0x5 Spring系列：深入聊一聊 Spring AOP 实现机制【二】

2： 扫描容器中的切面，创建 PointcutAdvisor对象

在spring ioc流程加载的过程中，会触发 beanPostProcessor 扩展接口，
而AnnotationAwareAspectJAutoProxyCreator又是SmartInstantiationAwareBeanPostProcessor的子类，所以该扩展接口正是 aop 实现的入口。

该接口的触发在实例化 bean 之后，初始化 bean之前，具体来看：

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

        if (!StringUtils.hasLength(beanName) || !this.targetSourcedBeans.contains(beanName)) {
            //advisedBeans用于存储不可代理的bean，如果包含直接返回
            if (this.advisedBeans.containsKey(cacheKey)) {
                return null;
            }
            //判断当前bean是否可以被代理，然后存入advisedBeans
            if (isInfrastructureClass(beanClass) || shouldSkip(beanClass, beanName)) {
                this.advisedBeans.put(cacheKey, Boolean.FALSE);
                return null;
            }
        }

        // Create proxy here if we have a custom TargetSource.
        // Suppresses unnecessary default instantiation of the target bean:
        // The TargetSource will handle target instances in a custom fashion.
        //到这里说明该bean可以被代理，所以去获取自定义目标类，如果没有定义，则跳过。
        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());
            //如果最终可以获得代理类，则返回代理类，直接执行实例化后置通知方法
            return proxy;
        }

        return null;
    }

来看一下判定 bean 是否被代理的方法依据：

    @Override
    protected boolean isInfrastructureClass(Class<?> beanClass) {
        return (super.isInfrastructureClass(beanClass) ||
                (this.aspectJAdvisorFactory != null && this.aspectJAdvisorFactory.isAspect(beanClass)));
    }

    private boolean hasAspectAnnotation(Class<?> clazz) {
        //判定当前类是否有 Aspect 注解，如果有，则不能被代理
        return (AnnotationUtils.findAnnotation(clazz, Aspect.class) != null);
    }

    protected boolean isInfrastructureClass(Class<?> beanClass) {
        //判定当前bean是否是 Advice、Pointcut、Advisor、AopInfrastructureBean等子类或实现类，如果是，则不能被代理
        boolean retVal = Advice.class.isAssignableFrom(beanClass) ||
                Pointcut.class.isAssignableFrom(beanClass) ||
                Advisor.class.isAssignableFrom(beanClass) ||
                AopInfrastructureBean.class.isAssignableFrom(beanClass);
        if (retVal && logger.isTraceEnabled()) {
            logger.trace("Did not attempt to auto-proxy infrastructure class [" + beanClass.getName() + "]");
        }
        return retVal;
    }

重点来看 shouldSkip方法：

    @Override
    protected boolean shouldSkip(Class<?> beanClass, String beanName) {
        // TODO: Consider optimization by caching the list of the aspect names
        //获取所有的候选顾问类 Advisor
        List<Advisor> candidateAdvisors = findCandidateAdvisors();
        for (Advisor advisor : candidateAdvisors) {
            if (advisor instanceof AspectJPointcutAdvisor &&
                    ((AspectJPointcutAdvisor) advisor).getAspectName().equals(beanName)) {
                return true;
            }
        }
        return super.shouldSkip(beanClass, beanName);
    }

上述代码通过findCandidateAdvisors()方法来获取所有的候选 advisor：

@Override
    protected List<Advisor> findCandidateAdvisors() {
        // Add all the Spring advisors found according to superclass rules.
        //获得 Advisor 实现类
        List<Advisor> advisors = super.findCandidateAdvisors();
        // Build Advisors for all AspectJ aspects in the bean factory.
        //将@Aspect注解类， 解析成Advisor 
        if (this.aspectJAdvisorsBuilder != null) {
            advisors.addAll(this.aspectJAdvisorsBuilder.buildAspectJAdvisors());
        }
        return advisors;
    }

