AOP源码解析之二-创建AOP代理前传，获取AOP信息。

Posted 2022-03-22 程序员田同学

AOP源码解析之二-创建AOP代理前传，获取AOP信息。

上篇文章对AOP的基本概念说清楚了，那么接下来的AOP还剩下两个大的步骤获取定义的AOP信息，生成代理对象扔到beanFactory中。

本篇文章重点对前半部分，如何获取到AOP信息的过程解读。

在Spring的核心方法Refresh方法中，aop是在

Object bean = resolveBeforeInstantiation(beanName, mbdToUse);

开始切入的，该文章就开始深入这个方法进行解析。

@Nullable
    protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) 
        Object bean = null;
        // 检测是否被解析过
        if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved)) 
            // Make sure bean class is actually resolved at this point.
            // hasInstantiationAwareBeanPostProcessors()是来判断容器中是否有InstantiationAwareBeanPostProcessor的实现bean
            // AOP切面后置处理器AspectJAwareAdvisorAutoProxyCreator就实现了InstantiationAwareBeanPostProcessor接口
            if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) 
                Class<?> targetType = determineTargetType(beanName, mbd);
                if (targetType != null) 
                    // 执行实现了InstantiationAwareBeanPostProcessor接口的BeanPostProcessor中的前置处理方法postProcessBeforeInstantiation方法
                    bean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName);
                    if (bean != null) 
                        // 执行实现了InstantiationAwareBeanPostProcessor接口的BeanPostProcessor中的后置处理方法postProcessAfterInitialization方法
                        bean = applyBeanPostProcessorsAfterInitialization(bean, beanName);
                    
                
            
            mbd.beforeInstantiationResolved = (bean != null);
        
        return bean;
    

见名知意，resolveBeforeInstantiation（执行初始化前方法），这一步主要判断一下工厂中是否有 InstantiationAwareBeanPostProcessor 的实现bean。InstantiationAwareBeanPostProcessor 应该是AOP最核心的接口了。

我们看一下InstantiationAwareBeanPostProcessor 的继承结构。

我们详细的说下InstantiationAwareBeanPostProcessor 这个接口。

public interface InstantiationAwareBeanPostProcessor extends BeanPostProcessor 

   Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) throws BeansException;

   boolean postProcessAfterInstantiation(Object bean, String beanName) throws BeansException;

   PropertyValues postProcessPropertyValues(
         PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeansException;

它和 BeanPostProcessor 的方法非常相似，而且它还继承了 BeanPostProcessor。

下面是 BeanPostProcessor 中的两个方法：

Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException;
Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException;

发现没有，InstantiationAwareBeanPostProcessor 是 Instantiation，BeanPostProcessor 是 Initialization，它代表的是 bean 在实例化完成并且属性注入完成，在执行 init-method 的前后进行作用的。

而 InstantiationAwareBeanPostProcessor 的执行时机要前面一些，我们回到refresh方法的doCreateBean中看一下。

看到这读者想必对于aop的执行时机已经模模糊糊的心里有个大概了。

我们定义的环绕通知，创建代理一定是在postProcessBeforeInitialization完成的。

我们的重点就是看看postProcessBeforeInitialization的方法中的通知怎么获取，怎么创建代理对象的进行详细的解读。

本文先对前半部分解读。

我们继续看postProcessBeforeInitialization有哪些实现类。

我们重点看AbstractAutoProxyCreator的实现类。

@Override
    public Object postProcessAfterInitialization(@Nullable Object bean, String beanName) 
        if (bean != null) 
            Object cacheKey = getCacheKey(bean.getClass(), beanName);
            // 1.判断当前bean是否需要被代理，如果需要则进行封装
            if (this.earlyProxyReferences.remove(cacheKey) != bean) 
                //1.判断当前bean是否需要被代理，如果需要则进行封装
                return wrapIfNecessary(bean, beanName, cacheKey);
            
        
        return bean;
    

如果需要代理执行继wrapIfNecessary方法。

//这个方法将返回代理类
    protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) 
        // 1.判断当前bean是否在targetSourcedBeans缓存中存在（已经处理过），如果存在，则直接返回当前bean
        if (StringUtils.hasLength(beanName) && this.targetSourcedBeans.contains(beanName)) 
            return bean;
        
        // 2.在advisedBeans缓存中存在，并且value为false，则代表无需处理
        if (Boolean.FALSE.equals(this.advisedBeans.get(cacheKey))) 
            return bean;
        
        // 3.bean的类是aop基础设施类 || bean应该跳过，则标记为无需处理，并返回
        if (isInfrastructureClass(bean.getClass()) || shouldSkip(bean.getClass(), beanName)) 
            this.advisedBeans.put(cacheKey, Boolean.FALSE);
            return bean;
        

