spring AOP 注解式 原理解析

开启注解需要在application.xml里配置<aop:aspectj-autoproxy/>
这个标签是spring AOP自定义的一个标签。我们从<aop:aspectj-autoproxy/>标签的解析入手，来分析 spring 对于注解实现AOP的装配过程和运行机制

<aop:aspectj-autoproxy/>标签解析

AopNamespaceHandler是解析AOP标签的入口，我们可以只关注第二个注册了一个，解析“aspectj-autoproxy”标签的AspectJAutoProxyBeanDefinitionParser解析器。

源码&&方法调用：

AopNamespaceHandler:

@Override
public void init() {
    // In 2.0 XSD as well as in 2.1 XSD.
    registerBeanDefinitionParser("config", new ConfigBeanDefinitionParser());
    registerBeanDefinitionParser("aspectj-autoproxy", new AspectJAutoProxyBeanDefinitionParser());
    registerBeanDefinitionDecorator("scoped-proxy", new ScopedProxyBeanDefinitionDecorator());
        
    // Only in 2.0 XSD: moved to context namespace as of 2.1
    registerBeanDefinitionParser("spring-configured", new SpringConfiguredBeanDefinitionParser());
}   

AspectJAutoProxyBeanDefinitionParser:

/**
parse方法的调用，是spring BeanDefinitionParser自定义标签的实现
*/
@Override
@Nullable
public BeanDefinition parse(Element element, ParserContext parserContext) {
    AopNamespaceUtils.registerAspectJAnnotationAutoProxyCreatorIfNecessary(parserContext, element);
    extendBeanDefinition(element, parserContext);
    return null;
}

AopNamespaceUtils:

public static void registerAspectJAnnotationAutoProxyCreatorIfNecessary(
            ParserContext parserContext, Element sourceElement) {

        BeanDefinition beanDefinition = AopConfigUtils.registerAspectJAnnotationAutoProxyCreatorIfNecessary(
                parserContext.getRegistry(), parserContext.extractSource(sourceElement));
        useClassProxyingIfNecessary(parserContext.getRegistry(), sourceElement);
        registerComponentIfNecessary(beanDefinition, parserContext);
    }

AopConfigUtils:

/**
注册AnnotationAwareAspectJAutoProxyCreator.class
*/
@Nullable
public static BeanDefinition registerAspectJAnnotationAutoProxyCreatorIfNecessary(BeanDefinitionRegistry registry,
        @Nullable Object source) {

    return registerOrEscalateApcAsRequired(AnnotationAwareAspectJAutoProxyCreator.class, registry, source);
}

@Nullable
private static BeanDefinition registerOrEscalateApcAsRequired(Class<?> cls, BeanDefinitionRegistry registry,
        @Nullable Object source) {

    Assert.notNull(registry, "BeanDefinitionRegistry must not be null");

    if (registry.containsBeanDefinition(AUTO_PROXY_CREATOR_BEAN_NAME)) {
        BeanDefinition apcDefinition = registry.getBeanDefinition(AUTO_PROXY_CREATOR_BEAN_NAME);
        if (!cls.getName().equals(apcDefinition.getBeanClassName())) {
            int currentPriority = findPriorityForClass(apcDefinition.getBeanClassName());
            int requiredPriority = findPriorityForClass(cls);
            if (currentPriority < requiredPriority) {
                apcDefinition.setBeanClassName(cls.getName());
            }
        }
        return null;
    }
    //注册bean
    RootBeanDefinition beanDefinition = new RootBeanDefinition(cls);
    beanDefinition.setSource(source);
    beanDefinition.getPropertyValues().add("order", Ordered.HIGHEST_PRECEDENCE);
    beanDefinition.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
    registry.registerBeanDefinition(AUTO_PROXY_CREATOR_BEAN_NAME, beanDefinition);
    return beanDefinition;
}

BeanDefinition中Role的定义值:
beanDefinition.setRole(BeanDefinition.ROLE_INFRASTRUCTURE)

/** 用户定义的bean
 * Role hint indicating that a {@code BeanDefinition} is a major part
 * of the application. Typically corresponds to a user-defined bean.
 */
int ROLE_APPLICATION = 0;

/** 某些复杂的配置
 * Role hint indicating that a {@code BeanDefinition} is a supporting
 * part of some larger configuration, typically an outer
 * {@link org.springframework.beans.factory.parsing.ComponentDefinition}.
 * {@code SUPPORT} beans are considered important enough to be aware
 * of when looking more closely at a particular
 * {@link org.springframework.beans.factory.parsing.ComponentDefinition},
 * but not when looking at the overall configuration of an application.
 */
int ROLE_SUPPORT = 1;

