Spring IoC容器的初始化，主要分为三段

1. 启动容器，扫描指定包路径下的Bean，封装BeanDefinition

   启动容器的三种方式：ClassPathXmlApplicationContext、AnnotationConfigApplicationContext、FileSystemXmlApplicationContext； 分别会走不同的扫描方式，最终完成BeanDefinition的封装。

2. 装配BeanFactory

3. 根据SpringBean的生命周期中定义的创建流程，创建Bean对象。无论以哪一种方式启动容器，最终都会交由AbstractApplicationContext类执行refresh方法进行Bean对象的创建

循环依赖

在Spring容器的初始化过程中，绕不开2个及以上Bean对象的相互依赖问题，Spring称之为循环依赖（circular references）

三级缓存

Spring对于循环依赖问题，提供了使用三级缓存的解决策略。分别为

• singletonFactories：三级缓存，存放的是ObjectFactory，注意此处存放的不是bean对象，而是生成bean对象的factory对象。
• earlySingletonObjects：二级缓存，存放的是由三级缓存中ObjectFactory对象执行getObject方法生成的代理对象，完成Spring通过动态代理的方式，对bean中的方法实现增强。此时该代理对象尚未开始属性注入。
• singletonObjects：一级缓存，即最终Spring容器中的单例池，存放最终完成所有属性注入的完整bean对象。

值得注意的是：Spring在初始化Bean的过程中，并不能预先确认该Bean是否存在循环依赖的情况。因此所有Bean对象（无论有无循环依赖），在被实例化之后，属性注入之前，都会被放入三级缓存singletonFactories中，走相同的初始化流程。

初始化Bean处理AutoWired.png

为什么要使用三级缓存

看了一些关于三级缓存的博客，大部分都有描述这个问题。简单回答：需要遵守设计的“单一原则”，各级缓存各司其职。

这又衍生出另一个问题：第三级缓存的存在，到底在设计上有何优雅或者必要之处？

实际上这个三级缓存中的ObjectFactory会在Bean对象第一次属性依赖（注意不一定是循环依赖的情况）注入时，就执行它的getObject方法，获取增强后的代理对象，放入二级缓存中；此后，在每一次循环依赖属性注入时，都会从二级缓存中取得代理对象进行注入。也就是说，如果根本就没有三级缓存，直接执行掉ObjectFactory.getObject方法里的操作，获取代理对象后放入二级缓存中。理论上也是可以的，这样似乎三级缓存就没必要了？不用怀疑ObjectFactory.getObject里的操作的执行时机问题，里面只是一些纯粹的增强。

下图是源码追踪Spring解决循环依赖问题的时序图，也是Spring Bean初始化与属性注入的大致流程：

Spring Bean循环依赖.png

以下源码分析的过程，对不关注的代码进行了删减

通过ClassPathXmlApplicationContext启动容器：

public ClassPathXmlApplicationContext(
            String[] configLocations, boolean refresh, @Nullable ApplicationContext parent)
            throws BeansException {

        super(parent);
        // sy 扫描配置文件，封装BeanDefinition对象 -- 所有的启动方式在构造函数中，都会先走配置扫描，做BeanDefinition封装。
        //      之后统一走AbstractApplicationContext：refresh();进行容器初始化
        // ClassPathXmlApplicationContext：setConfigLocations(configLocations);
        // FileSystemXmlApplicationContext：setConfigLocations(configLocations);
        // AnnotationConfigApplicationContext(Class<?>... componentClasses)：register(componentClasses);
        // AnnotationConfigApplicationContext(String... basePackages)：scan(basePackages);
        // -- AbstractApplicationContext：refresh();
        setConfigLocations(configLocations);
        if (refresh) {
            // sy 初始化IoC容器
            refresh();
        }
    }

ClassPathXmlApplicationContext的构造函数最终走AbstractApplicationContext.refresh方法，完成BeanFactory与Bean的建立

AbstractApplicationContext#refresh

public void refresh() throws BeansException, IllegalStateException {
        synchronized (this.startupShutdownMonitor) {
            // Prepare this context for refreshing.
            prepareRefresh();

            // Tell the subclass to refresh the internal bean factory.
            // sy 获取BeanFactory
            ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

            // Prepare the bean factory for use in this context.
            // sy 预装配BeanFactory
            prepareBeanFactory(beanFactory);

            try {
                // Allows post-processing of the bean factory in context subclasses.
                // sy 模板方法
                postProcessBeanFactory(beanFactory);

