这一次搞懂Spring代理创建及AOP链式调用过程操作

前言

AOP，也就是面向切面编程，它可以将公共的代码抽离出来，动态的织入到目标类、目标方法中，大大提高我们编程的效率，也使程序变得更加优雅。如事务、操作日志等都可以使用AOP实现。这种织入可以是在运行期动态生成代理对象实现，也可以在编译期、类加载时期静态织入到代码中。而Spring正是通过第一种方法实现，且在代理类的生成上也有两种方式：JDK Proxy和CGLIB，默认当类实现了接口时使用前者，否则使用后者；另外Spring AOP只能实现对方法的增强。

正文

基本概念

AOP的术语很多，虽然不清楚术语我们也能很熟练地使用AOP，但是要理解分析源码，术语就需要深刻体会其含义。

增强（Advice）：就是我们想要额外增加的功能

目标对象（Target）：就是我们想要增强的目标类，如果没有AOP，我们需要在每个目标对象中实现日志、事务管理等非业务逻辑

连接点（JoinPoint）：程序执行时的特定时机，如方法执行前、后以及抛出异常后等等。

切点（Pointcut）：连接点的导航，我们如何找到目标对象呢？切点的作用就在于此，在Spring中就是匹配表达式。

引介（Introduction）：引介是一种特殊的增强，它为类添加一些属性和方法。这样，即使一个业务类原本没有实现某个接口，通过AOP的引介功能，我们可以动态地为该业务类添加接口的实现逻辑，让业务类成为这个接口的实现类。

织入（Weaving）：即如何将增强添加到目标对象的连接点上，有动态（运行期生成代理）、静态（编译期、类加载时期）两种方式。

代理（Proxy）：目标对象被织入增强后，就会产生一个代理对象，该对象可能是和原对象实现了同样的一个接口（JDK），也可能是原对象的子类（CGLIB）。

切面（Aspect、Advisor）：切面由切点和增强组成，包含了这两者的定义。

代理对象的创建

在熟悉了AOP术语后，下面就来看看Spring是如何创建代理对象的，是否还记得上一篇提到的AOP的入口呢？在AbstractAutowireCapableBeanFactory类的applyBeanPostProcessorsAfterInitialization方法中循环调用了BeanPostProcessor的postProcessAfterInitialization方法，其中一个就是我们创建代理对象的入口。

这里是Bean实例化完成去创建代理对象，理所当然应该这样，但实际上在Bean实例化之前调用了一个resolveBeforeInstantiation方法，这里实际上我们也是有机会可以提前创建代理对象的，这里放到最后来分析，先来看主入口，进入到AbstractAutoProxyCreator类中：

 public Object postProcessAfterInitialization(@Nullable Object bean, String beanName) {
 if (bean != null) {
 Object cacheKey = getCacheKey(bean.getClass(), beanName);
 if (!this.earlyProxyReferences.contains(cacheKey)) {
 return wrapIfNecessary(bean, beanName, cacheKey);
 }
 }
 return bean;
 }

 protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {
 //创建当前bean的代理，如果这个bean有advice的话，重点看
 // Create proxy if we have advice.
 Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
 //如果有切面，则生成该bean的代理
 if (specificInterceptors != DO_NOT_PROXY) {
 this.advisedBeans.put(cacheKey, Boolean.TRUE);
 //把被代理对象bean实例封装到SingletonTargetSource对象中
 Object proxy = createProxy(
  bean.getClass(), beanName, specificInterceptors, new SingletonTargetSource(bean));
 this.proxyTypes.put(cacheKey, proxy.getClass());
 return proxy;
 }

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

先从缓存中拿，没有则调用wrapIfNecessary方法创建。在这个方法里面主要看两个地方：getAdvicesAndAdvisorsForBean和createProxy。简单一句话概括就是先扫描后创建，问题是扫描什么呢？你可以先结合上面的概念思考下，换你会怎么做。进入到子类AbstractAdvisorAutoProxyCreator的getAdvicesAndAdvisorsForBean方法中：

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

 //找到合格的切面
 List<Advisor> advisors = findEligibleAdvisors(beanClass, beanName);
 if (advisors.isEmpty()) {
 return DO_NOT_PROXY;
 }
 return advisors.toArray();
 }

 protected List<Advisor> findEligibleAdvisors(Class<?> beanClass, String beanName) {
 //找到候选的切面,其实就是一个寻找有@Aspectj注解的过程，把工程中所有有这个注解的类封装成Advisor返回
 List<Advisor> candidateAdvisors = findCandidateAdvisors();

 //判断候选的切面是否作用在当前beanClass上面，就是一个匹配过程。现在就是一个匹配
 List<Advisor> eligibleAdvisors = findAdvisorsThatCanApply(candidateAdvisors, beanClass, beanName);
 extendAdvisors(eligibleAdvisors);
 if (!eligibleAdvisors.isEmpty()) {
 //对有@Order@Priority进行排序
 eligibleAdvisors = sortAdvisors(eligibleAdvisors);
 }
 return eligibleAdvisors;
 }

在findEligibleAdvisors方法中可以看到有两个步骤，第一先找到所有的切面，即扫描所有带有@Aspect注解的类，并将其中的切点（表达式）和增强封装为切面，扫描完成后，自然是要判断哪些切面能够连接到当前Bean实例上。下面一步步来分析，首先是扫描过程，进入到AnnotationAwareAspectJAutoProxyCreator类中：

 protected List<Advisor> findCandidateAdvisors() {
 // 先通过父类AbstractAdvisorAutoProxyCreator扫描，这里不重要
 List<Advisor> advisors = super.findCandidateAdvisors();
 // 主要看这里
 if (this.aspectJAdvisorsBuilder != null) {
 advisors.addAll(this.aspectJAdvisorsBuilder.buildAspectJAdvisors());
 }
 return advisors;
 }

这里委托给了BeanFactoryAspectJAdvisorsBuilderAdapter类，并调用其父类的buildAspectJAdvisors方法创建切面对象：

