这一次搞懂Spring代理创建及AOP链式调用过程

文章目录

• 前言
• 正文
• 基本概念
• 代理对象的创建
• 小结
• AOP链式调用
• AOP扩展知识
• 一、自定义全局拦截器Interceptor
• 二、循环依赖三级缓存存在的必要性
• 三、如何在Bean创建之前提前创建代理对象
• 总结

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。

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;
		}
	}

这里的调用顺序就是前面创建切面时注解定义的顺序，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。光看代码可能会比较绕，我画了一张图帮助理解：



以上就是AOP的链式调用过程，你可以思考下如果有多个@Aspect类，这个调用过程又是怎样的？其核心思想和“栈”差不多，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？这个在学习了AOP之后就很清楚了，因为我们在@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。