Spring源码阅读--aop实现原理分析

aop实现原理简介

这里分析的是，在spring中是如何基于动态代理的思想实现aop的。为了方便了解接下来的源码分析，这里简单化了一个流程图分析aop的基本实现思想。

so，基于上面的流程，一步步分析spring源码中的aop实现方式。

采用一个简单的aop例子，利用基于注解配置方式的切面配置，分析一个简单的Before AOP例子。在spring boot下运行以下简单例子。

AOP的advisor和advice配置。

@Component
@Aspect
public class AopConfig {
    @Pointcut("execution(* com.garine.debug.testcase.model.AopObject..*(..))")
    public void mypoint(){
        //切面定义
    }
    @Before("mypoint()")
    public void doAround() throws Throwable {
        System.out.println("before logic");
    }
}

AopObject，被代理拦截对象。

@Component
public class AopObject {
    public void aoped(){
        System.out.println("logic");
    }
}

代理实现的处理器(BeanPostProcessor)

首先是第一步内容，对我们在AopConfig中的AOP配置内容进行解析并且保存到BeanFactory中，这个过程就是解析保存切面信息。

代理实现的源头–AnnotationAwareAspectJAutoProxyCreator

经过一遍的代码跟踪，我了解到注解方式的AOP配置，都离不开一个类–AnnotationAwareAspectJAutoProxyCreator，这个类继承了BeanPostProcessor接口，我们都知道BeanPostProcessor的实现类有多个执行处理节点，其中一个执行节点就是在Bean实例化之后。也就是在这个时机AnnotationAwareAspectJAutoProxyCreator拦截bean的初始化过程，根据提前解析得到的切面信息，对bean的方法进行尝试适配，如果有匹配则需要进行代理创建。

这里先分析的就是AnnotationAwareAspectJAutoProxyCreator，在bean实例化第一次查询所有切面信息时，就会解析保存Aop的信息到实例中，跟踪以下代码。

• AbstractApplicationContext#refresh （上下文初始化主干方法）

  • AbstractApplicationContext#registerBeanPostProcessors （执行实例化并保存所有实现BeanPostProcessor接口的类）

按照上面的逻辑，registerBeanPostProcessors 会比一般的bean实例化逻辑要早执行，因此我们接下来只需要分析AnnotationAwareAspectJAutoProxyCreator的初始化过程。

AnnotationAwareAspectJAutoProxyCreator的继承结构

通过上图可以知道，AnnotationAwareAspectJAutoProxyCreator是继承了BeanfactoryAware接口，所以在实例化时，会执行setFactory方法。而所有切面信息解析的执行者BeanFactoryAspectJAdvisorsBuilderAdapter初始化的时机也是在setFactory方法。

跟踪代码如下。

• AbstractAdvisorAutoProxyCreator#setBeanFactory

  • AnnotationAwareAspectJAutoProxyCreator#initBeanFactory

在这个方法里面会新建一个BeanFactoryAspectJAdvisorsBuilderAdapter，这个对象会根据Beanfactory内的aop配置信息，进行解析保存。但是需要注意，此时虽然新建了BeanFactoryAspectJAdvisorsBuilderAdapter对象.但是此时还不会马上解析aop配置，需要在第一次个普通bean实例化时才执行解析aop配置。解析的方法就是

BeanFactoryAspectJAdvisorsBuilder#buildAspectJAdvisors，会在初次执行AnnotationAwareAspectJAutoProxyCreator调用postProcessBeforeInitialization时开始执行。

protected void initBeanFactory(ConfigurableListableBeanFactory beanFactory) {
   super.initBeanFactory(beanFactory);
    //aspectJAdvisorsBuilder#buildAspectJAdvisors就是解析配置入口
   this.aspectJAdvisorsBuilder =
         new BeanFactoryAspectJAdvisorsBuilderAdapter(beanFactory, this.aspectJAdvisorFactory);
}

代理对象(Proxy)的创建

解析并缓存切面

上面提到继承结构图中，AnnotationAwareAspectJAutoProxyCreator是实现了InstantiationAwareBeanPostProcessor接口的，InstantiationAwareBeanPostProcessor接口定义的postProcessBeforeInitialization方法是一个可以对已经注入依赖属性的bean对象实例进行编辑操作的接口，会在

• AbstractAutowireCapableBeanFactory#doCreateBean

  • AbstractAutowireCapableBeanFactory#initializeBean(String, Object, RootBeanDefinition)

    • AbstractAutowireCapableBeanFactory#applyBeanPostProcessorsBeforeInstantiation

方法中执行InstantiationAwareBeanPostProcessor#postProcessBeforeInstantiation，初次初始化缓存切面信息的话就是在这个方法里面。。具体的调用链如上所示。这里的postProcessBeforeInstantiation方法实际上是AnnotationAwareAspectJAutoProxyCreator的实例进行调用，AnnotationAwareAspectJAutoProxyCreator实现InstantiationAwareBeanPostProcessor接口。