 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<>();
  //获取spring容器中的所有bean的名称BeanName
  String[] beanNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(
  this.beanFactory, Object.class, true, false);
  for (String beanName : beanNames) {
  if (!isEligibleBean(beanName)) {
  continue;
  }
  Class<?> beanType = this.beanFactory.getType(beanName);
  if (beanType == null) {
  continue;
  }
  //判断类上是否有@Aspect注解
  if (this.advisorFactory.isAspect(beanType)) {
  aspectNames.add(beanName);
  AspectMetadata amd = new AspectMetadata(beanType, beanName);
  if (amd.getAjType().getPerClause().getKind() == PerClauseKind.SINGLETON) {
  // 当@Aspect的value属性为""时才会进入到这里
  // 创建获取有@Aspect注解类的实例工厂，负责获取有@Aspect注解类的实例
  MetadataAwareAspectInstanceFactory factory =
   new BeanFactoryAspectInstanceFactory(this.beanFactory, beanName);

  //创建切面advisor对象
  List<Advisor> classAdvisors = this.advisorFactory.getAdvisors(factory);
  if (this.beanFactory.isSingleton(beanName)) {
   this.advisorsCache.put(beanName, classAdvisors);
  }
  else {
   this.aspectFactoryCache.put(beanName, factory);
  }
  advisors.addAll(classAdvisors);
  }
  else {
  MetadataAwareAspectInstanceFactory factory =
   new PrototypeAspectInstanceFactory(this.beanFactory, beanName);
  this.aspectFactoryCache.put(beanName, factory);
  advisors.addAll(this.advisorFactory.getAdvisors(factory));
  }
  }
  }
  this.aspectBeanNames = aspectNames;
  return advisors;
 }
 }
 }
 return advisors;
 }

这个方法里面首先从IOC中拿到所有Bean的名称，并循环判断该类上是否带有@Aspect注解，如果有则将BeanName和Bean的Class类型封装到BeanFactoryAspectInstanceFactory中，并调用ReflectiveAspectJAdvisorFactory.getAdvisors创建切面对象：

 public List<Advisor> getAdvisors(MetadataAwareAspectInstanceFactory aspectInstanceFactory) {
 //从工厂中获取有@Aspect注解的类Class
 Class<?> aspectClass = aspectInstanceFactory.getAspectMetadata().getAspectClass();
 //从工厂中获取有@Aspect注解的类的名称
 String aspectName = aspectInstanceFactory.getAspectMetadata().getAspectName();
 validate(aspectClass);

 // 创建工厂的装饰类，获取实例只会获取一次
 MetadataAwareAspectInstanceFactory lazySingletonAspectInstanceFactory =
 new LazySingletonAspectInstanceFactoryDecorator(aspectInstanceFactory);

 List<Advisor> advisors = new ArrayList<>();

 //这里循环没有@Pointcut注解的方法
 for (Method method : getAdvisorMethods(aspectClass)) {

 //非常重要重点看看
 Advisor advisor = getAdvisor(method, lazySingletonAspectInstanceFactory, advisors.size(), aspectName);
 if (advisor != null) {
 advisors.add(advisor);
 }
 }

 if (!advisors.isEmpty() && lazySingletonAspectInstanceFactory.getAspectMetadata().isLazilyInstantiated()) {
 Advisor instantiationAdvisor = new SyntheticInstantiationAdvisor(lazySingletonAspectInstanceFactory);
 advisors.add(0, instantiationAdvisor);
 }

 //判断属性上是否有引介注解，这里可以不看
 for (Field field : aspectClass.getDeclaredFields()) {
 //判断属性上是否有DeclareParents注解，如果有返回切面
 Advisor advisor = getDeclareParentsAdvisor(field);
 if (advisor != null) {
 advisors.add(advisor);
 }
 }

 return advisors;
 }

 private List<Method> getAdvisorMethods(Class<?> aspectClass) {
 final List<Method> methods = new ArrayList<>();
 ReflectionUtils.doWithMethods(aspectClass, method -> {
 // Exclude pointcuts
 if (AnnotationUtils.getAnnotation(method, Pointcut.class) == null) {
 methods.add(method);
 }
 });
 methods.sort(METHOD_COMPARATOR);
 return methods;
 }

根据Aspect的Class拿到所有不带@Pointcut注解的方法对象（为什么是不带@Pointcut注解的方法？仔细想想不难理解），另外要注意这里对method进行了排序，看看这个METHOD_COMPARATOR比较器：

 private static final Comparator<Method> METHOD_COMPARATOR;

 static {
 Comparator<Method> adviceKindComparator = new ConvertingComparator<>(
 new InstanceComparator<>(
  Around.class, Before.class, After.class, AfterReturning.class, AfterThrowing.class),
 (Converter<Method, Annotation>) method -> {
  AspectJAnnotation<?> annotation =
  AbstractAspectJAdvisorFactory.findAspectJAnnotationOnMethod(method);
  return (annotation != null ? annotation.getAnnotation() : null);
 });
 Comparator<Method> methodNameComparator = new ConvertingComparator<>(Method::getName);
 METHOD_COMPARATOR = adviceKindComparator.thenComparing(methodNameComparator);
 }

关注InstanceComparator构造函数参数，记住它们的顺序，这就是AOP链式调用中同一个@Aspect类中Advice的执行顺序。接着往下看，在getAdvisors方法中循环获取到的methods，分别调用getAdvisor方法，也就是根据方法逐个去创建切面：

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

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

 //获取pointCut对象，最重要的是从注解中获取表达式
 AspectJExpressionPointcut expressionPointcut = getPointcut(
 candidateAdviceMethod, aspectInstanceFactory.getAspectMetadata().getAspectClass());
 if (expressionPointcut == null) {
 return null;
 }

 //创建Advisor切面类，这才是真正的切面类，一个切面类里面肯定要有1、pointCut 2、advice
 //这里pointCut是expressionPointcut， advice 增强方法是 candidateAdviceMethod
 return new InstantiationModelAwarePointcutAdvisorImpl(expressionPointcut, candidateAdviceMethod,
 this, aspectInstanceFactory, declarationOrderInAspect, aspectName);
 }

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

 private AspectJExpressionPointcut getPointcut(Method candidateAdviceMethod, Class<?> candidateAspectClass) {
 //从候选的增强方法里面 candidateAdviceMethod 找有有注解
 //Pointcut.class, Around.class, Before.class, After.class, AfterReturning.class, AfterThrowing.class
 //并把注解信息封装成AspectJAnnotation对象
 AspectJAnnotation<?> aspectJAnnotation =
 AbstractAspectJAdvisorFactory.findAspectJAnnotationOnMethod(candidateAdviceMethod);
 if (aspectJAnnotation == null) {
 return null;
 }