        // Create proxy if we have advice.
        // 4.获取当前bean的Advices和Advisors
        Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
        // 5.如果存在增强器则创建代理
        if (specificInterceptors != DO_NOT_PROXY) 
            this.advisedBeans.put(cacheKey, Boolean.TRUE);
            // 创建代理...创建代理...创建代理...
            // 5.1 创建代理对象：这边SingletonTargetSource的target属性存放的就是我们原来的bean实例（也就是被代理对象），
            // 用于最后增加逻辑执行完毕后，通过反射执行我们真正的方法时使用（method.invoke(bean, args)）
            Object proxy = createProxy(
                    bean.getClass(), beanName, specificInterceptors, new SingletonTargetSource(bean));
            // 5.2 创建完代理后，将cacheKey -> 代理类的class放到缓存
            this.proxyTypes.put(cacheKey, proxy.getClass());
            return proxy;
        
        // 6.标记为无需处理
        this.advisedBeans.put(cacheKey, Boolean.FALSE);
        return bean;
    

4.1 我们先查看第一条主线，获取当前bean的Advices和Advisors

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

        // 1.找到符合条件的Advisor
        List<Advisor> advisors = findEligibleAdvisors(beanClass, beanName);
        if (advisors.isEmpty()) 
            // 2.如果没有符合条件的Advisor，则返回null
            return DO_NOT_PROXY;
        
        return advisors.toArray();
    

注：Advisors即是aop的环绕通知。

    protected List<Advisor> findEligibleAdvisors(Class<?> beanClass, String beanName) 
        // 1.查找所有的候选Advisor
        List<Advisor> candidateAdvisors = findCandidateAdvisors();
        // 2.从所有候选的Advisor中找出符合条件的
        List<Advisor> eligibleAdvisors = findAdvisorsThatCanApply(candidateAdvisors, beanClass, beanName);
        // 3.扩展方法，留个子类实现
        extendAdvisors(eligibleAdvisors);
        if (!eligibleAdvisors.isEmpty()) 
            // 4.对符合条件的Advisor进行排序
            eligibleAdvisors = sortAdvisors(eligibleAdvisors);
        
        return eligibleAdvisors;
    

这一步所做的事很简单，就是查找所有候选的Advisor，但是调用链路特别的长，如果将这些彻底搞明白，还是需要耗费一番功夫的，读者可以选择深入程度。

1、寻找可用的Advisor

public List<Advisor> findAdvisorBeans() 
        // Determine list of advisor bean names, if not cached already.
        // 1.确认advisor的beanName列表，优先从缓存中拿
        String[] 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!
            //  1.1 如果缓存为空，则获取class类型为Advisor的所有bean名称
            advisorNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(
                    this.beanFactory, Advisor.class, true, false);
            this.cachedAdvisorBeanNames = advisorNames;
        
        if (advisorNames.length == 0) 
            return new ArrayList<>();
        

        // 2.遍历处理advisorNames
        List<Advisor> advisors = new ArrayList<>();
        for (String name : advisorNames) 
            if (isEligibleBean(name)) 
                // 2.1 跳过当前正在创建的advisor
                if (this.beanFactory.isCurrentlyInCreation(name)) 
                    if (logger.isTraceEnabled()) 
                        logger.trace("Skipping currently created advisor " + name + "");
                    
                
                else 
                    try 
                        // 2.2 通过beanName获取对应的bean对象，并添加到advisors
                        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.isTraceEnabled()) 
                                    logger.trace("Skipping advisor " + name +
                                            " with dependency on currently created bean: " + ex.getMessage());
                                
                                // Ignore: indicates a reference back to the bean were trying to advise.
                                // We want to find advisors other than the currently created bean itself.
                                continue;
                            
                        
                        throw ex;
                    
                
            
        
        // 3.返回符合条件的advisor列表
        return advisors;
    

/**
 * 找到符合条件的Advisor
 * @return
 */
@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;

public List<Advisor> buildAspectJAdvisors() 
   List<String> aspectNames = this.aspectBeanNames;

   // 1.如果aspectNames为空，则进行解析
   if (aspectNames == null) 
      synchronized (this) 
         aspectNames = this.aspectBeanNames;
         if (aspectNames == null) 
            List<Advisor> advisors = new ArrayList<>();
            aspectNames = new ArrayList<>();
            // 1.1 获取所有的beanName
            String[] beanNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(
                  this.beanFactory, Object.class, true, false);
            // 1.2 循环遍历所有的beanName，找出对应的增强方法
            for (String beanName : beanNames) 
               // 1.3 不合法的beanName则跳过，默认返回true，子类可以覆盖实现，AnnotationAwareAspectJAutoProxyCreator
               // 实现了自己的逻辑，支持使用includePatterns进行筛选
               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beanName对应的bean的类型
               Class<?> beanType = this.beanFactory.getType(beanName);
               if (beanType == null) 
                  continue;
               
