// Eagerly cache singletons to be able to resolve circular references
		// even when triggered by lifecycle interfaces like BeanFactoryAware.
		//判斷當(dāng)前bean為單例，且允許循環(huán)依賴，且該bean正在創(chuàng)建中
		boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
				isSingletonCurrentlyInCreation(beanName));
		if (earlySingletonExposure) {
			if (logger.isTraceEnabled()) {
				logger.trace("Eagerly caching bean '" + beanName +
						"' to allow for resolving potential circular references");
			}
			addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
		}

protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {
		Assert.notNull(singletonFactory, "Singleton factory must not be null");
		synchronized (this.singletonObjects) {
			/*
			如果 singletonObjects 中不存在當(dāng)前bean（當(dāng)前bean還在創(chuàng)建，還沒生成最終的單例對象，顯然成立），則：
			1）把當(dāng)前的singletonFactory放入singletonFactories變量中（把獲取早期引用的lambda表達(dá)式放進(jìn)三級緩存）；
			2）把當(dāng)前bean從earlySingletonObjects變量中移除（清理二級緩存，確保在循環(huán)引用觸發(fā)時才生成早期引用；實際這里本來也沒有）；
			3）把當(dāng)前beanName放入registeredSingletons變量中。
			*/
			if (!this.singletonObjects.containsKey(beanName)) {
				this.singletonFactories.put(beanName, singletonFactory);
				this.earlySingletonObjects.remove(beanName);
				this.registeredSingletons.add(beanName);
			}
		}
	}

// Eagerly check singleton cache for manually registered singletons.
		Object sharedInstance = getSingleton(beanName);
		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 + "'");
				}
			}
			beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, null);
		}

protected Object getSingleton(String beanName, boolean allowEarlyReference) {
		// Quick check for existing instance without full singleton lock
		//先從singletonObjects中獲取bean A，之前說過顯然獲取不到
		Object singletonObject = this.singletonObjects.get(beanName);
		if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
			//再從earlySingletonObjects中獲取bean，剛開始也是沒有的
			singletonObject = this.earlySingletonObjects.get(beanName);
			if (singletonObject == null && allowEarlyReference) {
				synchronized (this.singletonObjects) {
					// Consistent creation of early reference within full singleton lock
					singletonObject = this.singletonObjects.get(beanName);
					if (singletonObject == null) {
						singletonObject = this.earlySingletonObjects.get(beanName);
						if (singletonObject == null) {
							//最后從singletonFactories里拿到singletonFactory
							ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
							if (singletonFactory != null) {
								//調(diào)用getObject方法獲取早期引用，放進(jìn)earlySingletonObjects，清理singletonFactories
								singletonObject = singletonFactory.getObject();
								this.earlySingletonObjects.put(beanName, singletonObject);
								this.singletonFactories.remove(beanName);
							}
						}
					}
				}
			}
		}
		return singletonObject;
	}

protected Object getEarlyBeanReference(String beanName, RootBeanDefinition mbd, Object bean) {
		Object exposedObject = bean;
		if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
			for (SmartInstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().smartInstantiationAware) {
				exposedObject = bp.getEarlyBeanReference(exposedObject, beanName);
			}
		}
		return exposedObject;
	}

protected void addSingleton(String beanName, Object singletonObject) {
		synchronized (this.singletonObjects) {
			this.singletonObjects.put(beanName, singletonObject);
			this.singletonFactories.remove(beanName);
			this.earlySingletonObjects.remove(beanName);
			this.registeredSingletons.add(beanName);
		}
	}