                // Invoke factory processors registered as beans in the context.
                // sy 设置工程中实现的BeanFactoryPostProcessors并执行postProcessBeanFactory方法
                // 最终通过DefaultListableBeanFactory.doGetBeanNamesForType遍历beanDefinitionNames获取BeanFactoryPostProcessors。
                invokeBeanFactoryPostProcessors(beanFactory);

                // Register bean processors that intercept bean creation.
                // sy 注册BeanPostProcessors
                // 调用时机：Bean实例化 -> Bean对象属性注入 -> BeanNameAware.setBeanName -> BeanFactoryAware.setBeanFactory -> ApplicationContextAware.setApplicationContext
                //          -> **** BeanPostProcessor.postProcessBeforeInitialization ****
                //              -> InitializingBean.afterPropertiesSet -> 调用xml定制的init-method方法或者Bean中用@PostConstruct注解的方法
                //          -> **** BeanPostProcessor.postProcessAfterInitialization  ****
                //              -> Singleton：放入单例池中，由Spring容器管理; Prototype：交给调用者，由JVM管理
                registerBeanPostProcessors(beanFactory);

                // Initialize message source for this context.
                // sy 国际化
                initMessageSource();

                // Initialize event multicaster for this context.
                initApplicationEventMulticaster();

                // Initialize other special beans in specific context subclasses.
                onRefresh();

                // Check for listener beans and register them.
                registerListeners();

                // Instantiate all remaining (non-lazy-init) singletons.
                // sy 最终完成IoC注入的调用
                finishBeanFactoryInitialization(beanFactory);

                // Last step: publish corresponding event.
                finishRefresh();
            }

            catch (BeansException ex) {
                if (logger.isWarnEnabled()) {
                    logger.warn("Exception encountered during context initialization - " +
                            "cancelling refresh attempt: " + ex);
                }

                // Destroy already created singletons to avoid dangling resources.
                destroyBeans();

                // Reset 'active' flag.
                cancelRefresh(ex);

                // Propagate exception to caller.
                throw ex;
            }

            finally {
                // Reset common introspection caches in Spring's core, since we
                // might not ever need metadata for singleton beans anymore...
                resetCommonCaches();
            }
        }
    }

执行AbstractApplicationContext#finishBeanFactoryInitialization

protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
        // 初始化Bean
        beanFactory.preInstantiateSingletons();
    }

执行DefaultListableBeanFactory#preInstantiateSingletons

public void preInstantiateSingletons() throws BeansException {
        if (logger.isTraceEnabled()) {
            logger.trace("Pre-instantiating singletons in " + this);
        }

        // Iterate over a copy to allow for init methods which in turn register new bean definitions.
        // While this may not be part of the regular factory bootstrap, it does otherwise work fine.
        List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);

        // Trigger initialization of all non-lazy singleton beans...
        // sy 通过遍历封装完毕的BeanDefinition，逐一创建Spring Bean
        for (String beanName : beanNames) {
            RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
            if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
                if (isFactoryBean(beanName)) {
                    // sy 处理FactoryBean
                    //  从容器中获取FactoryBean时，使用"&"+beanName的原因
                    Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
                    if (bean instanceof FactoryBean) {
                        final FactoryBean<?> factory = (FactoryBean<?>) bean;
                        boolean isEagerInit;
                        if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
                            isEagerInit = AccessController.doPrivileged((PrivilegedAction<Boolean>)
                                            ((SmartFactoryBean<?>) factory)::isEagerInit,
                                    getAccessControlContext());
                        }
                        else {
                            isEagerInit = (factory instanceof SmartFactoryBean &&
                                    ((SmartFactoryBean<?>) factory).isEagerInit());
                        }
                        if (isEagerInit) {
                            // sy 初始化bean
                            getBean(beanName);
                        }
                    }
                }
                else {
                    // sy 初始化bean
                    getBean(beanName);
                }
            }
        }

        // Trigger post-initialization callback for all applicable beans...
        for (String beanName : beanNames) {
            // 逐一从一级缓存、二级缓存、三级缓存中获取实例化对象
            Object singletonInstance = getSingleton(beanName);
            // 对单例bean对象的后置增强
            if (singletonInstance instanceof SmartInitializingSingleton) {
                final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
                if (System.getSecurityManager() != null) {
                    AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
                        smartSingleton.afterSingletonsInstantiated();
                        return null;
                    }, getAccessControlContext());
                }
                else {
                    smartSingleton.afterSingletonsInstantiated();
                }
            }
        }
    }

执行AbstractBeanFactory#getBean -> 调用内部AbstractBeanFactory#doGetBean方法

    protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
            @Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {

        final String beanName = transformedBeanName(name);
        Object bean;