/** 完全内部使用
 * Role hint indicating that a {@code BeanDefinition} is providing an
 * entirely background role and has no relevance to the end-user. This hint is
 * used when registering beans that are completely part of the internal workings
 * of a {@link org.springframework.beans.factory.parsing.ComponentDefinition}.
 */
int ROLE_INFRASTRUCTURE = 2;

至此spring通过对<aop:aspectj-autoproxy/>的解析，在容器中注册了一个beanName为org.springframework.aop.config.internalAutoProxyCreator,类为AnnotationAwareAspectJAutoProxyCreator.class ,只供内部使用的bean。

AnnotationAwareAspectJAutoProxyCreator运行机制

AnnotationAwareAspectJAutoProxyCreator.png

从继承的关系分析：
继承ProxyConfig：spring AOP代理工厂配置
继承Aware: 回调BeanClassLoaderAware，BeanFactory
继承BeanProcessor: 后置处理器；bean初始化前、后 aop逻辑的实现（重点）
继承Ordered: Advistor 列表的排序

AOP的装配过程是从bean初始化调用postProcessBeforeInitialization开始，postProcessAfterInitialization结束的。

源码&&方法调用：

AbstractAutoProxyCreator ：

/**
Bean初始化的前置处理
对切面类 打标记，如果是spring 基础类（isInfrastructureClass）或 忽略的类（shouldSkip）
存在advisedBeans列表里为false
**/
@Override
public Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) {
        Object cacheKey = getCacheKey(beanClass, beanName);

    if (!StringUtils.hasLength(beanName) || !this.targetSourcedBeans.contains(beanName)) {
        if (this.advisedBeans.containsKey(cacheKey)) {
            return null;
        }
        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.
    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;
}

/**
统一bean命名的一个规则定义
**/
protected Object getCacheKey(Class<?> beanClass, @Nullable String beanName) {
        if (StringUtils.hasLength(beanName)) {
            return (FactoryBean.class.isAssignableFrom(beanClass) ?
                    BeanFactory.FACTORY_BEAN_PREFIX + beanName : beanName);
        }
        else {
            return beanClass;
        }
    }

/**
Bean初始化的后置处理
*/
@Override
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;
}

/**
真正处理AOP的代理生成wrapIfNecessary
getAdvicesAndAdvisorsForBean是获取符合当前bean的Advistor
**/
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. 如查有匹配当前bean的advice，则创建代理
    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;
}

AbstractAdvisorAutoProxyCreator：

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

    List<Advisor> advisors = findEligibleAdvisors(beanClass, beanName);
    if (advisors.isEmpty()) {
        return DO_NOT_PROXY;
    }
    return advisors.toArray();
}

/**
findCandidateAdvisors是获取当前BeanFactory(DefaultListableBeanFactory)的所有Advistor
findAdvisorsThatCanApply从所有的Advistor中匹配适用当前bean的Advistor
**/
protected List<Advisor> findEligibleAdvisors(Class<?> beanClass, String beanName) {
    List<Advisor> candidateAdvisors = findCandidateAdvisors();
    List<Advisor> eligibleAdvisors = findAdvisorsThatCanApply(candidateAdvisors, beanClass, beanName);
    extendAdvisors(eligibleAdvisors);
    if (!eligibleAdvisors.isEmpty()) {
        eligibleAdvisors = sortAdvisors(eligibleAdvisors);
    }
    return eligibleAdvisors;
}

AnnotationAwareAspectJAutoProxyCreator:

/**
覆写了AbstractAdvisorAutoProxyCreator中的findCandidateAdvisors方法
获取所有Advistor，在这里就会扫描所有的bean寻找@Aspect类，进一步解析
**/
@Override
protected List<Advisor> findCandidateAdvisors() {
    // Add all the Spring advisors found according to superclass rules.
    List<Advisor> advisors = super.findCandidateAdvisors();
    // Build Advisors for all AspectJ aspects in the bean factory.
    if (this.aspectJAdvisorsBuilder != null) {
        advisors.addAll(this.aspectJAdvisorsBuilder.buildAspectJAdvisors());
    }
    return advisors;
}