               // 1.4 如果beanType存在Aspect注解则进行处理
               if (this.advisorFactory.isAspect(beanType)) 
                  // 将存在Aspect注解的beanName添加到aspectNames列表
                  aspectNames.add(beanName);
                  // 新建切面元数据
                  AspectMetadata amd = new AspectMetadata(beanType, beanName);
                  if (amd.getAjType().getPerClause().getKind() == PerClauseKind.SINGLETON) 
                     // 使用BeanFactory和beanName创建一个BeanFactoryAspectInstanceFactory，主要用来创建切面对象实例
                     MetadataAwareAspectInstanceFactory factory =
                           new BeanFactoryAspectInstanceFactory(this.beanFactory, beanName);
                     // 1.5 解析标记AspectJ注解中的增强方法*********************
                     List<Advisor> classAdvisors = this.advisorFactory.getAdvisors(factory);
                     // 1.6 放到缓存中
                     if (this.beanFactory.isSingleton(beanName)) 
                        // 如果beanName是单例则直接将解析的增强方法放到缓存
                        this.advisorsCache.put(beanName, classAdvisors);
                     
                     else 
                        // 如果不是单例，则将factory放到缓存，之后可以通过factory来解析增强方法
                        this.aspectFactoryCache.put(beanName, factory);
                     
                     // 1.7 将解析的增强器添加到advisors
                     advisors.addAll(classAdvisors);
                  
                  else 
                     // Per target or per this.
                     if (this.beanFactory.isSingleton(beanName)) 
                        // 名称为beanName的Bean是单例，但切面实例化模型不是单例，则抛异常
                        throw new IllegalArgumentException("Bean with name " + beanName +
                              " is a singleton, but aspect instantiation model is not singleton");
                     
                     MetadataAwareAspectInstanceFactory factory =
                           new PrototypeAspectInstanceFactory(this.beanFactory, beanName);
                     // 将factory放到缓存，之后可以通过factory来解析增强方法
                     this.aspectFactoryCache.put(beanName, factory);
                     // 解析标记AspectJ注解中的增强方法，并添加到advisors中
                     advisors.addAll(this.advisorFactory.getAdvisors(factory));
                  
               
            
            // 1.9 将解析出来的切面beanName放到缓存aspectBeanNames
            this.aspectBeanNames = aspectNames;
            return advisors;
         
      
   

   if (aspectNames.isEmpty()) 
      return Collections.emptyList();
   
   List<Advisor> advisors = new ArrayList<>();
   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));
      
   
   // 1.10 最后返回解析出来的增强器
   return advisors;

@Override
public List<Advisor> getAdvisors(MetadataAwareAspectInstanceFactory aspectInstanceFactory) 
   // 1.前面我们将beanClass和beanName封装成了aspectInstanceFactory的AspectMetadata属性，
   // 这边可以通过AspectMetadata属性重新获取到当前处理的切面类
   Class<?> aspectClass = aspectInstanceFactory.getAspectMetadata().getAspectClass();
   // 2.获取当前处理的切面类的名字
   String aspectName = aspectInstanceFactory.getAspectMetadata().getAspectName();
   // 3.校验切面类
   validate(aspectClass);

   // We need to wrap the MetadataAwareAspectInstanceFactory with a decorator
   // so that it will only instantiate once.
   // 4.使用装饰器包装MetadataAwareAspectInstanceFactory，以便它只实例化一次。
   MetadataAwareAspectInstanceFactory lazySingletonAspectInstanceFactory =
         new LazySingletonAspectInstanceFactoryDecorator(aspectInstanceFactory);

   List<Advisor> advisors = new ArrayList<>();
   // 5.获取切面类中的方法（也就是我们用来进行逻辑增强的方法，被@Around、@After等注解修饰的方法，使用@Pointcut的方法不处理）
   for (Method method : getAdvisorMethods(aspectClass)) 
      // 6.处理method，获取增强器
      Advisor advisor = getAdvisor(method, lazySingletonAspectInstanceFactory, advisors.size(), aspectName);
      if (advisor != null) 
         // 7.如果增强器不为空，则添加到advisors
         advisors.add(advisor);
      
   

   // If its a per target aspect, emit the dummy instantiating aspect.
   if (!advisors.isEmpty() && lazySingletonAspectInstanceFactory.getAspectMetadata().isLazilyInstantiated()) 
      // 8.如果寻找的增强器不为空而且又配置了增强延迟初始化，那么需要在首位加入同步实例化增强器（用以保证增强使用之前的实例化）
      Advisor instantiationAdvisor = new SyntheticInstantiationAdvisor(lazySingletonAspectInstanceFactory);
      advisors.add(0, instantiationAdvisor);
   

