【Spring源代码阅读之九】refresh刷新容器之finishBeanFactoryInitialization()方法对非延迟加载的单例Bean调用getBean()实例化流程

导图

AbstractApplicationContext#finishBeanFactoryInitialization()

/**
 * 对BeanFactory中的所有非延迟加载的单例Bean进行实例化
 */
protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
	// Initialize conversion service for this context.
	/**
	 * 如果容器中存在ConversionService接口的实现和conversionService名称的BeanDefinition
	 * 则实例化这个Bean放到容器中，默认是没有格式转换器的
	 * 这个接口是一个转换标准接口，可实现该接口注册到容器中进行数据的转换
	 * 如果加入了SpringMVC，则会自动注入一个该接口的实现，满足大部分MVC模型数据的转换
	 */
	if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&
			beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {
		beanFactory.setConversionService(
				beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class));
	}

	// Register a default embedded value resolver if no bean post-processor
	// (such as a PropertyPlaceholderConfigurer bean) registered any before:
	// at this point, primarily for resolution in annotation attribute values.
	/**
	 * 在这之前，容器中如果没有关于值处理的BeanPostProcessor，则在这注册默认的嵌入式值解析器
	 */
	if (!beanFactory.hasEmbeddedValueResolver()) {
		beanFactory.addEmbeddedValueResolver(strVal -> getEnvironment().resolvePlaceholders(strVal));
	}

	// Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.
	/**
	 * 实例化BeanFactory中的LoadTimeWeaverAware接口实现
	 * 这个接口可动态转化类，目前没有理解
	 */
	String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
	for (String weaverAwareName : weaverAwareNames) {
		getBean(weaverAwareName);
	}

	// Stop using the temporary ClassLoader for type matching.
	/**
	 * 将临时类加载器设置为null，即停止使用
	 */
	beanFactory.setTempClassLoader(null);

	// Allow for caching all bean definition metadata, not expecting further changes.
	/**
	 * 到这里BeanName不会再发生变化，将其缓存起来
	 * @see DefaultListableBeanFactory#freezeConfiguration()
	 */
	beanFactory.freezeConfiguration();

	// Instantiate all remaining (non-lazy-init) singletons.
	/**
	 * 这是实质性的对Bean进行实例化方法
	 *
	 * @see DefaultListableBeanFactory#preInstantiateSingletons()
	 */
	beanFactory.preInstantiateSingletons();
}

DefaultListableBeanFactory#preInstantiateSingletons()

/**
 * 对容器中的单例Bean进行创建
 */
@Override
public void preInstantiateSingletons() throws BeansException {
	if (logger.isDebugEnabled()) {
		logger.debug("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.
	/**
	 * 这里创建了一个全局BeanNames的副本，主要是为了在遍历副本时可为全局变量加入新的BeanDefinition
	 */
	List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);

	// Trigger initialization of all non-lazy singleton beans...
	/**
	 * 遍历
	 */
	for (String beanName : beanNames) {
		/**
		 * 进行BeanDefinition的合并
		 * 这里如果不需要合并，则返回对应的BeanDefinition
		 */
		RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
		/**
		 * BeanDefinition代表的类不是抽象类，是单例的且不是懒加载的，则进行创建
		 */
		if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
			/**
			 * 判断BeanName对应的BeanDefinition是不是FactoryBean
			 */
			if (isFactoryBean(beanName)) {
				/**
				 * 获得FactoryBean对象，即在BeanName前加上&符号
				 */
				Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
				/**
				 * Bean是FactoryBean类型的话，就进入if代码块
				 * java的安全管理器这里不阐述，如果Bean是FactoryBean类型的同时，也是SmartFactoryBean
				 * 类型(SmartFactoryBean接口是FactoryBean的子接口)，这个接口主要Spring内部使用，
				 * 如果实现这个接口并且isEagerInit方法返回true，证明这个FactoryBean需要急切初始化
				 * 这里会对这个FactoryBean进行实例创建
				 *
				 * 注意：FactoryBean默认是使用时调用getObject()进行实例创建
				 */
				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) {
						getBean(beanName);
					}
				}
			}
			/**
			 * 是普通类则直接进行创建
			 */
			else {
				getBean(beanName);
			}
		}
	}