        // Eagerly check singleton cache for manually registered singletons.
        // sy 依次从一级缓存、二级缓存、三级缓存中获取bean对象
        Object sharedInstance = getSingleton(beanName);
        // sy 如果已获得bean且其无参构造
        if (sharedInstance != null && args == null) {
            if (logger.isTraceEnabled()) {
                if (isSingletonCurrentlyInCreation(beanName)) {
                    logger.trace("Returning eagerly cached instance of singleton bean '" + beanName +
                            "' that is not fully initialized yet - a consequence of a circular reference");
                }
                else {
                    logger.trace("Returning cached instance of singleton bean '" + beanName + "'");
                }
            }
            // sy 处理factoryBean的情况
            bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
        }

        else {
            // Fail if we're already creating this bean instance:
            // We're assumably within a circular reference.
            // sy 如果当前的bean的scope为prototype，则抛出异常，prototype类型的bean不应该在初始化阶段生成
            if (isPrototypeCurrentlyInCreation(beanName)) {
                throw new BeanCurrentlyInCreationException(beanName);
            }

            // Check if bean definition exists in this factory.
            // sy 根据得到的BeanFactory做getBean递归
            BeanFactory parentBeanFactory = getParentBeanFactory();
            if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
                // Not found -> check parent.
                String nameToLookup = originalBeanName(name);
                if (parentBeanFactory instanceof AbstractBeanFactory) {
                    return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
                            nameToLookup, requiredType, args, typeCheckOnly);
                }
            }

            try {
                final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
                checkMergedBeanDefinition(mbd, beanName, args);

                // Guarantee initialization of beans that the current bean depends on.
                String[] dependsOn = mbd.getDependsOn();
                if (dependsOn != null) {
                    for (String dep : dependsOn) {
                        registerDependentBean(dep, beanName);
                        try {
                            // sy 递归创建依赖对象
                            getBean(dep);
                        }
                    }
                }

                // Create bean instance.
                if (mbd.isSingleton()) {
                    // sy 看命名应该能明白，该方法最终返回了一个singleton的bean对象，即存放在singletonObjects（一级缓存单例池中的对象）
                    sharedInstance = getSingleton(beanName, () -> {
                        try {
                            // sy 最终创建bean代理对象的地方
                            return createBean(beanName, mbd, args);
                        }
                    });
                    bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
                }
            }
        }
    
        return (T) bean;
    }

如果能使用DefaultListableBeanFactory#preInstantiateSingletons.getSingleton方法从缓存中得到bean，则之前已走过实例化流程。该方法很关键：

protected Object getSingleton(String beanName, boolean allowEarlyReference) {
        Object singletonObject = this.singletonObjects.get(beanName);
        // sy 如果对象不在单例池singletonObjects（一级缓存）中
        if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
            synchronized (this.singletonObjects) {
                singletonObject = this.earlySingletonObjects.get(beanName);
                // sy 如果对象不在二级缓存earlySingletonObjects中
                if (singletonObject == null && allowEarlyReference) {
                    // sy 从三级缓存singletonFactories中获取中获取ObjectFactory
                    //      注意三级缓存singletonFactories中存放的是ObjectFactory，并不是bean对象，即此时没有生成bean的代理对象
                    ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
                    if (singletonFactory != null) {
                        // sy 通过匿名内部类实现ObjectFactory接口的getObject，创建Bean对象
                        //      addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
                        singletonObject = singletonFactory.getObject();
                        // sy 将生成的Bean对象放入二级缓存中
                        this.earlySingletonObjects.put(beanName, singletonObject);
                        // sy 从一级缓存中移除使用beanName标记的ObjectFactory
                        this.singletonFactories.remove(beanName);
                    }
                }
            }
        }
        return singletonObject;
    }

如果从缓存中获取不到bean，说明该Bean对象尚未进行实例化，走org.springframework.beans.factory.support.DefaultSingletonBeanRegistry#getSingleton方法：

public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
   synchronized (this.singletonObjects) {
      Object singletonObject = this.singletonObjects.get(beanName);
      if (singletonObject == null) {
         beforeSingletonCreation(beanName);
         boolean newSingleton = false;
         boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
         if (recordSuppressedExceptions) {
            this.suppressedExceptions = new LinkedHashSet<>();
         }
         try {
            // sy 执行beanFactory的getObject方法即：
            //        () -> {
            //                try {
            //                   // sy 最终创建bean代理对象的地方(org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean)
            //                   return createBean(beanName, mbd, args);
            //                }
            //                catch (BeansException ex) {
            //                   // Explicitly remove instance from singleton cache: It might have been put there
            //                   // eagerly by the creation process, to allow for circular reference resolution.
            //                   // Also remove any beans that received a temporary reference to the bean.
            //                   destroySingleton(beanName);
            //                   throw ex;
            //                }
            //             }
            //    创建最终完成增强的完整的singleton代理对象
            //    并在下方执行addSingleton(beanName, singletonObject)，将其放进单例缓存池中，并将其从三级、二级缓存中删除
            singletonObject = singletonFactory.getObject();
            newSingleton = true;
         }
         finally {
            if (recordSuppressedExceptions) {
               this.suppressedExceptions = null;
            }
            afterSingletonCreation(beanName);
         }
         if (newSingleton) {
            // sy 将初始化完成的bean对象放入单例池中
            addSingleton(beanName, singletonObject);
         }
      }
      return singletonObject;
   }
}