继续跟进buildAspectJAdvisors方法，会触发
ReflectiveAspectJAdvisorFactory中的getAdvisors方法：

@Override
    public List<Advisor> getAdvisors(MetadataAwareAspectInstanceFactory aspectInstanceFactory) {
        //从 aspectMetadata 中获取 Aspect()标注的类 class对象
        Class<?> aspectClass = aspectInstanceFactory.getAspectMetadata().getAspectClass();
        //获取Aspect()标注的类名
        String aspectName = aspectInstanceFactory.getAspectMetadata().getAspectName();
        validate(aspectClass);

        // We need to wrap the MetadataAwareAspectInstanceFactory with a decorator
        // so that it will only instantiate once.
        MetadataAwareAspectInstanceFactory lazySingletonAspectInstanceFactory =
                new LazySingletonAspectInstanceFactoryDecorator(aspectInstanceFactory);

        List<Advisor> advisors = new LinkedList<>();
        //遍历该类所有方法，根据方法判断是否能获取到对应 pointCut，如果有，则生成 advisor 对象
        for (Method method : getAdvisorMethods(aspectClass)) {
            Advisor advisor = getAdvisor(method, lazySingletonAspectInstanceFactory, advisors.size(), aspectName);
            if (advisor != null) {
                advisors.add(advisor);
            }
        }

        // If it's a per target aspect, emit the dummy instantiating aspect.
        if (!advisors.isEmpty() && lazySingletonAspectInstanceFactory.getAspectMetadata().isLazilyInstantiated()) {
            Advisor instantiationAdvisor = new SyntheticInstantiationAdvisor(lazySingletonAspectInstanceFactory);
            advisors.add(0, instantiationAdvisor);
        }

        // Find introduction fields.
        //获取 @DeclareParents 注解修饰的属性（并不常用）
        for (Field field : aspectClass.getDeclaredFields()) {
            Advisor advisor = getDeclareParentsAdvisor(field);
            if (advisor != null) {
                advisors.add(advisor);
            }
        }

        return advisors;
    }

继续来看getAdvisor方法：

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

        validate(aspectInstanceFactory.getAspectMetadata().getAspectClass());
        //根据候选方法名，来获取对应的 pointCut
        AspectJExpressionPointcut expressionPointcut = getPointcut(
                candidateAdviceMethod, aspectInstanceFactory.getAspectMetadata().getAspectClass());
        if (expressionPointcut == null) {
            return null;
        }
        //如果能获取到 pointCut，则将切点表达式 expressionPointcut、当前
        对象ReflectiveAspectJAdvisorFactory、 方法名等包装成 advisor 对象
        return new InstantiationModelAwarePointcutAdvisorImpl(expressionPointcut, candidateAdviceMethod,
                this, aspectInstanceFactory, declarationOrderInAspect, aspectName);
    }

可以看到方法将切面相关的类，封装成InstantiationModelAwarePointcutAdvisorImpl对象，也就是Advisor 对象。

来看下上面获取切面的方法，规则就是遍历方法，根据注解判断：

    protected static AspectJAnnotation<?> findAspectJAnnotationOnMethod(Method method) {
        //定义class对象数组，如果方法中有以下注解中任何一种，则返回该注解
        Class<?>[] classesToLookFor = new Class<?>[] {
                Before.class, Around.class, After.class, AfterReturning.class, AfterThrowing.class, Pointcut.class};
        for (Class<?> c : classesToLookFor) {
            AspectJAnnotation<?> foundAnnotation = findAnnotation(method, (Class<Annotation>) c);
            if (foundAnnotation != null) {
                return foundAnnotation;
            }
        }
        return null;
    }