	// Trigger post-initialization callback for all applicable beans...
	/**
	 * 对于需要在Bean实例化之后处理的工作，可实现SmartInitializingSingleton接口，重写afterSingletonsInstantiated()
	 * 方法进行处理，这里会进行回调
	 */
	for (String beanName : beanNames) {
		Object singletonInstance = getSingleton(beanName);
		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();
			}
		}
	}
}

AbstractBeanFactory#getBean()

@Override
public Object getBean(String name) throws BeansException {
	return doGetBean(name, null, null, false);
}

doGetBean

/**
 * 创建BeanName对应的Bean实例
 */
@SuppressWarnings("unchecked")
protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
		@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {

	/**
	 * 名称规范替换，例如别名替换为正式名
	 */
	final String beanName = transformedBeanName(name);
	Object bean;

	// Eagerly check singleton cache for manually registered singletons.
	/**
	 * 首先从单例Map中获取一次
	 * 在Spring初始化阶段，这个get是没有意义的，但是当手动调用getBean()操作时，这个get很重要
	 */
	Object sharedInstance = getSingleton(beanName);
	/**
	 * 如果缓存中存在并且没有参数，则可以直接返回
	 */
	if (sharedInstance != null && args == null) {
		if (logger.isDebugEnabled()) {
			if (isSingletonCurrentlyInCreation(beanName)) {
				logger.debug("Returning eagerly cached instance of singleton bean '" + beanName +
						"' that is not fully initialized yet - a consequence of a circular reference");
			}
			else {
				logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
			}
		}
		/**
		 * 从一个Object中获取Bean实例(对于FactoryBean来说)
		 * 否则直接返回sharedInstance这个实例
		 */
		bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
	}

	else {
		// Fail if we're already creating this bean instance:
		// We're assumably within a circular reference.
		/**
		 * 如果我们已经在创建此bean实例，则失败：
		 * 因为此时发生了不可修复的循环引用
		 */
		if (isPrototypeCurrentlyInCreation(beanName)) {
			throw new BeanCurrentlyInCreationException(beanName);
		}

		// Check if bean definition exists in this factory.
		/**
		 * 如果指定了父工厂，则利用父工厂进行Bean的创建
		 */
		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);
			}
			else if (args != null) {
				// Delegation to parent with explicit args.
				return (T) parentBeanFactory.getBean(nameToLookup, args);
			}
			else {
				// No args -> delegate to standard getBean method.
				return parentBeanFactory.getBean(nameToLookup, requiredType);
			}
		}

		/**
		 * 存放到集合中，表示Bean即将创建
		 */
		if (!typeCheckOnly) {
			markBeanAsCreated(beanName);
		}

		try {
			/**
			 * 根据BeanName获取BeanDefinition并检查
			 */
			final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
			checkMergedBeanDefinition(mbd, beanName, args);

			// Guarantee initialization of beans that the current bean depends on.
			/**
			 * 获取@DependsOn注解内容，主要保证Bean所依赖的类已经被实例化
			 * 如果存在依赖类，则先实例化依赖类
			 */
			String[] dependsOn = mbd.getDependsOn();
			if (dependsOn != null) {
				for (String dep : dependsOn) {
					if (isDependent(beanName, dep)) {
						throw new BeanCreationException(mbd.getResourceDescription(), beanName,
								"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
					}
					registerDependentBean(dep, beanName);
					try {
						getBean(dep);
					}
					catch (NoSuchBeanDefinitionException ex) {
						throw new BeanCreationException(mbd.getResourceDescription(), beanName,
								"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
					}
				}
			}

