LinweiWang

protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
	// Initialize conversion service for this context.
	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 BeanFactoryPostProcessor
	// (such as a PropertySourcesPlaceholderConfigurer bean) registered any before:
	// at this point, primarily for resolution in annotation attribute values.
	if (!beanFactory.hasEmbeddedValueResolver()) {
		beanFactory.addEmbeddedValueResolver(strVal -> getEnvironment().resolvePlaceholders(strVal));
	}

	// Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.
	String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
	for (String weaverAwareName : weaverAwareNames) {
		getBean(weaverAwareName);
	}

	// Stop using the temporary ClassLoader for type matching.
	beanFactory.setTempClassLoader(null);

	// Allow for caching all bean definition metadata, not expecting further changes.
	beanFactory.freezeConfiguration();

	// 实例化所有立即加载的单例 Bean
	// Instantiate all remaining (non-lazy-init) singletons.
	beanFactory.preInstantiateSingletons();
}

@Override
public void preInstantiateSingletons() throws BeansException {
	if (logger.isTraceEnabled()) {
		logger.trace("Pre-instantiating singletons in " + this);
	}

	// 存放 BeanNames
	// 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.
	List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);

	// 触发所有非延迟加载单例 Bean 的初始化，主要步骤是 getBean
	// Trigger initialization of all non-lazy singleton beans...
	for (String beanName : beanNames) {
		// 合并父 BeanDefinition 对象
		RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
		if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
			// 工厂 Bean：&BeanName
			if (isFactoryBean(beanName)) {
				Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
				if (bean instanceof FactoryBean) {
					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);
					}
				}
			}
			else { // 实例化 Bean
				getBean(beanName);
			}
		}
	}

	// Trigger post-initialization callback for all applicable beans...
	for (String beanName : beanNames) {
		Object singletonInstance = getSingleton(beanName);
		if (singletonInstance instanceof SmartInitializingSingleton) {
			StartupStep smartInitialize = getApplicationStartup().start("spring.beans.smart-initialize")
					.tag("beanName", beanName);
			SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
			if (System.getSecurityManager() != null) {
				AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
					smartSingleton.afterSingletonsInstantiated();
					return null;
				}, getAccessControlContext());
			}
			else {
				smartSingleton.afterSingletonsInstantiated();
			}
			smartInitialize.end();
		}
	}
}

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

protected <T> T doGetBean(
		String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)
		throws BeansException {

	// 解析 BeanName，如果以 & 开头则去掉 & ，如果是别名则获取到真正的名字
	String beanName = transformedBeanName(name);
	Object beanInstance;

	// 从缓存获取 Bean（注意点：三级缓存）
	// Eagerly check singleton cache for manually registered singletons.
	Object sharedInstance = getSingleton(beanName);
	// 如果存在即返回
	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 + "'");
			}
		}
		// 针对 FactoryBean 做处理
		beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, null);
	}

	else {
		// prototype 类型的 Bean 不支持循环依赖
		// Fail if we're already creating this bean instance:
		// We're assumably within a circular reference.
		if (isPrototypeCurrentlyInCreation(beanName)) {
			throw new BeanCurrentlyInCreationException(beanName);
		}

		// 检查父工厂中是否已存在该对象
		// Check if bean definition exists in this factory.
		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 if (requiredType != null) {
				// No args -> delegate to standard getBean method.
				return parentBeanFactory.getBean(nameToLookup, requiredType);
			}
			else {
				return (T) parentBeanFactory.getBean(nameToLookup);
			}
		}

		// 标记
		if (!typeCheckOnly) {
			markBeanAsCreated(beanName);
		}

		StartupStep beanCreation = this.applicationStartup.start("spring.beans.instantiate")
				.tag("beanName", name);
		try {
			if (requiredType != null) {
				beanCreation.tag("beanType", requiredType::toString);
			}
			// 合并父子 Bean 属性
			RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
			checkMergedBeanDefinition(mbd, beanName, args);

			// 处理 dependsOn 配置
			// Guarantee initialization of beans that the current bean depends on.
			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);
					}
				}
			}

			// 创建 Bean 实例
			// Create bean instance.
			if (mbd.isSingleton()) {
				// 单例
				sharedInstance = getSingleton(beanName, () -> {
					try {
						// 创建 Bean
						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;
					}
				});
				beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
			}

			else if (mbd.isPrototype()) {
				// 原型
				// It's a prototype -> create a new instance.
				Object prototypeInstance = null;
				try {
					beforePrototypeCreation(beanName);
					prototypeInstance = createBean(beanName, mbd, args);
				}
				finally {
					afterPrototypeCreation(beanName);
				}
				beanInstance = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
			}