我们继续来看 Advisor 对象的构造方法。

InstantiationModelAwarePointcutAdvisorImpl的构造方法会触发构造通知对象：

public Advice getAdvice(Method candidateAdviceMethod, AspectJExpressionPointcut expressionPointcut,
            MetadataAwareAspectInstanceFactory aspectInstanceFactory, int declarationOrder, String aspectName) {
        //......
        //根据注解类型，匹配对应的通知类型
        switch (aspectJAnnotation.getAnnotationType()) {
            //前置通知
            case AtBefore:
                springAdvice = new AspectJMethodBeforeAdvice(
                        candidateAdviceMethod, expressionPointcut, aspectInstanceFactory);
                break;
            //最终通知
            case AtAfter:
                springAdvice = new AspectJAfterAdvice(
                        candidateAdviceMethod, expressionPointcut, aspectInstanceFactory);
                break;
            //后置通知
            case AtAfterReturning:
                springAdvice = new AspectJAfterReturningAdvice(
                        candidateAdviceMethod, expressionPointcut, aspectInstanceFactory);
                AfterReturning afterReturningAnnotation = (AfterReturning) aspectJAnnotation.getAnnotation();
                if (StringUtils.hasText(afterReturningAnnotation.returning())) {
                    springAdvice.setReturningName(afterReturningAnnotation.returning());
                }
                break;
            //异常通知
            case AtAfterThrowing:
                springAdvice = new AspectJAfterThrowingAdvice(
                        candidateAdviceMethod, expressionPointcut, aspectInstanceFactory);
                AfterThrowing afterThrowingAnnotation = (AfterThrowing) aspectJAnnotation.getAnnotation();
                if (StringUtils.hasText(afterThrowingAnnotation.throwing())) {
                    springAdvice.setThrowingName(afterThrowingAnnotation.throwing());
                }
                break;
            //环绕通知
            case AtAround:
                springAdvice = new AspectJAroundAdvice(
                        candidateAdviceMethod, expressionPointcut, aspectInstanceFactory);
                break;
            //切面
            case AtPointcut:
                if (logger.isDebugEnabled()) {
                    logger.debug("Processing pointcut '" + candidateAdviceMethod.getName() + "'");
                }
                return null;
            default:
                throw new UnsupportedOperationException(
                        "Unsupported advice type on method: " + candidateAdviceMethod);
        }

        //......
    }

可以看到，根据@Aspect类中方法的注解类型，生成对应的advice，并通过通知的构造方法，将通知增强方法，切面表达式传入到通知当中。

InstantiationModelAwarePointcutAdvisorImpl对象到这里构造完毕。

3： 生成代理类

上面创建advisor的逻辑发生在扩展接口中的postProcessBeforeInstantiation，实例化之前执行，如果有自定义的TargetSource指定类，则则直接生成代理类，并直接执行初始化之后的方法postProcessAfterInitialization。这种情况使用不多，常规代理类还是在postProcessAfterInitialization中创建，也就是 IOC 最后一个扩展方法。

    @Override
    public Object postProcessAfterInitialization(@Nullable Object bean, String beanName) throws BeansException {
        if (bean != null) {
            Object cacheKey = getCacheKey(bean.getClass(), beanName);
            //处理循环依赖的判断
            if (!this.earlyProxyReferences.contains(cacheKey)) {
                return wrapIfNecessary(bean, beanName, cacheKey);
            }
        }
        return bean;
    }

protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {
        if (StringUtils.hasLength(beanName) && this.targetSourcedBeans.contains(beanName)) {
            return bean;
        }
        if (Boolean.FALSE.equals(this.advisedBeans.get(cacheKey))) {
            return bean;
        }
        if (isInfrastructureClass(bean.getClass()) || shouldSkip(bean.getClass(), beanName)) {
            this.advisedBeans.put(cacheKey, Boolean.FALSE);
            return bean;
        }

        // Create proxy if we have advice.
        //获取到合适的advisor，如果为空。如果不为空，则生成代理类。
        Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
        if (specificInterceptors != DO_NOT_PROXY) {
            this.advisedBeans.put(cacheKey, Boolean.TRUE);
            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;
    }

上述方法通过调用getAdvicesAndAdvisorsForBean()方法来获取advisor，该方法最终会调用findEligibleAdvisors()，Eligible意为有资格的，合适的。具体来看下：

protected List<Advisor> findEligibleAdvisors(Class<?> beanClass, String beanName) {
        List<Advisor> candidateAdvisors = findCandidateAdvisors();
        //这里会对获取的advisor进行筛选
        List<Advisor> eligibleAdvisors = findAdvisorsThatCanApply(candidateAdvisors, beanClass, beanName);
        //添加一个默认的advisor，执行时用到。
        extendAdvisors(eligibleAdvisors);
        if (!eligibleAdvisors.isEmpty()) {
            eligibleAdvisors = sortAdvisors(eligibleAdvisors);
        }
        return eligibleAdvisors;
    }

