@Override
	@Nullable
	public Object getSingleton(String beanName) {
		return getSingleton(beanName, true);
	}

	/**
	 * singletonObjects 缓存：beanName -> 单例 bean 对象。
	 * earlySingletonObjects 缓存：beanName -> 单例 bean 对象，该缓存存放的是早期单例 bean 对象，可以理解成还未进行属性填充、初始化。
	 * singletonFactories 缓存：beanName -> ObjectFactory。
	 * singletonsCurrentlyInCreation 缓存：当前正在创建单例 bean 对象的 beanName 集合。
	 * singletonObjects、earlySingletonObjects、singletonFactories 在这边构成了一个类似于 “三级缓存” 的概念。
	 */
	@Nullable
	protected Object getSingleton(String beanName, boolean allowEarlyReference) {
		// 1.【从一级缓存里面获取】从单例对象缓存中获取beanName对应的单例对象
		Object singletonObject = this.singletonObjects.get(beanName);

		if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
			// 3.加锁进行操作
			synchronized (this.singletonObjects) {

				// 4.【从二级缓存里面获取】从早期单例对象缓存中获取单例对象
				// 之所称成为早期单例对象，是因为earlySingletonObjects里的对象的都是通过提前曝光的ObjectFactory创建出来的，还未进行属性填充等操作
				singletonObject = this.earlySingletonObjects.get(beanName);

				// 5.如果在早期单例对象缓存中也没有，并且允许创建早期单例对象引用
				if (singletonObject == null && allowEarlyReference) {

					// 6.【从三级缓存里面获取】从单例工厂缓存中获取beanName的单例工厂
					ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
					if (singletonFactory != null) {

						// 7.如果存在单例对象工厂，则通过工厂创建一个单例对象
						singletonObject = singletonFactory.getObject();

						// 8.将通过单例对象工厂创建的单例对象，放到早期单例对象缓存中
						this.earlySingletonObjects.put(beanName, singletonObject);

						// 9.移除该beanName对应的单例对象工厂，因为该单例工厂已经创建了一个实例对象，并且放到earlySingletonObjects缓存了，
						// 因此，后续获取beanName的单例对象，可以通过earlySingletonObjects缓存拿到，不需要在用到该单例工厂
						this.singletonFactories.remove(beanName);
					}
				}
			}
		}
		return singletonObject;
	}

    /**
	 * 真正创建bean的方法
	 */
	protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
			throws BeanCreationException {

		// 封装被创建的bean对象
		BeanWrapper instanceWrapper = null;
		if (mbd.isSingleton()) {
			instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
		}
		if (instanceWrapper == null) {
			// TODO【重要】 这一步创建了bean实例，只是一个早期的对象，还没有填充属性值
			instanceWrapper = createBeanInstance(beanName, mbd, args);
		}
		final Object bean = instanceWrapper.getWrappedInstance();

		// 获取实例化对象的类型
		Class<?> beanType = instanceWrapper.getWrappedClass();
		if (beanType != NullBean.class) {
			mbd.resolvedTargetType = beanType;
		}

		// 调用post-processors后置处理器
		synchronized (mbd.postProcessingLock) {
			if (!mbd.postProcessed) {
				try {
					applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
				}
				catch (Throwable ex) {
					throw new BeanCreationException(mbd.getResourceDescription(), beanName,
							"Post-processing of merged bean definition failed", ex);
				}
				mbd.postProcessed = true;
			}
		}

		// 向容器中缓存单例模式的bean，防止循环引用
		boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
				isSingletonCurrentlyInCreation(beanName));
		if (earlySingletonExposure) {
			if (logger.isDebugEnabled()) {
				logger.debug("Eagerly caching bean '" + beanName +
						"' to allow for resolving potential circular references");
			}
			// TODO【重要】 向三级缓存添加bean实例 匿名内部类，防止循环引用，尽早持有对象的引用
			addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
		}

		// 初始化bean实例，exposedObject在初始化完成之后返回依赖注入完成之后的bean
		Object exposedObject = bean;
		try {
			// TODO 【重要】 bean属性依赖注入，并且将bean定义中配置的属性值赋值给实例对象
			populateBean(beanName, mbd, instanceWrapper);

			// TODO 【重要】开始初始化bean对象 调用Aware接口方法 -> 调用BeanPostProcessor.before方法 -> 调用init-method方法 ->  调用BeanPostProcessor.after方法
			exposedObject = initializeBean(beanName, exposedObject, mbd);
		}
		catch (Throwable ex) {
			if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
				throw (BeanCreationException) ex;
			}
			else {
				throw new BeanCreationException(
						mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
			}
		}

		if (earlySingletonExposure) {
			// 获取指定名称的已注册的单例bean
			Object earlySingletonReference = getSingleton(beanName, false);
			if (earlySingletonReference != null) {
				// 根据名称获取已注册的bean和正在实例化的bean是同一个
				if (exposedObject == bean) {
					// 当前实例化的bean初始完成
					exposedObject = earlySingletonReference;
				}
				// 当前bean依赖其他bean，且当发送循环引用时，不允许创建新的实例对象
				else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
					// 获取当前bean所依赖的其他bean
					String[] dependentBeans = getDependentBeans(beanName);
					Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
					for (String dependentBean : dependentBeans) {
						// 对依赖bean进行类型检查
						if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
							actualDependentBeans.add(dependentBean);
						}
					}
					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.");
					}
				}
			}
		}

		// 注册完成依赖注入的bean
		try {
			registerDisposableBeanIfNecessary(beanName, bean, mbd);
		}
		catch (BeanDefinitionValidationException ex) {
			throw new BeanCreationException(
					mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
		}

		return exposedObject;
	}

/**
	 * 通过singletonFactory创建bean实例，并存入一级缓存singletonObjects
	 */
	public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
		Assert.notNull(beanName, "Bean name must not be null");
		synchronized (this.singletonObjects) {
			Object singletonObject = this.singletonObjects.get(beanName);
			if (singletonObject == null) {
				// 如果当前工厂单例正在销毁，则不允许创建，抛出异常
				if (this.singletonsCurrentlyInDestruction) {
					throw new BeanCreationNotAllowedException(beanName,
							"Singleton bean creation not allowed while singletons of this factory are in destruction " +
							"(Do not request a bean from a BeanFactory in a destroy method implementation!)");
				}
				if (logger.isDebugEnabled()) {
					logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
				}

				// 校验是否已经在创建中，抛出异常
				beforeSingletonCreation(beanName);

				boolean newSingleton = false;
				boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
				if (recordSuppressedExceptions) {
					this.suppressedExceptions = new LinkedHashSet<>();
				}

				try {
					// 【在这里创建实例】singletonFactory创建bean实例
					singletonObject = singletonFactory.getObject();
					newSingleton = true;
				}
				catch (IllegalStateException ex) {
					// Has the singleton object implicitly appeared in the meantime ->
					// if yes, proceed with it since the exception indicates that state.
					singletonObject = this.singletonObjects.get(beanName);
					if (singletonObject == null) {
						throw ex;
					}
				}
				catch (BeanCreationException ex) {
					if (recordSuppressedExceptions) {
						for (Exception suppressedException : this.suppressedExceptions) {
							ex.addRelatedCause(suppressedException);
						}
					}
					throw ex;
				}
				finally {
					if (recordSuppressedExceptions) {
						this.suppressedExceptions = null;
					}
					afterSingletonCreation(beanName);
				}
				if (newSingleton) {
					// TODO【重要】把bean实例加入缓存
					addSingleton(beanName, singletonObject);
				}
			}
			return singletonObject;
		}
	}