本文围绕 Spring AOP 原理解析的切面实现过程展开。先介绍 <aop> 配置标签解析，包括 <aop:config> 和 <aop:aspectj-autoproxy>。接着阐述注解切面代理创建类 AnnotationAwareAspectJAutoProxyCreator，分析其核心方法，最后总结了 AOP 从配置解析到代理创建的整体流程。
目录
3.1. AnnotationAwareAspectJAutoProxyCreator 类
3.2. postProcessBeforeInstantiation() 方法
3.3. postProcessAfterInitialization() 方法
一. 前言
在《（二）Spring 核心之控制反转（IoC）—— 体系结构设计及原理详解》中，我们分析了 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。本文主要介绍 Spring AOP 原理解析的切面实现过程（将切面类的所有切面方法根据使用的注解生成对应 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，为后续交给代理增强实现做准备的过程）。
在《（一）Spring 核心之面向切面编程（AOP）—— 配置及使用》中我们写了 AOP 的 Demo，根据其 XML 配置我们不难发现 AOP 是基于 IoC 的 Bean 加载来实现的。这便是我们的主要入口：
所以，理解 Spring AOP 的初始化必须要先理解 Spring IoC 的初始化。然后我们就能找到如下初始化的流程和 AOP 对应的 handler 类（即 parseCustomElement 方法找到 parse aop:aspectj-autoproxy 的 handler(org.springframework.aop.config.AopNamespaceHandler)）：
注：其实你会发现，最重要的是知识点的关联关系，而不是知识点本身。后续代码就是打个断点慢慢看了。
二. <aop> 配置标签的解析
上一节中，我们找到了 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 来处理。
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
// 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());
}
}2.1. <aop:config> 配置标签的解析
<aop:config /> 由 ConfigBeanDefinitionParser 这个类处理，作为 parser 类最重要的就是 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;
}打个断点看下：
parseAspect 的方法如下，处理逻辑不难，这里就不展开了：
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();
}
}2.2. <aop:aspectj-autoproxy> 配置标签的解析
<aop:aspectj-autoproxy /> 则由 AspectJAutoProxyBeanDefinitionParser 这个类处理的，我们看下 parse 方法：
@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 方法对应如下：
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，我们发现 AOP 的创建工作是交给 AnnotationAwareAspectJAutoProxyCreator 来完成的。
三. 注解切面代理创建类
3.1. AnnotationAwareAspectJAutoProxyCreator 类
注解切面代理创建类 AnnotationAwareAspectJAutoProxyCreator 类结构关系如下：
它实现了两类接口：
- BeanFactoryAware 属于 Bean 级生命周期接口方法；
- InstantiationAwareBeanPostProcessor 和 BeanPostProcessor 这两个接口实现，一般称它们的实现类为“后处理器”，是容器级生命周期接口方法。
结合 Spring Bean 生命周期的流程：《Spring 面试必考：Spring Bean 的生命周期和作用域》
我们就可以定位到核心的初始化方法肯定在 postProcessBeforeInstantiation 和postProcessAfterInitialization 中。
3.2. postProcessBeforeInstantiation() 方法
如下是上述类结构中 postProcessBeforeInstantiation() 的方法，读者在自己看代码的时候建议打个断点看，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)) {
// 如果已经在缓存中，则忽略
if (this.advisedBeans.containsKey(cacheKey)) {
return null;
}
// 是否是aop基础类？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;
}3.2.1. 判断是否是 AOP 基础类
是否是 AOP 基础类的判断方法 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.
// 父类判断它是aop基础类 or 使用@Aspect注解
return (super.isInfrastructureClass(beanClass) ||
(this.aspectJAdvisorFactory != null && this.aspectJAdvisorFactory.isAspect(beanClass)));
}父类判断它是否是 AOP 基础类的方法 super.isInfrastructureClass(beanClass)，本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;
}3.2.2. 是否应该跳过 shouldSkip
通过断点辅助，candidateAdvisors 就是和 xml 配置的通知是对应的：
@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);
}3.2.3. 切面方法转成 Advisor
findCandidateAdvisors() 方法如下：
@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注解的方式生成Advisor类
if (this.aspectJAdvisorsBuilder != null) {
advisors.addAll(this.aspectJAdvisorsBuilder.buildAspectJAdvisors());
}
return advisors;
}在当前的 Bean Factory 中通过 AspectJ 注解的方式生成 Advisor 类，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);
// 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;
}上述方法本质上的思路是：用 DCL（Double Check Lock） 双重锁的单例实现方式，拿到切面类里的切面方法，将其转换成 advisor 并放入缓存中。
转换的成 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;
}
// 封装成advisor
return new InstantiationModelAwarePointcutAdvisorImpl(expressionPointcut, candidateAdviceMethod,
this, aspectInstanceFactory, declarationOrderInAspect, aspectName);
}3.2.4. 获取表达式的切点
获取表达式的切点的方法 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() 获取注解方法如下：
/**
* 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);
}3.2.5. 封装成 Advisor
Advisor 是 advice的包装器，包含了 advice 及其它信息。
由 InstantiationModelAwarePointcutAdvisorImpl 构造完成：
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);
}
// 最后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;
}3.2.6. 小结
回头看，主要是处理使用了 @Aspect 注解的切面类，然后将切面类的所有切面方法根据使用的注解生成对应 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。
3.3. postProcessAfterInitialization() 方法
有了 Adisor，注入到合适的位置并交给代理（JDK 或 CGLIB ）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;
}后续文章将分别介绍代理的创建和实现：
《（三）Spring 核心之面向切面编程（AOP）—— 代理的创建》；
《Spring AOP 实现原理详解之 JDK 动态代理》；
《Spring AOP 实现原理详解之 CGLIB 动态代理》。
四. 总结
通过本文的分析，我们总结如下：
- 由 IoC Bean 加载方法栈中找到 parseCustomElement 方法，找到 parse <aop:aspectj-autoproxy> 的 handler（org.springframework.aop.config.AopNamespaceHandler）；
- AopNamespaceHandler 注册了 <aop:aspectj-autoproxy/> 的解析类是AspectJAutoProxyBeanDefinitionParser；
- AspectJAutoProxyBeanDefinitionParser 的 parse() 方法通过AspectJAwareAdvisorAutoProxyCreator 类去创建；
- AspectJAwareAdvisorAutoProxyCreator 实现了两类接口，BeanFactoryAware 和BeanPostProcessor。根据 Bean 生命周期方法找到两个核心方法 postProcessBeforeInstantiation() 和 postProcessAfterInitialization()
- postProcessBeforeInstantiation()：主要是处理使用了 @Aspect 注解的切面类，然后将切面类的所有切面方法根据使用的注解生成对应 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；
- postProcessAfterInitialization()：主要负责将 Advisor 注入到合适的位置，创建代理（JDK 或 CGLIB)，为后面给代理进行增强实现做准备。
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