			// Create bean instance.
			/**
			 * 实例化单例Bean
			 */
			if (mbd.isSingleton()) {
				sharedInstance = getSingleton(beanName, () -> {
					try {
						/**
						 * 此对象是lambda表达式
						 * @see AbstractAutowireCapableBeanFactory#createBean(java.lang.String, org.springframework.beans.factory.support.RootBeanDefinition, java.lang.Object[])
						 */
						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;
					}
				});
				bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
			}
			/**
			 * 实例化原型Bean
			 */
			else if (mbd.isPrototype()) {
				// It's a prototype -> create a new instance.
				Object prototypeInstance = null;
				try {
					beforePrototypeCreation(beanName);
					prototypeInstance = createBean(beanName, mbd, args);
				}
				finally {
					afterPrototypeCreation(beanName);
				}
				bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
			}

			else {
				String scopeName = mbd.getScope();
				final Scope scope = this.scopes.get(scopeName);
				if (scope == null) {
					throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
				}
				try {
					Object scopedInstance = scope.get(beanName, () -> {
						beforePrototypeCreation(beanName);
						try {
							return createBean(beanName, mbd, args);
						}
						finally {
							afterPrototypeCreation(beanName);
						}
					});
					bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
				}
				catch (IllegalStateException ex) {
					throw new BeanCreationException(beanName,
							"Scope '" + scopeName + "' is not active for the current thread; consider " +
							"defining a scoped proxy for this bean if you intend to refer to it from a singleton",
							ex);
				}
			}
		}
		catch (BeansException ex) {
			cleanupAfterBeanCreationFailure(beanName);
			throw ex;
		}
	}

	// Check if required type matches the type of the actual bean instance.
	if (requiredType != null && !requiredType.isInstance(bean)) {
		try {
			T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
			if (convertedBean == null) {
				throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
			}
			return convertedBean;
		}
		catch (TypeMismatchException ex) {
			if (logger.isDebugEnabled()) {
				logger.debug("Failed to convert bean '" + name + "' to required type '" +
						ClassUtils.getQualifiedName(requiredType) + "'", ex);
			}
			throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
		}
	}
	return (T) bean;
}

transformedBeanName

/**
 * 返回Bean名称，包括去掉&前缀、别名替换为正规名称等
 */
protected String transformedBeanName(String name) {
	return canonicalName(BeanFactoryUtils.transformedBeanName(name));
}

getObjectForBeanInstance

/**
 * 从一个Object中获取Bean实例
 *  注意：从Object中获取Bean实例是对于实现了FactoryBean接口的Bean
 */
protected Object getObjectForBeanInstance(
		Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd) {

	// Don't let calling code try to dereference the factory if the bean isn't a factory.
	/**
	 * 判断BeanName是否以&开头，是的话则进入if代码块
	 *   如果Bean实现了NullBean接口，则直接返回Bean
	 *   如果Bean没有实现FactoryBean接口，则抛异常
	 */
	if (BeanFactoryUtils.isFactoryDereference(name)) {
		if (beanInstance instanceof NullBean) {
			return beanInstance;
		}
		if (!(beanInstance instanceof FactoryBean)) {
			throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass());
		}
	}

	// Now we have the bean instance, which may be a normal bean or a FactoryBean.
	// If it's a FactoryBean, we use it to create a bean instance, unless the
	// caller actually wants a reference to the factory.
	/**
	 * 如果Bean没有实现FactoryBean接口或BeanName是以&开头，则直接返回Bean
	 */
	if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) {
		return beanInstance;
	}

	/**
	 * 定义返回的实例
	 * 到此证明获取的是FactoryBean中getObject方法返回的实例，而不是获取FactoryBean
	 */
	Object object = null;
	/**
	 * 首先从缓存中获取
	 */
	if (mbd == null) {
		object = getCachedObjectForFactoryBean(beanName);
	}
	/**
	 * 返回对象没有在缓存中时，则将其强转为FactoryBean类型，判断对应的
	 */
	if (object == null) {
		// Return bean instance from factory.
		/**
		 * 强转为FactoryBean类型
		 */
		FactoryBean<?> factory = (FactoryBean<?>) beanInstance;
		// Caches object obtained from FactoryBean if it is a singleton.
		/**
		 * 判断此Bean对应的BeanDefinition是否是合成的
		 */
		if (mbd == null && containsBeanDefinition(beanName)) {
			mbd = getMergedLocalBeanDefinition(beanName);
		}
		boolean synthetic = (mbd != null && mbd.isSynthetic());
		/**
		 * 调用方法创建Bean实例
		 */
		object = getObjectFromFactoryBean(factory, beanName, !synthetic);
	}
	return object;
}

