Spring AOP源码解读2 - 切面和增强的取得

ゝ一世哀愁。 2022-07-14 12:39 169阅读 0赞

# 前言 #

> 在[前一篇文章][Link 1] 中介绍了代理处理的入口，这一章介绍代理处理的流程之一：切面和增强的取得

# 正文 #

首先看看`getAdvicesAndAdvisorsForBean`方法的内容，这个方法的实现是在子类`AbstractAdvisorAutoProxyCreator`中完成的。这个方法里主要逻辑是在findEligibleAdvisors方法中：

protected Object[] getAdvicesAndAdvisorsForBean(Class<?> beanClass, String beanName, TargetSource targetSource) {
        List<Advisor> advisors = findEligibleAdvisors(beanClass, beanName);
        if (advisors.isEmpty()) {
            return DO_NOT_PROXY;
        }
        return advisors.toArray();
    }

findEligibleAdvisors中的主要逻辑是下面的这两个方法：

> 1.  findCandidateAdvisors：查找适合的Advisor（根据名字判断）
> 2.  findAdvisorsThatCanApply：看看Advisor能否被用在Bean上（根据Advisor的PointCut判断）

protected List<Advisor> findEligibleAdvisors(Class<?> beanClass, String beanName) {
        // 查找适合的Advisor
        List<Advisor> candidateAdvisors = findCandidateAdvisors();
        // 看看Advisor能否被用在Bean上
        List<Advisor> eligibleAdvisors = findAdvisorsThatCanApply(candidateAdvisors, beanClass, beanName);
        extendAdvisors(eligibleAdvisors);
        if (!eligibleAdvisors.isEmpty()) {
            eligibleAdvisors = sortAdvisors(eligibleAdvisors);
        }
        return eligibleAdvisors;
    }

## 1，findCandidateAdvisors取得的相关流程： ##

关于findCandidateAdvisors方法，因为是分析AspectJ的相关的处理，所以我们看一下这个方法在AnnotationAwareAspectAutoProxyCreator类中的实现

protected List<Advisor> findCandidateAdvisors() {
        // Add all the Spring advisors found according to superclass rules.
        // 当使用注释方式配置 AOP 的时候并不是丢弃了对 XML 配置的支持。
        // 在这里调用父类方法加载配置文件中的 AOP 声明
        List<Advisor> advisors = super.findCandidateAdvisors();
    
        // Build Advisors for all AspectJ aspects in the bean factory.
        // 把BeanFactory里面的所有定义切面，定义成Spring中用的切面（Advisor），并放到集合里
        advisors.addAll(this.aspectJAdvisorsBuilder.buildAspectJAdvisors());
        return advisors;
    }

public List<Advisor> buildAspectJAdvisors() {
        List<String> aspectNames = null;
    
        synchronized (this) {
            aspectNames = this.aspectBeanNames;
            if (aspectNames == null) {
                List<Advisor> advisors = new LinkedList<Advisor>();
                aspectNames = new LinkedList<String>();
    
                // beanNamesForTypeIncludingAncestors方法是通过循环所有的bean，
                // 来取得所有指定类型的子类（这里指定的类型是Object，所以是所有类）
                // 实现是DefaultListableBeanFactory#doGetBeanNamesForType方法中
                // 判断子类是使用的Class#isAssignableFrom方法
                String[] beanNames =
    BeanFactoryUtils.beanNamesForTypeIncludingAncestors(this.beanFactory, Object.class, true, false);
                for (String beanName : beanNames) {
                    // 根据Bean的名字，判断是否是适合
                    // 这个方法根据实现类不同，实现方法也不同。
                    // DefaultAdvisorAutoProxyCreator的话，判断方法内部，!isUsePrefix()永远返回True，所以这个判断总是True
                    // 具体的类型的判断是使用AbstractBeanFactory#isTypeMatch方法
                    if (!isEligibleBean(beanName)) {
                        continue;
                    }
                    // We must be careful not to instantiate beans eagerly as in this
                    // case they would be cached by the Spring container but would not
                    // have been weaved
                    // 获取对应的 bean 的类型
                    Class<?> beanType = this.beanFactory.getType(beanName);
                    if (beanType == null) {
                        continue;
                    }
    