 //创建一个PointCut类，并且把前面从注解里面解析的表达式设置进去
 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;
 }

之前就说过切面的定义，是切点和增强的组合，所以这里首先通过getPointcut获取到注解对象，然后new了一个Pointcut对象，并将表达式设置进去。然后在getAdvisor方法中最后new了一个InstantiationModelAwarePointcutAdvisorImpl对象：

 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;
 //这个方法重点看看，创建advice对象
 this.instantiatedAdvice = instantiateAdvice(this.declaredPointcut);
 }
 }

这个就是我们的切面类，在其构造方法的最后通过instantiateAdvice创建了Advice对象。注意这里传进来的declarationOrder参数，它就是循环method时的序号，其作用就是赋值给这里的declarationOrder属性以及Advice的declarationOrder属性，在后面排序时就会通过这个序号来比较，因此Advice的执行顺序是固定的，至于为什么要固定，后面分析完AOP链式调用过程自然就明白了。

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

 //获取有@Aspect注解的类
 Class<?> candidateAspectClass = aspectInstanceFactory.getAspectMetadata().getAspectClass();
 validate(candidateAspectClass);

 //找到candidateAdviceMethod方法上面的注解，并且包装成AspectJAnnotation对象，这个对象中就有注解类型
 AspectJAnnotation<?> aspectJAnnotation =
 AbstractAspectJAdvisorFactory.findAspectJAnnotationOnMethod(candidateAdviceMethod);
 if (aspectJAnnotation == null) {
 return null;
 }

 AbstractAspectJAdvice springAdvice;

 //根据不同的注解类型创建不同的advice类实例
 switch (aspectJAnnotation.getAnnotationType()) {
 case AtPointcut:
 if (logger.isDebugEnabled()) {
  logger.debug("Processing pointcut '" + candidateAdviceMethod.getName() + "'");
 }
 return null;
 case AtAround:
 //实现了MethodInterceptor接口
 springAdvice = new AspectJAroundAdvice(
  candidateAdviceMethod, expressionPointcut, aspectInstanceFactory);
 break;
 case AtBefore:
 //实现了MethodBeforeAdvice接口，没有实现MethodInterceptor接口
 springAdvice = new AspectJMethodBeforeAdvice(
  candidateAdviceMethod, expressionPointcut, aspectInstanceFactory);
 break;
 case AtAfter:
 //实现了MethodInterceptor接口
 springAdvice = new AspectJAfterAdvice(
  candidateAdviceMethod, expressionPointcut, aspectInstanceFactory);
 break;
 case AtAfterReturning:
 //实现了AfterReturningAdvice接口，没有实现MethodInterceptor接口
 springAdvice = new AspectJAfterReturningAdvice(
  candidateAdviceMethod, expressionPointcut, aspectInstanceFactory);
 AfterReturning afterReturningAnnotation = (AfterReturning) aspectJAnnotation.getAnnotation();
 if (StringUtils.hasText(afterReturningAnnotation.returning())) {
  springAdvice.setReturningName(afterReturningAnnotation.returning());
 }
 break;
 case AtAfterThrowing:
 //实现了MethodInterceptor接口
 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);
 }

 // Now to configure the advice...
 springAdvice.setAspectName(aspectName);
 springAdvice.setDeclarationOrder(declarationOrder);
 String[] argNames = this.parameterNameDiscoverer.getParameterNames(candidateAdviceMethod);
 if (argNames != null) {
 springAdvice.setArgumentNamesFromStringArray(argNames);
 }

 //计算argNames和类型的对应关系
 springAdvice.calculateArgumentBindings();

 return springAdvice;
 }

这里逻辑很清晰，就是拿到方法上的注解类型，根据类型创建不同的增强Advice对象：AspectJAroundAdvice、AspectJMethodBeforeAdvice、AspectJAfterAdvice、AspectJAfterReturningAdvice、AspectJAfterThrowingAdvice。完成之后通过calculateArgumentBindings方法进行参数绑定，感兴趣的可自行研究。这里主要看看几个Advice的继承体系：

可以看到有两个Advice是没有实现MethodInterceptor接口的：AspectJMethodBeforeAdvice和AspectJAfterReturningAdvice。而MethodInterceptor有一个invoke方法，这个方法就是链式调用的核心方法，但那两个没有实现该方法的Advice怎么处理呢？稍后会分析。

到这里切面对象就创建完成了，接下来就是判断当前创建的Bean实例是否和这些切面匹配以及对切面排序。匹配过程比较复杂，对理解主流程也没什么帮助，所以这里就不展开分析，感兴趣的自行分析（AbstractAdvisorAutoProxyCreator.findAdvisorsThatCanApply()）。

下面看看排序的过程，回到AbstractAdvisorAutoProxyCreator.findEligibleAdvisors方法：

 protected List<Advisor> findEligibleAdvisors(Class<?> beanClass, String beanName) {
 //找到候选的切面,其实就是一个寻找有@Aspectj注解的过程，把工程中所有有这个注解的类封装成Advisor返回
 List<Advisor> candidateAdvisors = findCandidateAdvisors();

 //判断候选的切面是否作用在当前beanClass上面，就是一个匹配过程。。现在就是一个匹配
 List<Advisor> eligibleAdvisors = findAdvisorsThatCanApply(candidateAdvisors, beanClass, beanName);
 extendAdvisors(eligibleAdvisors);
 if (!eligibleAdvisors.isEmpty()) {
 //对有@Order@Priority进行排序
 eligibleAdvisors = sortAdvisors(eligibleAdvisors);
 }
 return eligibleAdvisors;
 }

sortAdvisors方法就是排序，但这个方法有两个实现：当前类AbstractAdvisorAutoProxyCreator和子类AspectJAwareAdvisorAutoProxyCreator，应该走哪个呢？