getObjectFromFactoryBean

/**
 * 从单例FactoryBean中或缓存中获取对象
 */
protected Object getObjectFromFactoryBean(FactoryBean<?> factory, String beanName, boolean shouldPostProcess) {
	/**
	 * FactoryBean是单例的并且已经实例化完成
	 */
	if (factory.isSingleton() && containsSingleton(beanName)) {
		synchronized (getSingletonMutex()) {
			/**
			 * 从缓存中获取
			 */
			Object object = this.factoryBeanObjectCache.get(beanName);
			/**
			 * 缓存中没有
			 */
			if (object == null) {
				/**
				 * 调用getObject()方法获取
				 */
				object = doGetObjectFromFactoryBean(factory, beanName);
				// Only post-process and store if not put there already during getObject() call above
				// (e.g. because of circular reference processing triggered by custom getBean calls)
				/**
				 * 上面调用getObject()方法可能会触发循环引用，这里进行一次获取
				 * 暂时不理解
				 */
				Object alreadyThere = this.factoryBeanObjectCache.get(beanName);
				if (alreadyThere != null) {
					object = alreadyThere;
				}
				else {
					/**
					 * 如果是合成的，则调用执行所有BeanPostProcessor的postProcessAfterInitialization回调处理
					 * @see AbstractAutowireCapableBeanFactory#postProcessObjectFromFactoryBean(java.lang.Object, java.lang.String)
					 */
					if (shouldPostProcess) {
						if (isSingletonCurrentlyInCreation(beanName)) {
							// Temporarily return non-post-processed object, not storing it yet..
							return object;
						}
						beforeSingletonCreation(beanName);
						try {
							object = postProcessObjectFromFactoryBean(object, beanName);
						}
						catch (Throwable ex) {
							throw new BeanCreationException(beanName,
									"Post-processing of FactoryBean's singleton object failed", ex);
						}
						finally {
							afterSingletonCreation(beanName);
						}
					}
					/**
					 * 实例的BeanName已经注册了，则将此Bean实例存放到缓存中
					 */
					if (containsSingleton(beanName)) {
						this.factoryBeanObjectCache.put(beanName, object);
					}
				}
			}
			return object;
		}
	}
	else {
		/**
		 * 到这里证明
		 * 		FactoryBean不是单例的或
		 * 		getObject()方法返回的实例的BeanName并没有注册进容器中(没有执行过getObject方法)
		 * 则调用getObject()方法进行实例的创建
		 */
		Object object = doGetObjectFromFactoryBean(factory, beanName);
		/**
		 * 如果是合成的，则调用执行所有BeanPostProcessor的postProcessAfterInitialization回调处理
		 * @see AbstractAutowireCapableBeanFactory#postProcessObjectFromFactoryBean(java.lang.Object, java.lang.String)
		 */
		if (shouldPostProcess) {
			try {
				object = postProcessObjectFromFactoryBean(object, beanName);
			}
			catch (Throwable ex) {
				throw new BeanCreationException(beanName, "Post-processing of FactoryBean's object failed", ex);
			}
		}
		return object;
	}
}

getMergedLocalBeanDefinition

/**
 * 返回合并的RootBeanDefinition，如果指定的bean对应于子bean定义，则遍历父bean定义。
 * Spring2.5之前在XML中使用,如下情况B的定义需要合并A的定义
 * <bean id="A" class="xxx"></bean>
 * <bean id="B" parent="A"></bean>
 */
protected RootBeanDefinition getMergedLocalBeanDefinition(String beanName) throws BeansException {
	// Quick check on the concurrent map first, with minimal locking.
	/**
	 * 从合并BeanDefinition的集合中取出，能取出则直接返回
	 */
	RootBeanDefinition mbd = this.mergedBeanDefinitions.get(beanName);
	if (mbd != null) {
		return mbd;
	}
	/**
	 * 从集合中根据BeanName取出BeanDefinition{@link DefaultListableBeanFactory#getBeanDefinition(java.lang.String)}
	 * 然后执行可能会进行的合并BeanDefinition操作
	 */
	return getMergedBeanDefinition(beanName, getBeanDefinition(beanName));
}