                    // 如果beanType是Aspect类型的话
                    if (this.advisorFactory.isAspect(beanType)) {
                        aspectNames.add(beanName);
                        AspectMetadata amd = new AspectMetadata(beanType, beanName);
                        if (amd.getAjType().getPerClause().getKind() == PerClauseKind.SINGLETON) {
                            MetadataAwareAspectInstanceFactory factory =
                                    new BeanFactoryAspectInstanceFactory(this.beanFactory, beanName);
    
                            // 读取Aspect类的方法上的注解，从而生成Advisors。
                            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 {
                            // Per target or per this.
                            if (this.beanFactory.isSingleton(beanName)) {
                                throw new IllegalArgumentException("Bean with name '" + beanName +
                                        "' is a singleton, but aspect instantiation model is not singleton");
                            }
                            MetadataAwareAspectInstanceFactory factory =
                                    new PrototypeAspectInstanceFactory(this.beanFactory, beanName);
                            this.aspectFactoryCache.put(beanName, factory);
                            advisors.addAll(this.advisorFactory.getAdvisors(factory));
                        }
                    }
                }
                this.aspectBeanNames = aspectNames;
                return advisors;
            }
        }
    
        if (aspectNames.isEmpty()) {
            return Collections.emptyList();
        }
        List<Advisor> advisors = new LinkedList<Advisor>();
        for (String aspectName : aspectNames) {
            List<Advisor> cachedAdvisors = this.advisorsCache.get(aspectName);
            if (cachedAdvisors != null) {
                advisors.addAll(cachedAdvisors);
            }
            else {
                MetadataAwareAspectInstanceFactory factory = this.aspectFactoryCache.get(aspectName);
                advisors.addAll(this.advisorFactory.getAdvisors(factory));
            }
        }
        return advisors;
    }

在上面的步骤中最为重要也最为繁琐的就是通知器的获取。而这一功能看源码，是由 AspectJAdviosrFactory 接口的 getAdvisors 方法来完成的。  
我们根据 AspectJAdviosrFactory 接口的继承关系，在 ReflectiveAspectJAdvisorFactory 类中找到了他的 getAdvisors 实现类。

public List<Advisor> getAdvisors(MetadataAwareAspectInstanceFactory maaif) {
        final Class<?> aspectClass = maaif.getAspectMetadata().getAspectClass();
        final String aspectName = maaif.getAspectMetadata().getAspectName();
        validate(aspectClass);
    
        // We need to wrap the MetadataAwareAspectInstanceFactory with a decorator
        // so that it will only instantiate once.
        final MetadataAwareAspectInstanceFactory lazySingletonAspectInstanceFactory =
                new LazySingletonAspectInstanceFactoryDecorator(maaif);
    
        final List<Advisor> advisors = new LinkedList<Advisor>();
        // 对aspectClass的每一个带有注解的方法进行循环（带有PointCut注解的方法除外），取得Advisor，并添加到集合里。
        // (这是里应该是取得Advice，然后取得我们自己定义的切面类中PointCut，组合成Advisor)
        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.
        // 查找引入增强，并加到集合中。
        for (Field field : aspectClass.getDeclaredFields()) {
            Advisor advisor = getDeclareParentsAdvisor(field);
            if (advisor != null) {
                advisors.add(advisor);
            }
        }
    
        return advisors;
    }

// 这个方法，通过很巧妙的使用接口，定义一个匿名回调，把带有注解的Method都取得出来，放到集合里，有兴趣可以看看。
    private List<Method> getAdvisorMethods(Class<?> aspectClass) {
        final List<Method> methods = new LinkedList<Method>();
        ReflectionUtils.doWithMethods(aspectClass, new ReflectionUtils.MethodCallback() {
            @Override
            public void doWith(Method method) throws IllegalArgumentException {
                // Exclude pointcuts
                if (AnnotationUtils.getAnnotation(method, Pointcut.class) == null) {
                    methods.add(method);
                }
            }
        });
        Collections.sort(methods, METHOD_COMPARATOR);
        return methods;
    }

