/** 記錄已將創(chuàng)建的單例<beanName,singletonBean> */
  private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);

  /** 記錄singletonFactory<beanName,singletonFactory> singeletonFactory中存放beanName和上面的①階段的bean：Bean實例化實例對象（還未初始化DI）*/
  private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);

  /** 記錄早期的singletonBean 存放的也是① */
  private final Map<String, Object> earlySingletonObjects = new HashMap<>(16);

  /** 存放已經(jīng)初始化后的beanName，有序 */
  private final Set<String> registeredSingletons = new LinkedHashSet<>(256);

  /** 記錄正在初始化的bean的beanName */
  private final Set<String> singletonsCurrentlyInCreation = Collections.newSetFromMap(new ConcurrentHashMap<>(16));

/* org.springframework.beans.factory.support.DefaultSingletonBeanRegistry#getSingleton(java.lang.String, org.springframework.beans.factory.ObjectFactory<?>) */
  public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
    ...
    //循環(huán)依賴相關(guān)：初始化前先singletonsCurrentlyInCreation.add(beanName)
    beforeSingletonCreation(beanName);
    ...
    //lamda表達(dá)式：其實是調(diào)用createBean(beanName, mbd, args):Bean初始化
    singletonObject = singletonFactory.getObject();
    ...
    //循環(huán)依賴相關(guān)：初始化后singletonsCurrentlyInCreation.remove(beanName)
    afterSingletonCreation(beanName);
    ...//初始化完后
    //this.singletonObjects.put(beanName, singletonObject);放入到單例容器中
    //this.singletonFactories.remove(beanName);清空循環(huán)依賴的兩個打標(biāo)
    //this.earlySingletonObjects.remove(beanName);
    //this.registeredSingletons.add(beanName);放入單例beanName容器中
    addSingleton(beanName, singletonObject);
    ...
    }
  }

/* org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#doCreateBean */
  protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
      throws BeanCreationException {
    ...
    //循環(huán)依賴相關(guān)邏輯：
    //this.singletonFactories.put(beanName, singletonFactory);
    //將實例化bean（①階段）、beanName組裝成singletonFactory裝入singletonFactories容器
    //this.earlySingletonObjects.remove(beanName);
    //刪除earlySingletonObjects中beanName
    addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
    ...
    //實例初始化 就是在這里面實現(xiàn)依賴注入DI的:反射實現(xiàn)
    //調(diào)用AutowiredAnnotationBeanPostProcessor.postProcessProperties
    populateBean(beanName, mbd, instanceWrapper);
    ...
  }

/* org.springframework.beans.factory.support.DefaultSingletonBeanRegistry#getSingleton(java.lang.String, boolean) */
  protected Object getSingleton(String beanName, boolean allowEarlyReference) {
    Object singletonObject = this.singletonObjects.get(beanName);
　　　　 //循環(huán)依賴核心就在于這個判斷，由于打標(biāo)+記錄了①階段的bean，
    //循環(huán)依賴第二次調(diào)用getBean("a")時，這里會直接返回第一次調(diào)用getBean("a")創(chuàng)建的①階段的bean
    //而不會調(diào)用createBean("a")再次bean初始化（造成兩個bean的循環(huán)創(chuàng)建）
    if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
      synchronized (this.singletonObjects) {
        singletonObject = this.earlySingletonObjects.get(beanName);
        if (singletonObject == null && allowEarlyReference) {
          ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
          if (singletonFactory != null) {
            singletonObject = singletonFactory.getObject();
            this.earlySingletonObjects.put(beanName, singletonObject);
            this.singletonFactories.remove(beanName);
          }
        }
      }
    }
    return singletonObject;
  }

/* org.springframework.beans.factory.support.AbstractBeanFactory#doGetBean */
  protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
      @Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
    ...
    //假設(shè)A、B互相依賴
    //第一次getBean(A)，sharedInstance == null,走else,createBean
    //A正在創(chuàng)建打標(biāo)，①中beforeSingletonCreation()
    //A實例化后保存到singletonFactories中②中addSingletonFactory(beanName,singletonFactory)
    //DI依賴注入：②中populateBean(beanName,mbd,instanceWrapper),發(fā)現(xiàn)依賴B，調(diào)用getBean(B)初始化B的單例
    //調(diào)用getBean(B)重復(fù)上面步驟，DI依賴注入發(fā)現(xiàn)依賴A，調(diào)用getBean(A)
    //第二次getBean(A),③中if(singletonObject == null && isSingletonCurrentlyInCreation(A))由于打標(biāo)了所以返回singleFactory.getObject()
    //下面if條件直接返回bean,沒有走else破壞了循環(huán)
    Object sharedInstance = getSingleton(beanName);
    if (sharedInstance != null && args == null) {
      //
      bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
    }

    else {
      ...
      //
      createBean(beanName, mbd, args);
      bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
    }
    return bean;      
  }