DefaultSingletonBeanRegistry#getSingleton()

/**
 * 返回BeanName对应的单例对象，如果没有注册则注册一个并返回
 */
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
	Assert.notNull(beanName, "Bean name must not be null");
	synchronized (this.singletonObjects) {
		/**
		 * 从单例对象Map集合中get
		 */
		Object singletonObject = this.singletonObjects.get(beanName);
		/**
		 * 集合中不存在则进行创建
		 */
		if (singletonObject == null) {
			/**
			 * 判断工厂是否正在销毁单例对象，即工厂处于关闭过程中，这时进行创建操作，报错
			 */
			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 + "'");
			}
			/**
			 * 进行创建之前的必要操作
			 */
			beforeSingletonCreation(beanName);
			/**
			 * 是否正确创建对象的标识，初始为false，当正确创建对象后置为true
			 */
			boolean newSingleton = false;
			/**
			 * 抑制异常的集合，在对象创建过程中，会发生一些可处理的异常，会被存放进这个
			 * 集合中，会在catch代码块中进行提取并存放进异常对象中进行打印，
			 * 这些不是导致失败的直接原因，而是在创建bean实例之前发生的，例如引用链
			 * 如果这个集合不为null，则证明当前正在创建的对象是因为其它对象依赖本对象
			 * 而进行创建操作
			 */
			boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
			if (recordSuppressedExceptions) {
				this.suppressedExceptions = new LinkedHashSet<>();
			}
			try {
				/**
				 * 执行创建对象
				 * 创建成功后将标识置为true
				 */
				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 {
				/**
				 * 将抑制异常集合置为null
				 */
				if (recordSuppressedExceptions) {
					this.suppressedExceptions = null;
				}
				/**
				 * 执行创建对象后的必要操作
				 */
				afterSingletonCreation(beanName);
			}
			/**
			 * 判断标识，将实例进行必要存储
			 */
			if (newSingleton) {
				addSingleton(beanName, singletonObject);
			}
		}
		return singletonObject;
	}
}

AbstractAutowireCapableBeanFactory#createBean()

/**
 * 创建bean实例、填充bean实例、应用后处理器等
 */
@Override
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
		throws BeanCreationException {

	if (logger.isDebugEnabled()) {
		logger.debug("Creating instance of bean '" + beanName + "'");
	}
	/**
	 * 进行BeanDefinition的引用赋值，见名知意理解为即将使用的BeanDefinition
	 */
	RootBeanDefinition mbdToUse = mbd;

	// Make sure bean class is actually resolved at this point, and
	// clone the bean definition in case of a dynamically resolved Class
	// which cannot be stored in the shared merged bean definition.
	/**
	 * 获取BeanDefinition中Class
	 */
	Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
	/**
	 * 存在Class并且没有指定实例并且类名不为空则对BeanDefinition进行克隆
	 * 动态解析的类不能存储在共享合并bean定义中的情况下克隆bean定义。
	 */
	if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
		mbdToUse = new RootBeanDefinition(mbd);
		mbdToUse.setBeanClass(resolvedClass);
	}

	// Prepare method overrides.
	try {
		/**
		 * 做重写检查
		 */
		mbdToUse.prepareMethodOverrides();
	}
	catch (BeanDefinitionValidationException ex) {
		throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
				beanName, "Validation of method overrides failed", ex);
	}

	try {
		// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
		/**
		 * 让BeanPostProcessors有机会返回代理而不是目标bean实例。
		 * 可以通过BeanPostProcessor实例化bd，不需要进行后面的流程
		 * 框架内部使用，一般不会用
		 */
		Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
		if (bean != null) {
			return bean;
		}
	}
	catch (Throwable ex) {
		throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName,
				"BeanPostProcessor before instantiation of bean failed", ex);
	}