AspectJAwareAdvisorAutoProxyCreator：

/**
 * 覆盖了父类AbstractAdvisorAutoProxyCreator中的extendAdvisors方法
 * Adds an {@link ExposeInvocationInterceptor} to the beginning of the advice chain.
 * These additional advices are needed when using AspectJ expression pointcuts
 * and when using AspectJ-style advice.
 */
@Override
protected void extendAdvisors(List<Advisor> candidateAdvisors) {
    AspectJProxyUtils.makeAdvisorChainAspectJCapableIfNecessary(candidateAdvisors);
}

AspectJProxyUtils：

/**
自动加入 ExposeInvocationInterceptor.ADVISOR 到 advisors chain中
*/
public static boolean makeAdvisorChainAspectJCapableIfNecessary(List<Advisor> advisors) {
    // Don't add advisors to an empty list; may indicate that proxying is just not required
    if (!advisors.isEmpty()) {
        boolean foundAspectJAdvice = false;
        for (Advisor advisor : advisors) {
            // Be careful not to get the Advice without a guard, as
            // this might eagerly instantiate a non-singleton AspectJ aspect
            if (isAspectJAdvice(advisor)) {
                foundAspectJAdvice = true;
            }
        }
        if (foundAspectJAdvice && !advisors.contains(ExposeInvocationInterceptor.ADVISOR)) {
            advisors.add(0, ExposeInvocationInterceptor.ADVISOR);
            return true;
        }
    }
    return false;
}

BeanFactoryAdvisorRetrievalHelper:

/**
在缓存中匹配Advistor
**/
public List<Advisor> findAdvisorBeans() {
        // Determine list of advisor bean names, if not cached already.
        String[] advisorNames = null;
        synchronized (this) {
            advisorNames = this.cachedAdvisorBeanNames;
            if (advisorNames == null) {
                // Do not initialize FactoryBeans here: We need to leave all regular beans
                // uninitialized to let the auto-proxy creator apply to them!
                advisorNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(
                        this.beanFactory, Advisor.class, true, false);
                this.cachedAdvisorBeanNames = advisorNames;
            }
        }
        if (advisorNames.length == 0) {
            return new LinkedList<>();
        }

    List<Advisor> advisors = new LinkedList<>();
    for (String name : advisorNames) {
        if (isEligibleBean(name)) {
            if (this.beanFactory.isCurrentlyInCreation(name)) {
                if (logger.isDebugEnabled()) {
                    logger.debug("Skipping currently created advisor '" + name + "'");
                }
            }
            else {
                try {
                    advisors.add(this.beanFactory.getBean(name, Advisor.class));
                }
                catch (BeanCreationException ex) {
                    Throwable rootCause = ex.getMostSpecificCause();
                    if (rootCause instanceof BeanCurrentlyInCreationException) {
                        BeanCreationException bce = (BeanCreationException) rootCause;
                        String bceBeanName = bce.getBeanName();
                        if (bceBeanName != null && this.beanFactory.isCurrentlyInCreation(bceBeanName)) {
                            if (logger.isDebugEnabled()) {
                                logger.debug("Skipping advisor '" + name +
                                        "' with dependency on currently created bean: " + ex.getMessage());
                            }
                            // Ignore: indicates a reference back to the bean we're trying to advise.
                            // We want to find advisors other than the currently created bean itself.
                            continue;
                        }
                    }
                    throw ex;
                }
            }
        }
    }
    return advisors;
}

BeanFactoryAspectJAdvisorsBuilder :

    /**
    * 完成@Aspect和 @PointCut的解析，最终生成Advistor
    * Look for AspectJ-annotated aspect beans in the current bean factory,
    * and return to a list of Spring AOP Advisors representing them.
    * <p>Creates a Spring Advisor for each AspectJ advice method.
    * @return the list of {@link org.springframework.aop.Advisor} beans
    * @see #isEligibleBean
    */
    public List<Advisor> buildAspectJAdvisors() {
        List<String> aspectNames = this.aspectBeanNames;

        if (aspectNames == null) {
            synchronized (this) {
                aspectNames = this.aspectBeanNames;
                if (aspectNames == null) {
                    List<Advisor> advisors = new LinkedList<>();
                    aspectNames = new LinkedList<>();
                    String[] beanNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(
                            this.beanFactory, Object.class, true, false);
                    for (String beanName : beanNames) {
                        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.
                        Class<?> beanType = this.beanFactory.getType(beanName);
                        if (beanType == null) {
                            continue;
                        }
                        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);
                                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<>();
        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;
    }