的关联关系，而不是知识点自身，就后续代码而言不就是打个断点缓缓看的事了么。）

aop 配置标签的解析

上文中，咱们找到了 AopNamespaceHandler，其实就是注册 BeanDefinition 的解析器 BeanDefinitionParser，将 aop:xxxxxx 配置标签交给指定的 parser 来解决。

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
        // 注册解析 <aop:config> 配置
        registerBeanDefinitionParser("config", new ConfigBeanDefinitionParser());
        // 注册解析 <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 配置标签的解析

<aop:config/>由 ConfigBeanDefinitionParser 这个类解决，作为 parser 类最重要的就是 parse 办法

@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;
}

打个断点看下

parseAspect 的办法如下, 解决办法不难，我这里就不开展了

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 配置标签的解析

<aop:aspectj-autoproxy/>则由 AspectJAutoProxyBeanDefinitionParser 这个类解决的，咱们看下 parse 办法

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

AopNamespaceUtils.registerAspectJAnnotationAutoProxyCreatorIfNecessary 办法对应如下

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 对应如下

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

到这里，咱们发现 AOP 的创立工作是交给 AnnotationAwareAspectJAutoProxyCreator 来实现的。

注解切面代理创立类(AnnotationAwareAspectJAutoProxyCreator)

AnnotationAwareAspectJAutoProxyCreator 是如何工作的呢？这时候咱们就要看 AnnotationAwareAspectJAutoProxyCreator 类构造关系了。

如下是类构造关系

它实现了两类接口：

• BeanFactoryAware 属于Bean 级生命周期接口办法
• InstantiationAwareBeanPostProcessor 和 BeanPostProcessor 这两个接口实现，个别称它们的实现类为“后处理器”，是 容器级生命周期接口办法；

联合前文 Spring Bean 生命周期的流程

咱们就能够定位到外围的初始化办法必定在 postProcessBeforeInstantiation 和 postProcessAfterInitialization 中。

postProcessBeforeInstantiation

如下是上述类构造中 postProcessBeforeInstantiation 的办法，读者在本人看代码的时候倡议打个断点看，能够不便了解

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

    if (!StringUtils.hasLength(beanName) || !this.targetSourcedBeans.contains(beanName)) {
        // 如果曾经在缓存中，则疏忽
        if (this.advisedBeans.containsKey(cacheKey)) {return null;}
        // 是否是 aop 根底类？是否跳过？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;
}

判断是否是 aop 根底类

是否是 aop 根底类的判断办法 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.
    // 父类判断它是 aop 根底类 or 应用 @Aspect 注解
    return (super.isInfrastructureClass(beanClass) ||
            (this.aspectJAdvisorFactory != null && this.aspectJAdvisorFactory.isAspect(beanClass)));
}

父类判断它是否是 aop 根底类的办法 super.isInfrastructureClass(beanClass), 实质上就是判断该类是否实现了 Advice, Pointcut, Advisor 或者 AopInfrastructureBean 接口。

protected boolean isInfrastructureClass(Class<?> beanClass) {
    // 该类是否实现了 Advice, Pointcut, Advisor 或者 AopInfrastructureBean 接口
    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;
}

是否应该跳过 shouldSkip

通过断点辅助，candidateAdvisors 是就是 xml 配置的告诉是对应的

@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);
}

切面办法转成 Advisor

findCandidateAdvisors 办法如下：

@Override
protected List<Advisor> findCandidateAdvisors() {
    // 在父类中找到所有的 advisor：基于 xml 配置的 <aop:before/> 生成的
    List<Advisor> advisors = super.findCandidateAdvisors();
    // 为 bean Factory 中 AspectJ 切面构建 advistor：通过 AspectJ 注解的形式生成 Advisor 类
    if (this.aspectJAdvisorsBuilder != null) {advisors.addAll(this.aspectJAdvisorsBuilder.buildAspectJAdvisors());
    }
    return advisors;
}

在以后的 bean Factory 中通过 AspectJ 注解的形式生成 Advisor 类，buildAspectJAdvisors 办法如下

/**
    * 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);
                            // 单例加到 advisorsCache, 非单例加到 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 工厂获取 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;
}

上述办法实质上的思路是：用 DCL 双重锁的单例实现形式，拿到切面类里的切面办法，将其转换成 advisor（并放入缓存中）。

转换的成 advisor 的办法是：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 办法如下

@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;}

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

获取表达式的切点

获取表达式的切点的办法 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 的办法如下

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 办法如下

@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 获取注解办法如下

/**
    * 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);
}

封装成 Advisor

注：Advisor 是 advice 的包装器，蕴含了 advice 及其它信息

由 InstantiationModelAwarePointcutAdvisorImpl 结构实现

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);
    }
}

通过 pointcut 获取 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);
}

交给 aspectJAdvisorFactory 获取

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

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

    // 获取切面注解
    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);
    }

    // 切面注解转换成 advice
    AbstractAspectJAdvice springAdvice;

    switch (aspectJAnnotation.getAnnotationType()) {
        case AtPointcut: // AtPointcut 疏忽
            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);
    }

    // 最初将其它切面信息配置到 advice
    springAdvice.setAspectName(aspectName);
    springAdvice.setDeclarationOrder(declarationOrder);
    String[] argNames = this.parameterNameDiscoverer.getParameterNames(candidateAdviceMethod);
    if (argNames != null) {springAdvice.setArgumentNamesFromStringArray(argNames);
    }
    springAdvice.calculateArgumentBindings();

    return springAdvice;
}

小结

回头看，次要是解决应用了 @Aspect 注解的切面类，而后将切面类的所有切面办法依据应用的注解生成对应 Advice，并将 Advice 连同切入点匹配器和切面类等信息一并封装到 Advisor 的过程。

postProcessAfterInitialization

有了 Adisor, 注入到适合的地位并交给代理（cglib 和 jdk)实现了。

/**
* 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;
}

后文中将别离介绍代理的创立和实现：

• Spring 进阶 – Spring AOP 实现原理详解之 AOP 代理的创立
• Spring 进阶 – Spring AOP 实现原理详解之 Cglib 代理实现
• Spring 进阶 – Spring AOP 实现原理详解之 JDK 代理实现

总结

通过上文的剖析，咱们做下小结：

1. 由 IOC Bean 加载 办法栈中找到 parseCustomElement 办法，找到 parse aop:aspectj-autoproxy的 handler(org.springframework.aop.config.AopNamespaceHandler)
2. AopNamespaceHandler注册了 <aop:aspectj-autoproxy/> 的解析类是 AspectJAutoProxyBeanDefinitionParser
3. AspectJAutoProxyBeanDefinitionParser的 parse 办法 通过 AspectJAwareAdvisorAutoProxyCreator 类去创立

4. AspectJAwareAdvisorAutoProxyCreator实现了两类接口，BeanFactoryAware 和 BeanPostProcessor；依据 Bean 生命周期办法找到两个外围办法：postProcessBeforeInstantiation 和 postProcessAfterInitialization

   1. postProcessBeforeInstantiation：次要是解决应用了 @Aspect 注解的切面类，而后将切面类的所有切面办法依据应用的注解生成对应 Advice，并将 Advice 连同切入点匹配器和切面类等信息一并封装到 Advisor
   2. postProcessAfterInitialization：次要负责将 Advisor 注入到适合的地位，创立代理（cglib 或 jdk)，为前面给代理进行加强实现做筹备。

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