	try {
		/**
		 * 实际进行Bean的创建，也可能返回代理Bean
		 */
		Object beanInstance = doCreateBean(beanName, mbdToUse, args);
		if (logger.isDebugEnabled()) {
			logger.debug("Finished creating instance of bean '" + beanName + "'");
		}
		return beanInstance;
	}
	catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {
		// A previously detected exception with proper bean creation context already,
		// or illegal singleton state to be communicated up to DefaultSingletonBeanRegistry.
		throw ex;
	}
	catch (Throwable ex) {
		throw new BeanCreationException(
				mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
	}
}

doCreateBean

/**
 * 实际创建指定的bean。此时已经进行了预创建处理，例如，
 * 检查{@code postProcessBeforeInstantiation}回调。
 * <p>区分默认bean实例化、工厂方法的使用和自动连接构造函数。
 */
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
		throws BeanCreationException {

	// Instantiate the bean.
	/**
	 * 这是一个Bean实例包装类
	 */
	BeanWrapper instanceWrapper = null;
	/**
	 * BeanDefinition代表单例时，先从缓存中通过删除方法返回实例，
	 */
	if (mbd.isSingleton()) {
		instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
	}
	/**
	 * 返回为null则进行创建
	 */
	if (instanceWrapper == null) {
		instanceWrapper = createBeanInstance(beanName, mbd, args);
	}
	/**
	 * 到此为止，Bean已经完成了实例创建
	 * 获取原始实例和原始Class
	 */
	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.
	/**
	 * 如果Spring中存在MergedBeanDefinitionPostProcessors类型的BeanPostProcessor时，执行
	 * 允许后处理器修改合并的bean定义。
	 */
	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;
		}
	}

	// Eagerly cache singletons to be able to resolve circular references
	// even when triggered by lifecycle interfaces like BeanFactoryAware.
	/**
	 * 急切地缓存singleton，以便能够解析循环引用，
	 * 即使是由生命周期接口（如BeanFactoryAware）触发时也是如此。
	 * 提前缓存ObjectFactory以解决bean之间的循环依赖
	 * mbd.isSingleton()->是否单例
	 * allowCircularReferences->是否允许循环依赖
	 * isSingletonCurrentlyInCreation->该bean是否创建中
	 */
	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");
		}
		addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
	}

	// Initialize the bean instance.
	/**
	 * 正式开始初始化Bean实例
	 */
	Object exposedObject = bean;
	try {
		/**
		 * 对Bean中的属性进行填充
		 */
		populateBean(beanName, mbd, instanceWrapper);
		/**
		 * Bean的初始化
		 */
		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);
		}
	}
	/**
	 * 确实发生了循环依赖，则进入本if代码块
	 */
	if (earlySingletonExposure) {
		/**
		 * 获取BeanName对应的Bean实例，也就是从集合中获取的
		 */
		Object earlySingletonReference = getSingleton(beanName, false);
		if (earlySingletonReference != null) {
			/**
			 * 集合中存在并且实例是相等的(没有发生重新创建操作)
			 * 则将返回的实例替换成刚提取的实例
			 */
			if (exposedObject == bean) {
				exposedObject = earlySingletonReference;
			}
			/**
			 * 如果Bean发生了变化，且有其它Bean依赖了此Bean
			 */
			else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
				/**
				 * 获取这些Bean的BeanName进行遍历
				 */
				String[] dependentBeans = getDependentBeans(beanName);
				Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
				for (String dependentBean : dependentBeans) {
					/**
					 * 如果这些Bean已经实例化完成了，会返回false，添加到集合中下面会抛异常
					 * 如果没有实例化完成，则将其从集合中删除实例，以后再重新创建实例
					 */
					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.");
				}
			}
		}
	}

	// Register bean as disposable.
	try {
		/**
		 * 注册容器销毁时此实例的销毁方法
		 */
		registerDisposableBeanIfNecessary(beanName, bean, mbd);
	}
	catch (BeanDefinitionValidationException ex) {
		throw new BeanCreationException(
				mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
	}

	return exposedObject;
}