   // Find introduction fields.
   // 9.获取DeclareParents注解
   for (Field field : aspectClass.getDeclaredFields()) 
      Advisor advisor = getDeclareParentsAdvisor(field);
      if (advisor != null) 
         advisors.add(advisor);
      
   

   return advisors;

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

   // 1.校验切面类
   validate(aspectInstanceFactory.getAspectMetadata().getAspectClass());

   // 2.AspectJ切点信息的获取（例如：表达式），就是指定注解的表达式信息的获取，
   // 如：@Around("execution(* com.joonwhee.open.aop.*.*(..))")
   AspectJExpressionPointcut expressionPointcut = getPointcut(
         candidateAdviceMethod, aspectInstanceFactory.getAspectMetadata().getAspectClass());
   // 3.如果expressionPointcut为null，则直接返回null
   if (expressionPointcut == null) 
      return null;
   

   // 4.根据切点信息生成增强器
   return new InstantiationModelAwarePointcutAdvisorImpl(expressionPointcut, candidateAdviceMethod,
         this, aspectInstanceFactory, declarationOrderInAspect, aspectName);

获取到@Before, @Around, @After, @AfterReturning, @AfterThrowing, @Pointcut定义注解信息

@Nullable
private AspectJExpressionPointcut getPointcut(Method candidateAdviceMethod, Class<?> candidateAspectClass) 
   // 1.查找并返回给定方法的第一个AspectJ注解（@Before, @Around, @After, @AfterReturning, @AfterThrowing, @Pointcut）
   // 因为我们之前把@Pointcut注解的方法跳过了，所以这边必然不会获取到@Pointcut注解
   AspectJAnnotation<?> aspectJAnnotation =
         AbstractAspectJAdvisorFactory.findAspectJAnnotationOnMethod(candidateAdviceMethod);
   // 2.如果方法没有使用AspectJ的注解，则返回null
   if (aspectJAnnotation == null) 
      return null;
   

   // 3.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，
   // 例如：@Around("execution(* com.joonwhee.open.aop.*.*(..))")，得到：execution(* com.joonwhee.open.aop.*.*(..))
   ajexp.setExpression(aspectJAnnotation.getPointcutExpression());
   if (this.beanFactory != null) 
      ajexp.setBeanFactory(this.beanFactory);
   
   return ajexp;

@Nullable
protected static AspectJAnnotation<?> findAspectJAnnotationOnMethod(Method method) 
   // 设置要查找的注解类
   for (Class<?> clazz : ASPECTJ_ANNOTATION_CLASSES) 
      // 查找方法上是否存在当前遍历的注解，如果有则返回
      AspectJAnnotation<?> foundAnnotation = findAnnotation(method, (Class<Annotation>) clazz);
      if (foundAnnotation != null) 
         return foundAnnotation;
      
   
   return null;

2、获取切点以后就需要生成增强器了。

 new InstantiationModelAwarePointcutAdvisorImpl(expressionPointcut, candidateAdviceMethod,
                this, aspectInstanceFactory, declarationOrderInAspect, aspectName)

/**
 * 根据切点信息生成增强器
 * @param declaredPointcut
 * @param aspectJAdviceMethod
 * @param aspectJAdvisorFactory
 * @param aspectInstanceFactory
 * @param declarationOrder
 * @param aspectName
 */
public InstantiationModelAwarePointcutAdvisorImpl(AspectJExpressionPointcut declaredPointcut,
      Method aspectJAdviceMethod, AspectJAdvisorFactory aspectJAdvisorFactory,
      MetadataAwareAspectInstanceFactory aspectInstanceFactory, int declarationOrder, String aspectName) 

   // 1.简单的将信息封装在类的实例中
   this.declaredPointcut = declaredPointcut;
   this.declaringClass = aspectJAdviceMethod.getDeclaringClass();
   this.methodName = aspectJAdviceMethod.getName();
   this.parameterTypes = aspectJAdviceMethod.getParameterTypes();
   // aspectJAdviceMethod保存的是我们用来进行逻辑增强的方法（@Around、@After等修饰的方法）
   this.aspectJAdviceMethod = aspectJAdviceMethod;
   this.aspectJAdvisorFactory = aspectJAdvisorFactory;
   this.aspectInstanceFactory = aspectInstanceFactory;
   this.declarationOrder = declarationOrder;
   this.aspectName = aspectName;

   // 2.是否需要延迟实例化
   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 its 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;
      // 3.实例化增强器：根据注解中的信息初始化对应的增强器
      this.instantiatedAdvice = instantiateAdvice(this.declaredPointcut);
   

经过以上长长的源码分析过程，就将aop的第一个大过程，获取到我们定义的@Before、@After的方法以后，进行增强，下一步就要拿到这些获取的信息去创建代理对象了。