由singletonObject = singletonFactory.getObject()执行org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean方法：这里删除了异常处理的代码，主要关注2个方法：resolveBeforeInstantiation、doCreateBean（最终创建Bean对象）

    protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
            throws BeanCreationException {

        RootBeanDefinition mbdToUse = mbd;

        try {
            // Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
            // sy 听名字有点疑惑，
            //  设置BeanPostProcessors的postProcessBeforeInitialization，postProcessAfterInitialization。
            //  这样似乎说明会在Bean依赖注入完毕就立马执行BeanPostProcessor ？
            Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
            if (bean != null) {
                return bean;
            }
        }

        try {
            // sy 创建bean对象
            Object beanInstance = doCreateBean(beanName, mbdToUse, args);
            return beanInstance;
        }
    }

org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#doCreateBean这个方法做了3件事：

• addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean))
  

  将ObjectFactory加入三级缓存中

• populateBean(beanName, mbd, instanceWrapper)
  

  进行属性注入

• exposedObject = initializeBean(beanName, exposedObject, mbd)
  

  在bean装配完成后，执行BeanPostProcessor与init-method

源码详解：

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
      throws BeanCreationException {

   // Instantiate the bean.
   BeanWrapper instanceWrapper = null;
   if (mbd.isSingleton()) {
      // sy factoryBeanInstanceCache.put前会将beanName做一步createBeanInstance
      instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
   }
   // sy 如果为空说明当前尚未创建过Bean对象
   if (instanceWrapper == null) {
      // sy 最终都会走createBeanInstance方法去实例化对象，仅仅只是实例化原生对象，尚未对bean对象进行属性注入，也未生成代理对象且增强
      instanceWrapper = createBeanInstance(beanName, mbd, args);
   }
   final Object bean = instanceWrapper.getWrappedInstance();
   Class<?> beanType = instanceWrapper.getWrappedClass();
   if (beanType != NullBean.class) {
      mbd.resolvedTargetType = beanType;
   }

   // Allow post-processors to modify the merged bean definition.
   synchronized (mbd.postProcessingLock) {
      if (!mbd.postProcessed) {
         try {
            applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
         }
         mbd.postProcessed = true;
      }
   }
   boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
         isSingletonCurrentlyInCreation(beanName));
   if (earlySingletonExposure) {
      // sy 在bean实例化之后，将其BeanFactory：() -> getEarlyBeanReference(beanName, mbd, bean)放入三级缓存中，
      //        可解决循环依赖问题（默认开启允许循环依赖：this.allowCircularReferences的默认值为true）
      addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
   }

   // Initialize the bean instance.
   Object exposedObject = bean;
   try {
      // sy 为bean对象执行属性注入。
      populateBean(beanName, mbd, instanceWrapper);
      // sy 在bean装配完成后，执行BeanPostProcessor与init-method
      exposedObject = initializeBean(beanName, exposedObject, mbd);
   }

   // sy 如果在二级缓存中
   if (earlySingletonExposure) {
      Object earlySingletonReference = getSingleton(beanName, false);
      if (earlySingletonReference != null) {
         if (exposedObject == bean) {
            exposedObject = earlySingletonReference;
         }
         else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
            String[] dependentBeans = getDependentBeans(beanName);
            Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
            for (String dependentBean : dependentBeans) {
               if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
                  actualDependentBeans.add(dependentBean);
               }
            }
         }
      }
   }

   // Register bean as disposable.
   try {
      registerDisposableBeanIfNecessary(beanName, bean, mbd);
   }

   return exposedObject;
}

重点关注属性装配：org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#populateBean：

protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
  
   // 前面做了一系列处理去获取pvs：propertyValues
   if (pvs != null) {
      // sy 解决@AutoWired注入
      applyPropertyValues(beanName, mbd, bw, pvs);
   }
}

最终调用了org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#applyPropertyValues：

