AOP源码解析之二-创建AOP代理前传，获取AOP信息

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 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 findEligibleAdvisors(Class<?> beanClass, String beanName) {
// 1.查找所有的候选Advisor
List candidateAdvisors = findCandidateAdvisors();
// 2.从所有候选的Advisor中找出符合条件的
List 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 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 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 we’re 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 findCandidateAdvisors() {
  // Add all the Spring advisors found according to superclass rules.
  List 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 buildAspectJAdvisors() {
List 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 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 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 advisors = new ArrayList<>();
for (String aspectName : aspectNames) {
List 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 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 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 it’s 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) 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 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;
  // 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。