Spring源码阅读--aop实现原理分析

悠悠 2023-10-13 08:12 35阅读 0赞

## 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。

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzIwNTk3NzI3_size_16_color_FFFFFF_t_70]

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 ###

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzIwNTk3NzI3_size_16_color_FFFFFF_t_70 1]

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。

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzIwNTk3NzI3_size_16_color_FFFFFF_t_70 2]

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。

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzIwNTk3NzI3_size_16_color_FFFFFF_t_70 3]

### 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<Advisor> findEligibleAdvisors(Class<?> beanClass, String beanName) {
        
        //从缓存取出所有切面信息
       List<Advisor> candidateAdvisors = findCandidateAdvisors();
        //根据advisor信息中的表达式进行方法对class的匹配
       List<Advisor> 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。

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzIwNTk3NzI3_size_16_color_FFFFFF_t_70 4]

\#\#　　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);
          }
       }
    }

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