这里面主要做了2件事

• Object resolvedValue = valueResolver.resolveValueIfNecessary(pv, originalValue);
  

  通过调用BeanDefinitionValueResolver.resolveValueIfNecessary获取该属性的值

• bw.setPropertyValues(new MutablePropertyValues(deepCopy));
  

  通过BeanWrapper.setPropertyValues方法将获取的属性值，注入到属性对象中。内部使用了PropertyAccessor（属性存取器）做了一些处理，最终在org.springframework.beans.BeanWrapperImpl.BeanPropertyHandler#setValue方法中使用反射进行属性值注入：

  public void setValue(final @Nullable Object value) throws Exception {
     final Method writeMethod = (this.pd instanceof GenericTypeAwarePropertyDescriptor ?
           ((GenericTypeAwarePropertyDescriptor) this.pd).getWriteMethodForActualAccess() :
           this.pd.getWriteMethod());
     if (System.getSecurityManager() != null) {
        AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
           ReflectionUtils.makeAccessible(writeMethod);
           return null;
        });
        try {
           AccessController.doPrivileged((PrivilegedExceptionAction<Object>) () ->
                 writeMethod.invoke(getWrappedInstance(), value), acc);
        }
        catch (PrivilegedActionException ex) {
           throw ex.getException();
        }
     }
     else {
        // sy 暴力调用
        ReflectionUtils.makeAccessible(writeMethod);
        writeMethod.invoke(getWrappedInstance(), value);
     }
  }
  

最终该方法的源码详情：

protected void applyPropertyValues(String beanName, BeanDefinition mbd, BeanWrapper bw, PropertyValues pvs) {
        if (pvs.isEmpty()) {
            return;
        }
        BeanDefinitionValueResolver valueResolver = new BeanDefinitionValueResolver(this, beanName, mbd, converter);

        // Create a deep copy, resolving any references for values.
        List<PropertyValue> deepCopy = new ArrayList<>(original.size());
        boolean resolveNecessary = false;
        for (PropertyValue pv : original) {
            if (pv.isConverted()) {
                deepCopy.add(pv);
            }
            else {
                String propertyName = pv.getName();
                Object originalValue = pv.getValue();
                // sy 处理property的注入值
                Object resolvedValue = valueResolver.resolveValueIfNecessary(pv, originalValue);
                Object convertedValue = resolvedValue;
                boolean convertible = bw.isWritableProperty(propertyName) &&
                        !PropertyAccessorUtils.isNestedOrIndexedProperty(propertyName);
                if (convertible) {
                    convertedValue = convertForProperty(resolvedValue, propertyName, bw, converter);
                }
                if (resolvedValue == originalValue) {
                    if (convertible) {
                        pv.setConvertedValue(convertedValue);
                    }
                    deepCopy.add(pv);
                }
            }
        }

        // Set our (possibly massaged) deep copy.
        try {
            // sy 将获取的属性值，通过反射注入到属性对象中
            bw.setPropertyValues(new MutablePropertyValues(deepCopy));
        }
    }

进入org.springframework.beans.factory.support.BeanDefinitionValueResolver#resolveValueIfNecessary方法：

public Object resolveValueIfNecessary(Object argName, @Nullable Object value) {
   // 这方法其实东西挺多的，处理的情况也比较多。大部分直接会走第一个if条件，就不看其他的了
   if (value instanceof RuntimeBeanReference) {
      RuntimeBeanReference ref = (RuntimeBeanReference) value;
      // sy 处理引用（即对象引用）
      return resolveReference(argName, ref);
   }
}

进入org.springframework.beans.factory.support.BeanDefinitionValueResolver#resolveReference方法：

这个方法很关键，是下一轮递归的入口：

private Object resolveReference(Object argName, RuntimeBeanReference ref) {
   try {
      Object bean;
      String refName = ref.getBeanName();
      refName = String.valueOf(doEvaluate(refName));
      if (ref.isToParent()) {
         bean = this.beanFactory.getParentBeanFactory().getBean(refName);
      }
      else {
         // sy 很熟悉吧。又回到了最初的getBean，算是一种递归吧。
         //     递归结束的条件即在getBean方法中从三级到一级缓存内获取到了bean对象。此时该bean对象即增强后的代理对象了。
         //       如果在缓存内未取得bean对象，则继续向下递归，若取得，则向上回归
         bean = this.beanFactory.getBean(refName);
         this.beanFactory.registerDependentBean(refName, this.beanName);
      }
      return bean;
   }
}

最终在最后一层递归完成，向上回归到第一次getBean方法内的getSingleton方法中，从一级缓存（单例池）中获取完整对象后，实例化完成。