通过类图我们可以肯定是进入的AspectJAwareAdvisorAutoProxyCreator类，因为AnnotationAwareAspectJAutoProxyCreator的父类是它。

 protected List<Advisor> sortAdvisors(List<Advisor> advisors) {
 List<PartiallyComparableAdvisorHolder> partiallyComparableAdvisors = new ArrayList<>(advisors.size());
 for (Advisor element : advisors) {
 partiallyComparableAdvisors.add(
  new PartiallyComparableAdvisorHolder(element, DEFAULT_PRECEDENCE_COMPARATOR));
 }
 List<PartiallyComparableAdvisorHolder> sorted = PartialOrder.sort(partiallyComparableAdvisors);
 if (sorted != null) {
 List<Advisor> result = new ArrayList<>(advisors.size());
 for (PartiallyComparableAdvisorHolder pcAdvisor : sorted) {
 result.add(pcAdvisor.getAdvisor());
 }
 return result;
 }
 else {
 return super.sortAdvisors(advisors);
 }
 }

这里排序主要是委托给PartialOrder进行的，而在此之前将所有的切面都封装成了PartiallyComparableAdvisorHolder对象，注意传入的DEFAULT_PRECEDENCE_COMPARATOR参数，这个就是比较器对象：

private static final Comparator<Advisor> DEFAULT_PRECEDENCE_COMPARATOR = new AspectJPrecedenceComparator();

所以我们直接看这个比较器的compare方法：

 public int compare(Advisor o1, Advisor o2) {
 int advisorPrecedence = this.advisorComparator.compare(o1, o2);
 if (advisorPrecedence == SAME_PRECEDENCE && declaredInSameAspect(o1, o2)) {
 advisorPrecedence = comparePrecedenceWithinAspect(o1, o2);
 }
 return advisorPrecedence;
 }

 private final Comparator<? super Advisor> advisorComparator;
 public AspectJPrecedenceComparator() {
 this.advisorComparator = AnnotationAwareOrderComparator.INSTANCE;
 }

第一步先通过AnnotationAwareOrderComparator去比较，点进去看可以发现是对实现了PriorityOrdered和Ordered接口以及标记了Priority和Order注解的非同一个@Aspect类中的切面进行排序。这个和之前分析BeanFacotryPostProcessor类是一样的原理。而对同一个@Aspect类中的切面排序主要是comparePrecedenceWithinAspect方法：

 private int comparePrecedenceWithinAspect(Advisor advisor1, Advisor advisor2) {
 boolean oneOrOtherIsAfterAdvice =
 (AspectJAopUtils.isAfterAdvice(advisor1) || AspectJAopUtils.isAfterAdvice(advisor2));
 int adviceDeclarationOrderDelta = getAspectDeclarationOrder(advisor1) - getAspectDeclarationOrder(advisor2);

 if (oneOrOtherIsAfterAdvice) {
 // the advice declared last has higher precedence
 if (adviceDeclarationOrderDelta < 0) {
 // advice1 was declared before advice2
 // so advice1 has lower precedence
 return LOWER_PRECEDENCE;
 }
 else if (adviceDeclarationOrderDelta == 0) {
 return SAME_PRECEDENCE;
 }
 else {
 return HIGHER_PRECEDENCE;
 }
 }
 else {
 // the advice declared first has higher precedence
 if (adviceDeclarationOrderDelta < 0) {
 // advice1 was declared before advice2
 // so advice1 has higher precedence
 return HIGHER_PRECEDENCE;
 }
 else if (adviceDeclarationOrderDelta == 0) {
 return SAME_PRECEDENCE;
 }
 else {
 return LOWER_PRECEDENCE;
 }
 }
 }

 private int getAspectDeclarationOrder(Advisor anAdvisor) {
 AspectJPrecedenceInformation precedenceInfo =
 AspectJAopUtils.getAspectJPrecedenceInformationFor(anAdvisor);
 if (precedenceInfo != null) {
 return precedenceInfo.getDeclarationOrder();
 }
 else {
 return 0;
 }
 }

这里就是通过precedenceInfo.getDeclarationOrder拿到在创建InstantiationModelAwarePointcutAdvisorImpl对象时设置的declarationOrder属性，这就验证了之前的说法（实际上这里排序过程非常复杂，不是简单的按照这个属性进行排序）。

当上面的一切都进行完成后，就该创建代理对象了，回到AbstractAutoProxyCreator.wrapIfNecessary，看关键部分代码：

 //如果有切面，则生成该bean的代理
 if (specificInterceptors != DO_NOT_PROXY) {
 this.advisedBeans.put(cacheKey, Boolean.TRUE);
 //把被代理对象bean实例封装到SingletonTargetSource对象中
 Object proxy = createProxy(
 bean.getClass(), beanName, specificInterceptors, new SingletonTargetSource(bean));
 this.proxyTypes.put(cacheKey, proxy.getClass());
 return proxy;
 }

注意这里将被代理对象封装成了一个SingletonTargetSource对象，它是TargetSource的实现类。

 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);
 }

 //创建代理工厂
 ProxyFactory proxyFactory = new ProxyFactory();
 proxyFactory.copyFrom(this);

 if (!proxyFactory.isProxyTargetClass()) {
 if (shouldProxyTargetClass(beanClass, beanName)) {
 //proxyTargetClass 是否对类进行代理，而不是对接口进行代理，设置为true时，使用CGLib代理。
 proxyFactory.setProxyTargetClass(true);
 }
 else {
 evaluateProxyInterfaces(beanClass, proxyFactory);
 }
 }

 //把advice类型的增强包装成advisor切面
 Advisor[] advisors = buildAdvisors(beanName, specificInterceptors);
 proxyFactory.addAdvisors(advisors);
 proxyFactory.setTargetSource(targetSource);
 customizeProxyFactory(proxyFactory);

 用来控制代理工厂被配置后，是否还允许修改代理的配置,默认为false
 proxyFactory.setFrozen(this.freezeProxy);
 if (advisorsPreFiltered()) {
 proxyFactory.setPreFiltered(true);
 }

 //获取代理实例
 return proxyFactory.getProxy(getProxyClassLoader());
 }

这里通过ProxyFactory对象去创建代理实例，这是工厂模式的体现，但在创建代理对象之前还有几个准备动作：需要判断是JDK代理还是CGLIB代理以及通过buildAdvisors方法将扩展的Advice封装成Advisor切面。准备完成则通过getProxy创建代理对象：

public Object getProxy(@Nullable ClassLoader classLoader) {
 //根据目标对象是否有接口来判断采用什么代理方式，cglib代理还是jdk动态代理
 return createAopProxy().getProxy(classLoader);
 }