下面进入InstantiationAwareBeanPostProcessor#postProcessBeforeInitialization方法分析代码。

• AbstractAutoProxyCreator#postProcessBeforeInstantiation

  • AspectJAwareAdvisorAutoProxyCreator#shouldSkip （关键代码）

进入如下代码AbstractAutoProxyCreator，这个实例也就是之前一开始初始化的AnnotationAwareAspectJAutoProxyCreator实例，进入实例的shouldSkip 方法，

@Override
    protected boolean shouldSkip(Class<?> beanClass, String beanName) {
        // TODO: Consider optimization by caching the list of the aspect names
        //预先解析缓存切面信息
        List<Advisor> candidateAdvisors = findCandidateAdvisors();
        for (Advisor advisor : candidateAdvisors) {
            if (advisor instanceof AspectJPointcutAdvisor) {
                if (((AbstractAspectJAdvice) advisor.getAdvice()).getAspectName().equals(beanName)) {
                    return true;
                }
            }
        }
        return super.shouldSkip(beanClass, beanName);
    }

• AnnotationAwareAspectJAutoProxyCreator#findCandidateAdvisors

方法findCandidateAdvisors代码如下，这里是预先解析缓存所有切面advisor信息，注意这一步操作是在AbstractAutoProxyCreator#postProcessBeforeInitialization处理，也就是开头提到的切面解析操作，解析完成就进行缓存。

@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.
    //这里就是前面提到的BeanFactoryAspectJAdvisorsBuilder解析所有切面信息的调用点
   advisors.addAll(this.aspectJAdvisorsBuilder.buildAspectJAdvisors());
   return advisors;
}

然后继续在这里先提前看一下是如何解析aop配置的。跟踪BeanFactoryAspectJAdvisorsBuilder#buildAspectJAdvisors

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>();
          //查询出Beanfactory中所有已经注册的BeanName
         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;
            }
             //判断Bean是否是切面Bean，isAspect方法判断[标注1]
            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);
                   //解析aop class的配置，包返回Advisor对象[标注2]
                  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;
}

**[标注1]如何判断类是否是aop切面配置类？ **

通过以下代码。

@Override
public boolean isAspect(Class<?> clazz) {
   return (hasAspectAnnotation(clazz) && !compiledByAjc(clazz));
}
private boolean hasAspectAnnotation(Class<?> clazz) {
    //包含@Aspect注解
   return (AnnotationUtils.findAnnotation(clazz, Aspect.class) != null);
}

[标注2]如何解析为Advisor对象？

• ReflectiveAspectJAdvisorFactory#getAdvisors 遍历所有没被@PointCut注解标注的方法，也就是遍历切面内容方法

  • ReflectiveAspectJAdvisorFactory#getAdvisor 处理所有没被@PointCut注解标注的方法，候选切面内容方法

代码如下。

@Override
public Advisor getAdvisor(Method candidateAdviceMethod, MetadataAwareAspectInstanceFactory aspectInstanceFactory,
      int declarationOrderInAspect, String aspectName) {
   validate(aspectInstanceFactory.getAspectMetadata().getAspectClass());
   //解析判断候选方法是否有@Before，@After，@Around等注解，如果有，就继续执行新建Advisor对象。
   AspectJExpressionPointcut expressionPointcut = getPointcut(
         candidateAdviceMethod, aspectInstanceFactory.getAspectMetadata().getAspectClass());
   if (expressionPointcut == null) {
      return null;
   }
//创建advisor
   return new InstantiationModelAwarePointcutAdvisorImpl(expressionPointcut, candidateAdviceMethod,
         this, aspectInstanceFactory, declarationOrderInAspect, aspectName);
}

最终循环解析，@Before，@After，@Around等标注的方法都会新建一个Advisor对象。新建的Advisor对象都保存在BeanFactoryAspectJAdvisorsBuilder#advisorsCache中，当AnnotationAwareAspectJAutoProxyCreator拦截bean的创建过程时，从这里面适配是否有切面可用。

这里解析得到的Advisor，大概有以下信息。下面的信息中，并没有对@PointCut注解做处理，pointCut属性只得出一个”mypoint()”，此时还不知道Advisor实际对应的拦截表达式。

拦截表达式还是空的，会在AnnotationAwareAspectJAutoProxyCreator#postProcessAfterInstantiation第一次执行时解析拦截表达式。

适配切面

在AbstractAutoProxyCreator#postProcessAfterInitialization执行时，找到上面AbstractAutoProxyCreator#postProcessBeforeInitialization缓存的所有的切面信息，之后是如何进行切面适配，从而决定是否需要进行代理对象的创建呢？