普通增强器的获取逻辑通过 getAdvisor 方法来实现，实现步骤包括对切点的注解以及根据注解信息生成增强。

public Advisor getAdvisor(Method candidateAdviceMethod, MetadataAwareAspectInstanceFactory aif,
            int declarationOrderInAspect, String aspectName) {
    
        validate(aif.getAspectMetadata().getAspectClass());
        // 获取PointCut信息（主要是PointCut里的表达式）
        // 把Method对象也传进去的目的是，比较Method对象上的注解，是不是下面注解其中一个
        // 如果不是，返回null；如果是，就把取得PointCut内容包装返回
        // 被比较注解：Before.class, Around.class, After.class, AfterReturning.class, AfterThrowing.class, Pointcut.class
        AspectJExpressionPointcut ajexp =
                getPointcut(candidateAdviceMethod, aif.getAspectMetadata().getAspectClass());
        if (ajexp == null) {
            return null;
        }
        // 根据PointCut信息生成增强器
        return new InstantiationModelAwarePointcutAdvisorImpl(
                this, ajexp, aif, candidateAdviceMethod, declarationOrderInAspect, aspectName);
    }

private AspectJExpressionPointcut getPointcut(Method candidateAdviceMethod, Class<?> candidateAspectClass) {
        // 获取方法上的注解
        // 比较Method对象上的注解，是不是下面注解其中一个，如果不是返回null
        // 被比较注解：Before.class, Around.class, After.class, AfterReturning.class, AfterThrowing.class, Pointcut.class
        AspectJAnnotation<?> aspectJAnnotation =
    AbstractAspectJAdvisorFactory.findAspectJAnnotationOnMethod(candidateAdviceMethod);
        if (aspectJAnnotation == null) {
            return null;
        }
        // 使用AspectJExpressionPointcut 实例封装获取的信息
        AspectJExpressionPointcut ajexp =
                new AspectJExpressionPointcut(candidateAspectClass, new String[0], new Class<?>[0]);
    
        // 提取得到的注解中的表达式如：
        // @Pointcut（"execution（* test.TestBean.*(..)）"）
        ajexp.setExpression(aspectJAnnotation.getPointcutExpression());
        return ajexp;
    }

protected static AspectJAnnotation<?> findAspectJAnnotationOnMethod(Method method) {
        // 设置要查找的注解类，看看方法的上注解是不是这些注解其中之一
        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;
    }

// 获取指定方法上的注解并使用 AspectJAnnotation 封装
    private static <A extends Annotation> AspectJAnnotation<A> findAnnotation(Method method, Class<A> toLookFor) {
        A result = AnnotationUtils.findAnnotation(method, toLookFor);
        if (result != null) {
            return new AspectJAnnotation<A>(result);
        }
        else {
            return null;
        }
    }