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

 public AopProxy createAopProxy(AdvisedSupport config) throws AopConfigException {
 if (config.isOptimize() || config.isProxyTargetClass() || hasNoUserSuppliedProxyInterfaces(config)) {
 Class<?> targetClass = config.getTargetClass();
 if (targetClass == null) {
 throw new AopConfigException("TargetSource cannot determine target class: " +
  "Either an interface or a target is required for proxy creation.");
 }
 if (targetClass.isInterface() || Proxy.isProxyClass(targetClass)) {
 return new JdkDynamicAopProxy(config);
 }
 return new ObjenesisCglibAopProxy(config);
 }
 else {
 return new JdkDynamicAopProxy(config);
 }
 }

首先通过配置拿到对应的代理类：ObjenesisCglibAopProxy和JdkDynamicAopProxy，然后再通过getProxy创建Bean的代理，这里以JdkDynamicAopProxy为例：

 public Object getProxy(@Nullable ClassLoader classLoader) {
 //advised是代理工厂对象
 Class<?>[] proxiedInterfaces = AopProxyUtils.completeProxiedInterfaces(this.advised, true);
 findDefinedEqualsAndHashCodeMethods(proxiedInterfaces);
 return Proxy.newProxyInstance(classLoader, proxiedInterfaces, this);
 }

这里的代码你应该不陌生了，就是JDK的原生API，newProxyInstance方法传入的InvocationHandler对象是this，因此，最终AOP代理的调用就是从该类中的invoke方法开始。至此，代理对象的创建就完成了，下面来看下整个过程的时序图：

小结

代理对象的创建过程整体来说并不复杂，首先找到所有带有@Aspect注解的类，并获取其中没有@Pointcut注解的方法，循环创建切面，而创建切面需要切点和增强两个元素，其中切点可简单理解为我们写的表达式，增强则是根据@Before、@Around、@After等注解创建的对应的Advice类。切面创建好后则需要循环判断哪些切面能对当前的Bean实例的方法进行增强并排序，最后通过ProxyFactory创建代理对象。

AOP链式调用

熟悉JDK动态代理的都知道通过代理对象调用方法时，会进入到InvocationHandler对象的invoke方法，所以我们直接从JdkDynamicAopProxy的这个方法开始：

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

 //从代理工厂中拿到TargetSource对象，该对象包装了被代理实例bean
 TargetSource targetSource = this.advised.targetSource;
 Object target = null;

 try {
 //被代理对象的equals方法和hashCode方法是不能被代理的，不会走切面
 .......

 Object retVal;

 // 可以从当前线程中拿到代理对象
 if (this.advised.exposeProxy) {
 // Make invocation available if necessary.
 oldProxy = AopContext.setCurrentProxy(proxy);
 setProxyContext = true;
 }

 //这个target就是被代理实例
 target = targetSource.getTarget();
 Class<?> targetClass = (target != null ? target.getClass() : null);

 //从代理工厂中拿过滤器链 Object是一个MethodInterceptor类型的对象，其实就是一个advice对象
 List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass);

 //如果该方法没有执行链，则说明这个方法不需要被拦截，则直接反射调用
 if (chain.isEmpty()) {
 Object[] argsToUse = AopProxyUtils.adaptArgumentsIfNecessary(method, args);
 retVal = AopUtils.invokeJoinpointUsingReflection(target, method, argsToUse);
 }
 else {
 invocation = new ReflectiveMethodInvocation(proxy, target, method, args, targetClass, chain);
 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())) {
 retVal = proxy;
 }
 return retVal;
 }
 finally {
 if (target != null && !targetSource.isStatic()) {
 // Must have come from TargetSource.
 targetSource.releaseTarget(target);
 }
 if (setProxyContext) {
 // Restore old proxy.
 AopContext.setCurrentProxy(oldProxy);
 }
 }
 }

这段代码比较长，我删掉了不关键的地方。首先来看this.advised.exposeProxy这个属性，这在@EnableAspectJAutoProxy注解中可以配置，当为true时，会将该代理对象设置到当前线程的ThreadLocal对象中，这样就可以通过AopContext.currentProxy拿到代理对象。这个有什么用呢？我相信有经验的Java开发都遇到过这样一个BUG，在Service实现类中调用本类中的另一个方法时，事务不会生效，这是因为直接通过this调用就不会调用到代理对象的方法，而是原对象的，所以事务切面就没有生效。因此这种情况下就可以从当前线程的ThreadLocal对象拿到代理对象，不过实际上直接使用@Autowired注入自己本身也可以拿到代理对象。

接下来就是通过getInterceptorsAndDynamicInterceptionAdvice拿到执行链，看看具体做了哪些事情：

 public List<Object> getInterceptorsAndDynamicInterceptionAdvice(
 Advised config, Method method, @Nullable Class<?> targetClass) {

 AdvisorAdapterRegistry registry = GlobalAdvisorAdapterRegistry.getInstance();
 //从代理工厂中获得该被代理类的所有切面advisor，config就是代理工厂对象
 Advisor[] advisors = config.getAdvisors();
 List<Object> interceptorList = new ArrayList<>(advisors.length);
 Class<?> actualClass = (targetClass != null ? targetClass : method.getDeclaringClass());
 Boolean hasIntroductions = null;

 for (Advisor advisor : advisors) {
 //大部分走这里
 if (advisor instanceof PointcutAdvisor) {
 // Add it conditionally.
 PointcutAdvisor pointcutAdvisor = (PointcutAdvisor) advisor;
 //如果切面的pointCut和被代理对象是匹配的，说明是切面要拦截的对象
 if (config.isPreFiltered() || pointcutAdvisor.getPointcut().getClassFilter().matches(actualClass)) {
  MethodMatcher mm = pointcutAdvisor.getPointcut().getMethodMatcher();
  boolean match;
  if (mm instanceof IntroductionAwareMethodMatcher) {
  if (hasIntroductions == null) {
  hasIntroductions = hasMatchingIntroductions(advisors, actualClass);
  }
  match = ((IntroductionAwareMethodMatcher) mm).matches(method, actualClass, hasIntroductions);
  }
  else {
  //接下来判断方法是否是切面pointcut需要拦截的方法
  match = mm.matches(method, actualClass);
  }
  //如果类和方法都匹配
  if (match) {

