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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，欧博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），皇冠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： 代码语言：javascript 复制 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： 代码语言：javascript 复制 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，DG游戏进入到AnnotationAwareAspectJAutoProxyCreator类中： 代码语言：javascript 复制 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： 代码语言：javascript 复制 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： 代码语言：javascript 复制 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： 代码语言：javascript 复制 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： 代码语言：javascript 复制 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，欧博注册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： 代码语言：javascript 复制 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。 代码语言：javascript 复制 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：
在这里插入图片描述 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： 代码语言：javascript 复制 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，欧博代理但这个方法有两个实现：当前类AbstractAdvisorAutoProxyCreator和子类AspectJAwareAdvisorAutoProxyCreator，应该走哪个呢？
在这里插入图片描述 通过类图我们可以肯定是进入的AspectJAwareAdvisorAutoProxyCreator类，因为AnnotationAwareAspectJAutoProxyCreator的父类是它。 代码语言：javascript 复制 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： 代码语言：javascript 复制 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： 代码语言：javascript 复制 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： 代码语言：javascript 复制 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： 代码语言：javascript 复制 //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。 代码语言：javascript 复制 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： 代码语言：javascript 复制 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： 代码语言：javascript 复制 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：
在这里插入图片描述 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： 代码语言：javascript 复制 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： 代码语言：javascript 复制 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： 代码语言：javascript 复制 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。 代码语言：javascript 复制 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： 代码语言：javascript 复制 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 代码语言：javascript 复制 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 代码语言：javascript 复制 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 代码语言：javascript 复制 public Object invoke(MethodInvocation mi) throws Throwable { try { return mi.proceed(); } finally { invokeAdviceMethod(getJoinPointMatch(), null, null); } } AfterReturningAdviceInterceptor -> AspectJAfterReturningAdvice 代码语言：javascript 复制 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 代码语言：javascript 复制 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。
在这里插入图片描述 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： 代码语言：javascript 复制 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： 代码语言：javascript 复制 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： 代码语言：javascript 复制 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。 代码语言：javascript 复制 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： 代码语言：javascript 复制 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： 代码语言：javascript 复制 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： 代码语言：javascript 复制 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： 代码语言：javascript 复制 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： 代码语言：javascript 复制 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： 代码语言：javascript 复制 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： 代码语言：javascript 复制 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： 代码语言：javascript 复制 @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： 代码语言：javascript 复制 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： 代码语言：javascript 复制 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。 (责任编辑：) |
