/** Cache of singleton objects: bean name to bean instance. */
Map<String, Object> singletonObjects;

/** Cache of early singleton objects: bean name to bean instance. */
Map<String, Object> earlySingletonObjects ;

/** Cache of singleton factories: bean name to ObjectFactory. */
Map<String, ObjectFactory<?>> singletonFactories;

org.springframework.beans.factory.support.AbstractBeanFactory#doGetBean(beanName:A)
　　org.springframework.beans.factory.support.DefaultSingletonBeanRegistry#getSingleton(beanName:A)
　　　　org.springframework.beans.factory.support.DefaultSingletonBeanRegistry#getSingleton(beanName:A, allowEarlyReference:true)

org.springframework.beans.factory.support.AbstractBeanFactory#doGetBean(beanName:A)
　　org.springframework.beans.factory.support.DefaultSingletonBeanRegistry#getSingleton(beanName:A, ObjectFactory:lamda表達，調用AbstractBeanFactory#createBean(beanName:A))
　　org.springframework.beans.factory.support.DefaultSingletonBeanRegistry#addSingleton(beanName:A,singletonObject:A)

org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#createBean(beanName:A, RootBeanDefinition:mbd, Object[]:args)
　　org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#doCreateBean(beanName:A, RootBeanDefinition:mbd, Object[]:args)
　　　　org.springframework.beans.factory.support.DefaultSingletonBeanRegistry#addSingletonFactory(beanName:A, () -> getEarlyBeanReference(beanName:A, mbd, bean:A))
　　　　//放入三級緩存，beanName:A -> ObjectFactory( () -> getEarlyBeanReference(beanName:A, mbd, bean:A) )
　　org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#populateBean　　//屬性填充
　　org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#initializeBean　　 //初始化

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
      throws BeanCreationException {
   // Instantiate the bean.
   BeanWrapper instanceWrapper = createBeanInstance(beanName, mbd, args);
   }
   final Object bean = instanceWrapper.getWrappedInstance();
   boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
         isSingletonCurrentlyInCreation(beanName));
      addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
   }
   // Initialize the bean instance.
   Object exposedObject = bean;
   populateBean(beanName, mbd, instanceWrapper);
   exposedObject = initializeBean(beanName, exposedObject, mbd);

   if (earlySingletonExposure) {
      Object earlySingletonReference = getSingleton(beanName, false);
      if (earlySingletonReference != null) {
         if (exposedObject == bean) {
            exposedObject = earlySingletonReference;
         }
         else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
         }
       }   
   }
}

protected Object doCreateBean( ... ){
    ...
    boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences && isSingletonCurrentlyInCreation(beanName));
    if (earlySingletonExposure) {
        addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
    }
    ...

    Object exposedObject = bean;

    /**
    *假如A實例有方法注有@Async註解，A實例依賴B實例，B實例依賴A實例。創建A實例時先走到populateBean方法，然後開始填充屬性B實例，B實例也會走到populateBean方法，然後從三級緩存中通過A流程中的getEarlyBeanReference()方法，從而拿到A的早期引用*。執行A的getEarlyBeanReference()方法的時候，會執行自動代理創建器，這裡最終得到是可能是原始A對象，也可能是代理後的A對象（注意哦，A實例的@Async註解這裡還沒有被處理呢）。exposedObject此時指向的是原始A實例。
    */
    populateBean(beanName, mbd, instanceWrapper);

     /**
     *標註有@Async的A實例的代理對象在此處會被生成， 參照類：AsyncAnnotationBeanPostProcessor。執行完之後，exposedObject指向的是個代理對象而非原始A實例了。
     */
    exposedObject = initializeBean(beanName, exposedObject, mbd);
    
    ...
    // 這裡是報錯的重點。
    if (earlySingletonExposure) {
        /**
        *因為A被B循環依賴進去了，所以此時A是被放進了二級緩存的，所以此處earlySingletonReference指向的是通過創建A實例流程中的getEarlyBeanReference()返回的A實例（再強調用一下，可能是原始對象，也可能是代理對象）。
        *說到這裡，什麼情況下earlySingletonReference==null？也就是getSingleton(beanName:A, false)==null，只有當A實例沒有牽涉到循環依賴的時候（即不存在A依賴B且B依賴A的場景；單獨存在B依賴A是沒有問題，A的三級緩存根本不會執行，所以二級緩存就不會有值，A創建並初始化完成之後直接放到了一級緩存）。
        */
        Object earlySingletonReference = getSingleton(beanName, false);
        if (earlySingletonReference != null) {
            //這裡exposedObject指向的是被@Aysnc代理過的對象，而bean是原始對象，所以此處不相等，走else邏輯。
            if (exposedObject == bean) {
                exposedObject = earlySingletonReference;
            }
            /**
            *allowRawInjectionDespiteWrapping 標註是否允許此Bean的原始類型被注入到其它Bean裏面，即使自己最終會被包裝（代理）。
            *默認是false表示不允許，如果改為true表示允許，就不會報錯啦。這是我們後面講的決方案的其中一個方案。
            *另外dependentBeanMap是記錄著每個Bean它所依賴的Bean的Map。
            */
            else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
                //因為A依賴於B，所以此處拿到了B實例的beanName
                String[] dependentBeans = getDependentBeans(beanName);
                Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);

                /**
                *B實例經過removeSingletonIfCreatedForTypeCheckOnly最終返返回false  因為alreadyCreated裏面已經有它了表示B已經完全創建完成了。
                *既然B實例已經創建完成了，通過創建A實例流程中的getEarlyBeanReference()返回的A實例已經注入到了B實例中，此時B實例注入的和exposedObject指向的不是同一個A實例，那肯定就有問題了。
                */
                for (String dependentBean : dependentBeans) {
                    if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
                        actualDependentBeans.add(dependentBean);
                    }
                }
    
                // 若存在這種真正的依賴，那就報錯了~~~  則個異常就是上面看到的異常信息
                if (!actualDependentBeans.isEmpty()) {
                    throw new BeanCurrentlyInCreationException(beanName,
                            "Bean with name '" + beanName + "' has been injected into other beans [" +
                            StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
                            "] in its raw version as part of a circular reference, but has eventually been " +
                            "wrapped. This means that said other beans do not use the final version of the " +
                            "bean. This is often the result of over-eager type matching - consider using " +
                            "'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.");
                }
            }
        }
    }
    ...
}