  //获取到切面advisor中的advice，并且包装成MethodInterceptor类型的对象
  MethodInterceptor[] interceptors = registry.getInterceptors(advisor);
  if (mm.isRuntime()) {
  for (MethodInterceptor interceptor : interceptors) {
  interceptorList.add(new InterceptorAndDynamicMethodMatcher(interceptor, mm));
  }
  }
  else {
  interceptorList.addAll(Arrays.asList(interceptors));
  }
  }
 }
 }
 //如果是引介切面
 else if (advisor instanceof IntroductionAdvisor) {
 IntroductionAdvisor ia = (IntroductionAdvisor) advisor;
 if (config.isPreFiltered() || ia.getClassFilter().matches(actualClass)) {
  Interceptor[] interceptors = registry.getInterceptors(advisor);
  interceptorList.addAll(Arrays.asList(interceptors));
 }
 }
 else {
 Interceptor[] interceptors = registry.getInterceptors(advisor);
 interceptorList.addAll(Arrays.asList(interceptors));
 }
 }

 return interceptorList;
 }

这也是个长方法，看关键的部分，因为之前我们创建的基本上都是InstantiationModelAwarePointcutAdvisorImpl对象，该类是PointcutAdvisor的实现类，所以会进入第一个if判断里，这里首先进行匹配，看切点和当前对象以及该对象的哪些方法匹配，如果能匹配上，则调用getInterceptors获取执行链：

 private final List<AdvisorAdapter> adapters = new ArrayList<>(3);
 public DefaultAdvisorAdapterRegistry() {
 registerAdvisorAdapter(new MethodBeforeAdviceAdapter());
 registerAdvisorAdapter(new AfterReturningAdviceAdapter());
 registerAdvisorAdapter(new ThrowsAdviceAdapter());
 }

 public MethodInterceptor[] getInterceptors(Advisor advisor) throws UnknownAdviceTypeException {
 List<MethodInterceptor> interceptors = new ArrayList<>(3);
 Advice advice = advisor.getAdvice();
 //如果是MethodInterceptor类型的，如：AspectJAroundAdvice
 //AspectJAfterAdvice
 //AspectJAfterThrowingAdvice
 if (advice instanceof MethodInterceptor) {
 interceptors.add((MethodInterceptor) advice);
 }

 //处理 AspectJMethodBeforeAdvice AspectJAfterReturningAdvice
 for (AdvisorAdapter adapter : this.adapters) {
 if (adapter.supportsAdvice(advice)) {
 interceptors.add(adapter.getInterceptor(advisor));
 }
 }
 if (interceptors.isEmpty()) {
 throw new UnknownAdviceTypeException(advisor.getAdvice());
 }
 return interceptors.toArray(new MethodInterceptor[0]);
 }

这里我们可以看到如果是MethodInterceptor的实现类，则直接添加到链中，如果不是，则需要通过适配器去包装后添加，刚好这里有MethodBeforeAdviceAdapter和AfterReturningAdviceAdapter两个适配器对应上文两个没有实现MethodInterceptor接口的类。最后将Interceptors返回。

if (chain.isEmpty()) {
 Object[] argsToUse = AopProxyUtils.adaptArgumentsIfNecessary(method, args);
 retVal = AopUtils.invokeJoinpointUsingReflection(target, method, argsToUse);
}
else {
 // We need to create a method invocation...
 invocation = new ReflectiveMethodInvocation(proxy, target, method, args, targetClass, chain);
 // Proceed to the joinpoint through the interceptor chain.
 retVal = invocation.proceed();
}

返回到invoke方法后，如果执行链为空，说明该方法不需要被增强，所以直接反射调用原对象的方法（注意传入的是TargetSource封装的被代理对象）；反之，则通过ReflectiveMethodInvocation类进行链式调用，关键方法就是proceed：

 private int currentInterceptorIndex = -1;

 public Object proceed() throws Throwable {
 //如果执行链中的advice全部执行完，则直接调用joinPoint方法，就是被代理方法
 if (this.currentInterceptorIndex == this.interceptorsAndDynamicMethodMatchers.size() - 1) {
 return invokeJoinpoint();
 }

 Object interceptorOrInterceptionAdvice =
 this.interceptorsAndDynamicMethodMatchers.get(++this.currentInterceptorIndex);
 if (interceptorOrInterceptionAdvice instanceof InterceptorAndDynamicMethodMatcher) {
 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 {
 return proceed();
 }
 }
 else {
 //调用MethodInterceptor中的invoke方法
 return ((MethodInterceptor) interceptorOrInterceptionAdvice).invoke(this);
 }
 }

这个方法的核心就在两个地方：invokeJoinpoint和interceptorOrInterceptionAdvice.invoke(this)。当增强方法调用完后就会通过前者调用到被代理的方法，否则则是依次调用Interceptor的invoke方法。下面就分别看看每个Interceptor是怎么实现的。

AspectJAroundAdvice
 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);
 }

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

 public void before(Method method, Object[] args, @Nullable Object target) throws Throwable {
 invokeAdviceMethod(getJoinPointMatch(), null, null);
 }

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

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

 public void afterReturning(@Nullable Object returnValue, Method method, Object[] args, @Nullable Object target) throws Throwable {
 if (shouldInvokeOnReturnValueOf(method, returnValue)) {
 invokeAdviceMethod(getJoinPointMatch(), returnValue, null);
 }
 }

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

这里的调用顺序是怎样的呢？其核心就是通过proceed方法控制流程，每执行完一个Advice就会回到proceed方法中调用下一个Advice。可以思考一下，怎么才能让调用结果满足如下图的执行顺序。

以上就是AOP的链式调用过程，但是这只是只有一个切面类的情况，如果有多个@Aspect类呢，这个调用过程又是怎样的？其核心思想和“栈”一样，就是“先进后出，后进先出”。