在调用AbstractAutoProxyCreator#postProcessAfterInitialization方法时，进行切面适配，并且会根据适配创建代理对象。根据以下调用链。

• AbstractAutoProxyCreator#postProcessAfterInitialization

  • AbstractAutoProxyCreator#wrapIfNecessary

    protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {
       if (beanName != null && 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.
        //查找匹配切面
       Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
       if (specificInterceptors != DO_NOT_PROXY) {
          this.advisedBeans.put(cacheKey, Boolean.TRUE);
           //创建代理对象
          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#getAdvicesAndAdvisorsForBean

      • AbstractAdvisorAutoProxyCreator#findEligibleAdvisors

  protected List findEligibleAdvisors(Class<?> beanClass, String beanName) {

  //从缓存取出所有切面信息

  List candidateAdvisors = findCandidateAdvisors();

  //根据advisor信息中的表达式进行方法对class的匹配

  List eligibleAdvisors = findAdvisorsThatCanApply(candidateAdvisors, beanClass, beanName);
  extendAdvisors(eligibleAdvisors);
  if (!eligibleAdvisors.isEmpty()) {

    eligibleAdvisors = sortAdvisors(eligibleAdvisors);

  }
  return eligibleAdvisors;
  }

此时如果是第一次执行适配方法findAdvisorsThatCanApply的话，candidateAdvisors中的拦截表达式还是空的，需要进行表达式获取，也就是@Pointcut的value。spring的操作的在第一次执行findAdvisorsThatCanApply时解析获取拦截表达式的值，获得拦截表达式值之后就跟当前class的方法进行匹配看是否需要进行代理。继续往下跟踪代码

• AopUtils#canApply(org.springframework.aop.Advisor, java.lang.Class<?>, boolean)

  • AopUtils#canApply(org.springframework.aop.Pointcut, java.lang.Class<?>, boolean)

    • AspectJExpressionPointcut#getClassFilter

      • AspectJExpressionPointcut#checkReadyToMatch

  private void checkReadyToMatch() {

  if (getExpression() == null) {

    throw new IllegalStateException("Must set property 'expression' before attempting to match");

  }
  if (this.pointcutExpression == null) {

    this.pointcutClassLoader = (this.beanFactory instanceof ConfigurableBeanFactory ?
          ((ConfigurableBeanFactory) this.beanFactory).getBeanClassLoader() :
          ClassUtils.getDefaultClassLoader());
     //解析得到拦截表达式，例如根据@Before的value来关联查询出对应的表达式
    this.pointcutExpression = buildPointcutExpression(this.pointcutClassLoader);

  }
  }

最终解析完之后，advisor中的表达式信息结构如下图。包含在pointcut属性中，匹配时就根据pointcutExpression循环进行匹配class的方法。有兴趣的可以继续调试看看是如何实现匹配表达式的。

##　　创建代理对象

如果在上面的匹配切面过程中，发现适配的切面，那就需要进行代理对象的创建了。

我们回到上面的AbstractAutoProxyCreator#wrapIfNecessary，主要看代码如下。

// Create proxy if we have advice.
    //查找匹配切面
   Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
   if (specificInterceptors != DO_NOT_PROXY) {
      this.advisedBeans.put(cacheKey, Boolean.TRUE);
       //创建代理对象  
      Object proxy = createProxy(
            bean.getClass(), beanName, specificInterceptors, new SingletonTargetSource(bean));
      this.proxyTypes.put(cacheKey, proxy.getClass());
      return proxy;
   }

所以，继续看

• AbstractAutoProxyCreator#createProxy

的创建代理对象方法。设置ProxyFactory创建Proxy需要的一切信息。

protected Object createProxy(
      Class<?> beanClass, String beanName, Object[] specificInterceptors, TargetSource targetSource) {
   if (this.beanFactory instanceof ConfigurableListableBeanFactory) {
      AutoProxyUtils.exposeTargetClass((ConfigurableListableBeanFactory) this.beanFactory, beanName, beanClass);
   }
    //新建代理对象工厂
   ProxyFactory proxyFactory = new ProxyFactory();
   proxyFactory.copyFrom(this);
    //设置工厂代理类
   if (!proxyFactory.isProxyTargetClass()) {
      if (shouldProxyTargetClass(beanClass, beanName)) {
         proxyFactory.setProxyTargetClass(true);
      }
      else {
         evaluateProxyInterfaces(beanClass, proxyFactory);
      }
   }
    //设置拦截切面
   Advisor[] advisors = buildAdvisors(beanName, specificInterceptors);
   for (Advisor advisor : advisors) {
      proxyFactory.addAdvisor(advisor);
   }
    //设置被代理对象
   proxyFactory.setTargetSource(targetSource);
   customizeProxyFactory(proxyFactory);
   proxyFactory.setFrozen(this.freezeProxy);
   if (advisorsPreFiltered()) {
      proxyFactory.setPreFiltered(true);
   }
//创建代理对象
   return proxyFactory.getProxy(getProxyClassLoader());
}