上面的`getAdvisor`方法的最后一行代码是  
`return new InstantiationModelAwarePointcutAdvisorImpl( this, ajexp, aif, candidateAdviceMethod, declarationOrderInAspect, aspectName)`  
这行代码作用是根据切点信息生成增强。所有的增强都由 `Advisor` 的实现类`InstantiationModelAwarePointcutAdvisorImpl` 统一封装的。

public InstantiationModelAwarePointcutAdvisorImpl(AspectJAdvisorFactory af, AspectJExpressionPointcut ajexp,
            MetadataAwareAspectInstanceFactory aif, Method method, int declarationOrderInAspect, String aspectName) {
    
        this.declaredPointcut = ajexp;
        this.method = method;
        this.atAspectJAdvisorFactory = af;
        this.aspectInstanceFactory = aif;
        this.declarationOrder = declarationOrderInAspect;
        this.aspectName = aspectName;
    
        if (aif.getAspectMetadata().isLazilyInstantiated()) {
            // Static part of the pointcut is a lazy type.
            Pointcut preInstantiationPointcut =
                    Pointcuts.union(aif.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, aif);
            this.lazy = true;
        }
        else {
            // A singleton aspect.
            // 初始化Advice
            this.instantiatedAdvice = instantiateAdvice(this.declaredPointcut);
            this.pointcut = declaredPointcut;
            this.lazy = false;
        }
    }

在封装过程中只是简单地将信息封装在类的实例中而已，所有的信息单纯地赋值，在实例初始化的过程中还完成了对于增强器的初始化。因为不同的增强所体现的逻辑是不同的，而根据注解中的信息初始化对应的增强器是在 instantiateAdvice 方法中完成的。

private Advice instantiateAdvice(AspectJExpressionPointcut pcut) {
        return this.atAspectJAdvisorFactory.getAdvice(
            this.method, pcut, this.aspectInstanceFactory, this.declarationOrder, this.aspectName);
    }

public Advice getAdvice(Method candidateAdviceMethod, AspectJExpressionPointcut ajexp,
        MetadataAwareAspectInstanceFactory aif, int declarationOrderInAspect, String aspectName) {
    
        Class<?> candidateAspectClass = aif.getAspectMetadata().getAspectClass();
        validate(candidateAspectClass);
    
        AspectJAnnotation<?> aspectJAnnotation =
                AbstractAspectJAdvisorFactory.findAspectJAnnotationOnMethod(candidateAdviceMethod);
        if (aspectJAnnotation == null) {
            return null;
        }
    
        // If we get here, we know we have an AspectJ method.
        // Check that it's an AspectJ-annotated class
        if (!isAspect(candidateAspectClass)) {
            throw new AopConfigException("Advice must be declared inside an aspect type: " +
                    "Offending method '" + candidateAdviceMethod + "' in class [" +
                    candidateAspectClass.getName() + "]");
        }
    
        if (logger.isDebugEnabled()) {
            logger.debug("Found AspectJ method: " + candidateAdviceMethod);
        }
    
        AbstractAspectJAdvice springAdvice;
    
        // 根据不同的注解类封装增强器
        switch (aspectJAnnotation.getAnnotationType()) {
            case AtBefore:
                springAdvice = new AspectJMethodBeforeAdvice(candidateAdviceMethod, ajexp, aif);
                break;
            case AtAfter:
                springAdvice = new AspectJAfterAdvice(candidateAdviceMethod, ajexp, aif);
                break;
            case AtAfterReturning:
                springAdvice = new AspectJAfterReturningAdvice(candidateAdviceMethod, ajexp, aif);
                AfterReturning afterReturningAnnotation = (AfterReturning) aspectJAnnotation.getAnnotation();
                if (StringUtils.hasText(afterReturningAnnotation.returning())) {
                    springAdvice.setReturningName(afterReturningAnnotation.returning());
                }
                break;
            case AtAfterThrowing:
                springAdvice = new AspectJAfterThrowingAdvice(candidateAdviceMethod, ajexp, aif);
                AfterThrowing afterThrowingAnnotation = (AfterThrowing) aspectJAnnotation.getAnnotation();
                if (StringUtils.hasText(afterThrowingAnnotation.throwing())) {
                    springAdvice.setThrowingName(afterThrowingAnnotation.throwing());
                }
                break;
            case AtAround:
                springAdvice = new AspectJAroundAdvice(candidateAdviceMethod, ajexp, aif);
                break;
            case AtPointcut:
                if (logger.isDebugEnabled()) {
                    logger.debug("Processing pointcut '" + candidateAdviceMethod.getName() + "'");
                }
                return null;
            default:
                throw new UnsupportedOperationException(
                        "Unsupported advice type on method " + candidateAdviceMethod);
    }