AOP扩展知识

一、自定义全局拦截器Interceptor

在上文创建代理对象的时候有这样一个方法：

 protected Advisor[] buildAdvisors(@Nullable String beanName, @Nullable Object[] specificInterceptors) {
 //自定义MethodInterceptor.拿到setInterceptorNames方法注入的Interceptor对象
 Advisor[] commonInterceptors = resolveInterceptorNames();

 List<Object> allInterceptors = new ArrayList<>();
 if (specificInterceptors != null) {
 allInterceptors.addAll(Arrays.asList(specificInterceptors));
 if (commonInterceptors.length > 0) {
 if (this.applyCommonInterceptorsFirst) {
  allInterceptors.addAll(0, Arrays.asList(commonInterceptors));
 }
 else {
  allInterceptors.addAll(Arrays.asList(commonInterceptors));
 }
 }
 }

 Advisor[] advisors = new Advisor[allInterceptors.size()];
 for (int i = 0; i < allInterceptors.size(); i++) {
 //对自定义的advice要进行包装，把advice包装成advisor对象，切面对象
 advisors[i] = this.advisorAdapterRegistry.wrap(allInterceptors.get(i));
 }
 return advisors;
 }

这个方法的作用就在于我们可以扩展我们自己的Interceptor，首先通过resolveInterceptorNames方法获取到通过setInterceptorNames方法设置的Interceptor，然后调用DefaultAdvisorAdapterRegistry.wrap方法将其包装为DefaultPointcutAdvisor对象并返回：

 public Advisor wrap(Object adviceObject) throws UnknownAdviceTypeException {
 if (adviceObject instanceof Advisor) {
 return (Advisor) adviceObject;
 }
 if (!(adviceObject instanceof Advice)) {
 throw new UnknownAdviceTypeException(adviceObject);
 }
 Advice advice = (Advice) adviceObject;
 if (advice instanceof MethodInterceptor) {
 return new DefaultPointcutAdvisor(advice);
 }
 for (AdvisorAdapter adapter : this.adapters) {
 if (adapter.supportsAdvice(advice)) {
 return new DefaultPointcutAdvisor(advice);
 }
 }
 throw new UnknownAdviceTypeException(advice);
 }

 public DefaultPointcutAdvisor(Advice advice) {
 this(Pointcut.TRUE, advice);
 }

需要注意DefaultPointcutAdvisor构造器里面传入了一个Pointcut.TRUE，表示这种扩展的Interceptor是全局的拦截器。下面来看看如何使用：

public class MyMethodInterceptor implements MethodInterceptor {
 @Override
 public Object invoke(MethodInvocation invocation) throws Throwable {

  System.out.println("自定义拦截器");
  return invocation.proceed();
 }
}

首先写一个类实现MethodInterceptor 接口，在invoke方法中实现我们的拦截逻辑，然后通过下面的方式测试，只要UserService 有AOP拦截就会发现自定义的MyMethodInterceptor也生效了。

 public void costomInterceptorTest() {
  AnnotationAwareAspectJAutoProxyCreator bean = applicationContext.getBean(AnnotationAwareAspectJAutoProxyCreator.class);
  bean.setInterceptorNames("myMethodInterceptor ");

  UserService userService = applicationContext.getBean(UserService.class);
  userService.queryUser("dark");
 }

但是如果换个顺序，像下面这样：

 public void costomInterceptorTest() {

  UserService userService = applicationContext.getBean(UserService.class);

  AnnotationAwareAspectJAutoProxyCreator bean = applicationContext.getBean(AnnotationAwareAspectJAutoProxyCreator.class);
  bean.setInterceptorNames("myMethodInterceptor ");

  userService.queryUser("dark");
 }

这时自定义的全局拦截器就没有作用了，这是为什么呢？因为当执行getBean的时候，如果有切面匹配就会通过ProxyFactory去创建代理对象，注意Interceptor是存到这个Factory对象中的，而这个对象和代理对象是一一对应的，因此调用getBean时，还没有myMethodInterceptor这个对象，自定义拦截器就没有效果了，也就是说要想自定义拦截器生效，就必须在代理对象生成之前注册进去。

二、循环依赖三级缓存存在的必要性

在上一篇文章我分析了Spring是如何通过三级缓存来解决循环依赖的问题的，但你是否考虑过第三级缓存为什么要存在？我直接将bean存到二级不就行了么，为什么还要存一个ObjectFactory对象到第三级缓存中？一个是因为不是每个Bean都会出现循环依赖，所以三级缓存只存了一个工厂对象；二是我们在@Autowired对象时，想要注入的不一定是Bean本身，而是想要注入一个修改过后的对象，如代理对象。在AbstractAutowireCapableBeanFactory.getEarlyBeanReference方法中循环调用了SmartInstantiationAwareBeanPostProcessor.getEarlyBeanReference方法，AbstractAutoProxyCreator对象就实现了该方法：

 public Object getEarlyBeanReference(Object bean, String beanName) {
 Object cacheKey = getCacheKey(bean.getClass(), beanName);
 if (!this.earlyProxyReferences.contains(cacheKey)) {
 this.earlyProxyReferences.add(cacheKey);
 }
 // 创建代理对象
 return wrapIfNecessary(bean, beanName, cacheKey);
 }

因此，当我们想要对循坏依赖的Bean做出修改时，就可以像AOP这样做。

三、如何在Bean创建之前提前创建代理对象

Spring的代理对象基本上都是在Bean实例化完成之后创建的，但在文章开始我就说过，Spring也提供了一个机会在创建Bean对象之前就创建代理对象，在AbstractAutowireCapableBeanFactory.resolveBeforeInstantiation方法中：

 protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) {
 Object bean = null;
 if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved)) {
 // Make sure bean class is actually resolved at this point.
 if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
 Class<?> targetType = determineTargetType(beanName, mbd);
 if (targetType != null) {
  bean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName);
  if (bean != null) {
  bean = applyBeanPostProcessorsAfterInitialization(bean, beanName);
  }
 }
 }
 mbd.beforeInstantiationResolved = (bean != null);
 }
 return bean;
 }

 protected Object applyBeanPostProcessorsBeforeInstantiation(Class<?> beanClass, String beanName) {
 for (BeanPostProcessor bp : getBeanPostProcessors()) {
 if (bp instanceof InstantiationAwareBeanPostProcessor) {
 InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
 Object result = ibp.postProcessBeforeInstantiation(beanClass, beanName);
 if (result != null) {
  return result;
 }
 }
 }
 return null;
 }