下面看ProxyFactory是如何创建代理对象，继续跟踪proxyFactory.getProxy(getProxyClassLoader());

public Object getProxy(ClassLoader classLoader) {
   return createAopProxy().getProxy(classLoader);
}

createAopProxy()作用是根据class的种类判断采用的代理方式，看如下实现

• DefaultAopProxyFactory#createAopProxy

  @Override
  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.");
          }
          if (targetClass.isInterface() || Proxy.isProxyClass(targetClass)) {
              //采用jdk动态代理必须基于接口
              return new JdkDynamicAopProxy(config);
          }
          //基于cglib实现代理不需要接口
          return new ObjenesisCglibAopProxy(config);
      }
      else {
          return new JdkDynamicAopProxy(config);
      }
  }

所以在当前调试的例子中，使用cglib代理。所以执行如下代理。

@Override
public Object getProxy(ClassLoader classLoader) {
     //。。。。。。
      // Configure CGLIB Enhancer...
      Enhancer enhancer = createEnhancer();
      if (classLoader != null) {
         enhancer.setClassLoader(classLoader);
         if (classLoader instanceof SmartClassLoader &&
               ((SmartClassLoader) classLoader).isClassReloadable(proxySuperClass)) {
            enhancer.setUseCache(false);
         }
      }
      enhancer.setSuperclass(proxySuperClass);
      enhancer.setInterfaces(AopProxyUtils.completeProxiedInterfaces(this.advised));
      enhancer.setNamingPolicy(SpringNamingPolicy.INSTANCE);
      enhancer.setStrategy(new ClassLoaderAwareUndeclaredThrowableStrategy(classLoader));
    //获取拦截回调函数
      Callback[] callbacks = getCallbacks(rootClass);
      Class<?>[] types = new Class<?>[callbacks.length];
      for (int x = 0; x < types.length; x++) {
         types[x] = callbacks[x].getClass();
      }
      // fixedInterceptorMap only populated at this point, after getCallbacks call above
      enhancer.setCallbackFilter(new ProxyCallbackFilter(
            this.advised.getConfigurationOnlyCopy(), this.fixedInterceptorMap, this.fixedInterceptorOffset));
      enhancer.setCallbackTypes(types);
      // Generate the proxy class and create a proxy instance.
    //返回一个cglib代理对象
      return createProxyClassAndInstance(enhancer, callbacks);
   }
   catch (CodeGenerationException ex) {
       //、、、、、、
   }
}

getCallbacks(rootClass);在这个获取回调函数的方法中，普通的aop采用的回调函数是如下的方式。

// Choose an "aop" interceptor (used for AOP calls).
Callback aopInterceptor = new DynamicAdvisedInterceptor(this.advised);

cglib 的aop回调函数如下。

public Object intercept(Object proxy, Method method, Object[] args, MethodProxy methodProxy) throws Throwable {
   Object oldProxy = null;
   boolean setProxyContext = false;
   Class<?> targetClass = null;
   Object target = null;
   try {
       //这里注入的advised就是之前创建的ProxyFactory对象
      if (this.advised.exposeProxy) {
         // Make invocation available if necessary.
         oldProxy = AopContext.setCurrentProxy(proxy);
         setProxyContext = true;
      }
      // May be null. Get as late as possible to minimize the time we
      // "own" the target, in case it comes from a pool...
      target = getTarget();
      if (target != null) {
         targetClass = target.getClass();
      }
       //根据切面信息创建切面内容调用链
      List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass);
      Object retVal;
      // Check whether we only have one InvokerInterceptor: that is,
      // no real advice, but just reflective invocation of the target.
      if (chain.isEmpty() && Modifier.isPublic(method.getModifiers())) {
         // We can skip creating a MethodInvocation: just invoke the target directly.
         // Note that the final invoker must be an InvokerInterceptor, so we know
         // it does nothing but a reflective operation on the target, and no hot
         // swapping or fancy proxying.
         Object[] argsToUse = AopProxyUtils.adaptArgumentsIfNecessary(method, args);
         retVal = methodProxy.invoke(target, argsToUse);
      }
      else {
         // We need to create a method invocation...
          //创建一个方法调用对象，具体调用实现没分析，Before逻辑大概是先调用切面，在反射调用目标方法
         retVal = new CglibMethodInvocation(proxy, target, method, args, targetClass, chain, methodProxy).proceed();
      }
      retVal = processReturnType(proxy, target, method, retVal);
      return retVal;
   }
   finally {
      if (target != null) {
         releaseTarget(target);
      }
      if (setProxyContext) {
         // Restore old proxy.
         AopContext.setCurrentProxy(oldProxy);
      }
   }
}