最终的筛选规则在AopUtils中：

public static List<Advisor> findAdvisorsThatCanApply(List<Advisor> candidateAdvisors, Class<?> clazz) {
        //......
        for (Advisor candidate : candidateAdvisors) {
            if (candidate instanceof IntroductionAdvisor) {
                // already processed
                continue;
            } 
            //调用 canApply 方法，遍历所有的方法进行匹配
            if (canApply(candidate, clazz, hasIntroductions)) {
                eligibleAdvisors.add(candidate);
            }
        }
        //......
    }

调用canApply方法，遍历被代理类的所有的方法，跟进切面表达式进行匹配，如果有一个方法匹配到，也就意味着该类会被代理。
匹配方法是借助org.aspectj.weaver.internal.tools实现，也就是AspectJ框架中的工具类，有兴趣的可以自行查看。

重点来看一下代理生成方式：

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.");
            }
            //如果代理目标是接口或者Proxy类型，则走jdk类型
            if (targetClass.isInterface() || Proxy.isProxyClass(targetClass)) {
                return new JdkDynamicAopProxy(config);
            }
            return new ObjenesisCglibAopProxy(config);
        }
        else {
            return new JdkDynamicAopProxy(config);
        }
    }

上述方法通过三个变量来进行筛选代理方法：

·      optimize：官方文档翻译为设置代理是否应执行积极的优化，默认为false。

·      proxyTargetClass：这个在上面已经提到了，AopAutoConfiguration中指定，默认为true，也就是选择使用 cglib 代理。可以看到该变量和optimize意义一样，之所以这么做，个人理解是为了可以在不同的场景中使用。

·      hasNoUserSuppliedProxyInterfaces：是否指定了实现接口。什么意思呢？

hasNoUserSuppliedProxyInterfaces方法如下：

private boolean hasNoUserSuppliedProxyInterfaces(AdvisedSupport config) {

            Class<?>[] ifcs = config.getProxiedInterfaces();

            return (ifcs.length == 0 || (ifcs.length == 1 && SpringProxy.class.isAssignableFrom(ifcs[0])));

      }

主要就是判断AdvisedSupport中interfaces变量中是否设置了接口，

意思是如果一个类实现了接口，把接口设置到该方法的变量中，但是不是一定会设置到该变量中，具体设置接口的代码如下：

protected Object createProxy(

                  Class<?> beanClass, String beanName, Object[] specificInterceptors, TargetSource targetSource) {

            //......

            if (!proxyFactory.isProxyTargetClass()) {

                  if (shouldProxyTargetClass(beanClass, beanName)) {

                        proxyFactory.setProxyTargetClass(true);

                  }

                  else {

                        evaluateProxyInterfaces(beanClass, proxyFactory);

                  }

            }

            //......

      }

可以看到如果proxyTargetClass为 true，上述方法将不再执行，也就意味着interfaces变量不再赋值。同时，只要为类代理，默认会走 CGLIB 方式。

三：Spring Boot 1.x 版本和 2.x 版本中 AOP配置变动

配置类AopAutoConfiguration：

1.5x版本：

    @Configuration

    @EnableAspectJAutoProxy(proxyTargetClass = true)

    @ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class", havingValue = "true", matchIfMissing = false)

    public static class CglibAutoProxyConfiguration {

    }

2.x版本：

    @Configuration

    @EnableAspectJAutoProxy(proxyTargetClass = true)

    @ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class", havingValue = "true", matchIfMissing = true)

    public static class CglibAutoProxyConfiguration {

    }

在SpringBoot2.x中最主要的变化就是proxy-target-class默认为true，意味着类代理的时候全部走cglib代理方式，只有为接口代理时才走jdk代理(注意：这里为接口代理，不是指代理目标类是否实现了接口)。所以，在使用springboot2.x的版本中，除了代理目标类是接口外，其余的代理方式全部采用cglib类型。

总结 

Springboot通过自动装配AopAutoConfiguration配置类，默认自动开启 AOP 功能。通过注册AnnotationAwareAspectJAutoProxyCreator类，来扫描创建所有的Advisor，再通过 Advisor在 Spring IOC的扩展接口中，通过各种设置的匹配规则，来判断是否设置代理，最终生成代理类，注入容器 Spring中

具体代理类如何调用执行呢？后面会详细介绍。