主要是InstantiationAwareBeanPostProcessor.postProcessBeforeInstantiation方法中，这里又会进入到AbstractAutoProxyCreator类中：

public Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) {
 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;
 }

 protected TargetSource getCustomTargetSource(Class<?> beanClass, String beanName) {
 // We can't create fancy target sources for directly registered singletons.
 if (this.customTargetSourceCreators != null &&
 this.beanFactory != null && this.beanFactory.containsBean(beanName)) {
 for (TargetSourceCreator tsc : this.customTargetSourceCreators) {
 TargetSource ts = tsc.getTargetSource(beanClass, beanName);
 if (ts != null) {
  return ts;
 }
 }
 }

 // No custom TargetSource found.
 return null;
 }

看到这里大致应该明白了，先是获取到一个自定义的TargetSource对象，然后创建代理对象，所以我们首先需要自己实现一个TargetSource类，这里直接继承一个抽象类，getTarget方法则返回原始对象：

public class MyTargetSource extends AbstractBeanFactoryBasedTargetSource {
 @Override
 public Object getTarget() throws Exception {
  return getBeanFactory().getBean(getTargetBeanName());
 }
}

但这还不够，上面首先判断了customTargetSourceCreators!=null，而这个属性是个数组，可以通过下面这个方法设置进来：

 public void setCustomTargetSourceCreators(TargetSourceCreator... targetSourceCreators) {
 this.customTargetSourceCreators = targetSourceCreators;
 }

所以我们还要实现一个TargetSourceCreator类，同样继承一个抽象类实现，并只对userServiceImpl对象进行拦截：

public class MyTargetSourceCreator extends AbstractBeanFactoryBasedTargetSourceCreator {
 @Override
 protected AbstractBeanFactoryBasedTargetSource createBeanFactoryBasedTargetSource(Class<?> beanClass, String beanName) {

  if (getBeanFactory() instanceof ConfigurableListableBeanFactory) {
   if(beanName.equalsIgnoreCase("userServiceImpl")) {
    return new MyTargetSource();
   }
  }

  return null;
 }
}

createBeanFactoryBasedTargetSource方法是在AbstractBeanFactoryBasedTargetSourceCreator.getTargetSource中调用的，而getTargetSource就是在上面getCustomTargetSource中调用的。以上工作做完后，还需要将其设置到AnnotationAwareAspectJAutoProxyCreator对象中，因此需要我们注入这个对象：

@Configuration
public class TargetSourceCreatorBean {

 @Autowired
 private BeanFactory beanFactory;

 @Bean
 public AnnotationAwareAspectJAutoProxyCreator annotationAwareAspectJAutoProxyCreator() {
  AnnotationAwareAspectJAutoProxyCreator creator = new AnnotationAwareAspectJAutoProxyCreator();
  MyTargetSourceCreator myTargetSourceCreator = new MyTargetSourceCreator();
  myTargetSourceCreator.setBeanFactory(beanFactory);
  creator.setCustomTargetSourceCreators(myTargetSourceCreator);
  return creator;
 }
}

这样，当我们通过getBean获取userServiceImpl的对象时，就会优先生成代理对象，然后在调用执行链的过程中再通过TargetSource.getTarget获取到被代理对象。但是，为什么我们在getTarget方法中调用getBean就能拿到被代理对象呢？

继续探究，通过断点我发现从getTarget进入时，在resolveBeforeInstantiation方法中返回的bean就是null了，而getBeanPostProcessors方法返回的Processors中也没有了AnnotationAwareAspectJAutoProxyCreator对象，也就是没有进入到AbstractAutoProxyCreator.postProcessBeforeInstantiation方法中，所以不会再次获取到代理对象，那AnnotationAwareAspectJAutoProxyCreator对象是在什么时候移除的呢？

带着问题，我开始反推，发现在AbstractBeanFactoryBasedTargetSourceCreator类中有这样一个方法buildInternalBeanFactory：

 protected DefaultListableBeanFactory buildInternalBeanFactory(ConfigurableBeanFactory containingFactory) {
 DefaultListableBeanFactory internalBeanFactory = new DefaultListableBeanFactory(containingFactory);

 // Required so that all BeanPostProcessors, Scopes, etc become available.
 internalBeanFactory.copyConfigurationFrom(containingFactory);

 // Filter out BeanPostProcessors that are part of the AOP infrastructure,
 // since those are only meant to apply to beans defined in the original factory.
 internalBeanFactory.getBeanPostProcessors().removeIf(beanPostProcessor ->
 beanPostProcessor instanceof AopInfrastructureBean);

 return internalBeanFactory;
 }

在这里移除掉了所有AopInfrastructureBean的子类，而AnnotationAwareAspectJAutoProxyCreator就是其子类，那这个方法是在哪里调用的呢？继续反推：

 protected DefaultListableBeanFactory getInternalBeanFactoryForBean(String beanName) {
 synchronized (this.internalBeanFactories) {
 DefaultListableBeanFactory internalBeanFactory = this.internalBeanFactories.get(beanName);
 if (internalBeanFactory == null) {
 internalBeanFactory = buildInternalBeanFactory(this.beanFactory);
 this.internalBeanFactories.put(beanName, internalBeanFactory);
 }
 return internalBeanFactory;
 }
 }

 public final TargetSource getTargetSource(Class<?> beanClass, String beanName) {
 AbstractBeanFactoryBasedTargetSource targetSource =
 createBeanFactoryBasedTargetSource(beanClass, beanName);

 // 创建完targetSource后就移除掉AopInfrastructureBean类型的BeanPostProcessor对象，如AnnotationAwareAspectJAutoProxyCreator
 DefaultListableBeanFactory internalBeanFactory = getInternalBeanFactoryForBean(beanName);

 ......
 return targetSource;
 }

至此，关于TargetSource接口扩展的原理就搞明白了。

总结

本篇篇幅比较长，主要搞明白Spring代理对象是如何创建的以及AOP链式调用过程，而后面的扩展则是对AOP以及Bean创建过程中一些疑惑的补充，可根据实际情况学习掌握。希望大家多多支持我们。