## 2，findAdvisorsThatCanApply相关流程 ##

findCandidateAdvisors通过名字匹配，找到了一些适合的Advisor，我们再来看看findAdvisorsThatCanApply方法。这个方法是作用就是根据PointCut，看目标类是不是符合代理对象的条件，如果符合就把Advisor加到集合中，最后返回集合。

public static List<Advisor> findAdvisorsThatCanApply(List<Advisor> candidateAdvisors, Class<?> clazz) {
        if (candidateAdvisors.isEmpty()) {
            return candidateAdvisors;
        }
        List<Advisor> eligibleAdvisors = new LinkedList<Advisor>();
        // 这个循环主要是处理IntroductionAdvisor类型的Advisor，看适合不适合
        for (Advisor candidate : candidateAdvisors) {
            if (candidate instanceof IntroductionAdvisor && canApply(candidate, clazz)) {
                eligibleAdvisors.add(candidate);
            }
        }
        boolean hasIntroductions = !eligibleAdvisors.isEmpty();
        // 这个循环处理非IntroductionAdvisor类型的Advisor
        for (Advisor candidate : candidateAdvisors) {
            if (candidate instanceof IntroductionAdvisor) {
                // already processed
                continue;
            }
            // 在这里判断是否适合
            if (canApply(candidate, clazz, hasIntroductions)) {
                eligibleAdvisors.add(candidate);
            }
        }
        return eligibleAdvisors;
    }
    
       // 根据Advisor类型不同，判断方法也不同，我们看一下最常用的PointCut类型
    public static boolean canApply(Advisor advisor, Class<?> targetClass, boolean hasIntroductions) {
        if (advisor instanceof IntroductionAdvisor) {
            return ((IntroductionAdvisor) advisor).getClassFilter().matches(targetClass);
        }
        // PointCut类型的Advisor的判断方法
        else if (advisor instanceof PointcutAdvisor) {
            PointcutAdvisor pca = (PointcutAdvisor) advisor;
            return canApply(pca.getPointcut(), targetClass, hasIntroductions);
        }
        else {
            // It doesn't have a pointcut so we assume it applies.
            return true;
        }
    }

// 看了实现方式，判断方式就是通过PointCut进行判断的
    public static boolean canApply(Pointcut pc, Class<?> targetClass, boolean hasIntroductions) {
        Assert.notNull(pc, "Pointcut must not be null");
        if (!pc.getClassFilter().matches(targetClass)) {
            return false;
        }
    
        MethodMatcher methodMatcher = pc.getMethodMatcher();
        IntroductionAwareMethodMatcher introductionAwareMethodMatcher = null;
        if (methodMatcher instanceof IntroductionAwareMethodMatcher) {
            introductionAwareMethodMatcher = (IntroductionAwareMethodMatcher) methodMatcher;
        }
    
        Set<Class<?>> classes = new LinkedHashSet<Class<?>>(ClassUtils.getAllInterfacesForClassAsSet(targetClass));
        classes.add(targetClass);
        for (Class<?> clazz : classes) {
            Method[] methods = clazz.getMethods();
            for (Method method : methods) {
                if ((introductionAwareMethodMatcher != null &&
                        introductionAwareMethodMatcher.matches(method, targetClass, hasIntroductions)) ||
                        methodMatcher.matches(method, targetClass)) {
                    return true;
                }
            }
        }
    
        return false;
    }

到此为止，切面和增强的取得相关逻辑讲完了，下一章讲解如何使用找到切面，来创建代理。

[Link 1]: http://blog.csdn.net/hotdust/article/details/53524990