			else {
				String scopeName = mbd.getScope();
				if (!StringUtils.hasLength(scopeName)) {
					throw new IllegalStateException("No scope name defined for bean '" + beanName + "'");
				}
				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);
						}
					});
					beanInstance = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
				}
				catch (IllegalStateException ex) {
					throw new ScopeNotActiveException(beanName, scopeName, ex);
				}
			}
		}
		catch (BeansException ex) {
			beanCreation.tag("exception", ex.getClass().toString());
			beanCreation.tag("message", String.valueOf(ex.getMessage()));
			cleanupAfterBeanCreationFailure(beanName);
			throw ex;
		}
		finally {
			beanCreation.end();
		}
	}

	return adaptBeanInstance(name, beanInstance, requiredType);
}

public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
	Assert.notNull(beanName, "Bean name must not be null");
	synchronized (this.singletonObjects) {
		// 丛单例池中获取 Bean
		Object singletonObject = this.singletonObjects.get(beanName);
		if (singletonObject == null) {
			// 是否正在销毁，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 + "'");
			}
			// 验证完真正开始创建对象，先标识该 Bean 正在被创建
			beforeSingletonCreation(beanName);
			boolean newSingleton = false;
			boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
			if (recordSuppressedExceptions) {
				this.suppressedExceptions = new LinkedHashSet<>();
			}
			try {
				// 传进来的 Lambda 表达式： singletonFactory ，并调用 getObject
				// （即上一步骤中的 () -> { try { return createBean(beanName, mbd, args); } catch (BeansException ex) { ... } }）
				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) {
				addSingleton(beanName, singletonObject);
			}
		}
		return singletonObject;
	}
}

protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
		throws BeanCreationException {

	if (logger.isTraceEnabled()) {
		logger.trace("Creating instance of bean '" + beanName + "'");
	}
	// 拿到 mbd
	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.
	Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
	if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
		mbdToUse = new RootBeanDefinition(mbd);
		mbdToUse.setBeanClass(resolvedClass);
	}

	// Prepare method overrides.
	try {
		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.
		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 {
		// doCreateBean 方法进行创建 Bean
		Object beanInstance = doCreateBean(beanName, mbdToUse, args);
		if (logger.isTraceEnabled()) {
			logger.trace("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);
	}
}

protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
		throws BeanCreationException {

	// Instantiate the bean.
	BeanWrapper instanceWrapper = null;
	if (mbd.isSingleton()) {
		instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
	}
	if (instanceWrapper == null) {
		// 创建 Bean 实例，仅调用构造方法，尚未设置属性
		instanceWrapper = createBeanInstance(beanName, mbd, args);
	}
	Object bean = instanceWrapper.getWrappedInstance();
	Class<?> beanType = instanceWrapper.getWrappedClass();
	if (beanType != NullBean.class) {
		mbd.resolvedTargetType = beanType;
	}

	// Allow post-processors to modify the merged bean definition.
	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.
	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));
	}

	// 初始化 Bean 实例：属性填充
	// Initialize the bean instance.
	Object exposedObject = bean;
	try {
		// 属性填充
		populateBean(beanName, mbd, instanceWrapper);
		// 调用初始化方法，引用 BeanPostProcessor 后置处理器
		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) {
		Object earlySingletonReference = getSingleton(beanName, false);
		if (earlySingletonReference != null) {
			if (exposedObject == bean) {
				exposedObject = earlySingletonReference;
			}
			else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
				String[] dependentBeans = getDependentBeans(beanName);
				Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
				for (String dependentBean : dependentBeans) {
					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 " +
							"'getBeanNamesForType' with the 'allowEagerInit' flag turned off, for example.");
				}
			}
		}
	}

	// Register bean as disposable.
	try {
		registerDisposableBeanIfNecessary(beanName, bean, mbd);
	}
	catch (BeanDefinitionValidationException ex) {
		throw new BeanCreationException(
				mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
	}

	return exposedObject;
}

@Nullable
private <T> NamedBeanHolder<T> resolveNamedBean(
		String beanName, ResolvableType requiredType, @Nullable Object[] args) throws BeansException {
       // AbstractBeanFactory#getBean 里面会调用 doGetBean
	Object bean = getBean(beanName, null, args);
	if (bean instanceof NullBean) {
		return null;
	}
	return new NamedBeanHolder<T>(beanName, adaptBeanInstance(beanName, bean, requiredType.toClass()));
}

// 处理循环依赖：单例、允许循环依赖
// Eagerly cache singletons to be able to resolve circular references
// even when triggered by lifecycle interfaces like BeanFactoryAware.
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");
	}
	// 加入三级缓存
	addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
// 初始化 Bean 实例：属性填充
// Initialize the bean instance.
Object exposedObject = bean;
try {
	// 属性填充
	populateBean(beanName, mbd, instanceWrapper);
	// 调用初始化方法，引用 BeanPostProcessor 后置处理器
	exposedObject = initializeBean(beanName, exposedObject, mbd);
}

if (pvs != null) {
	// 处理属性
	applyPropertyValues(beanName, mbd, bw, pvs);
}

String propertyName = pv.getName();
Object originalValue = pv.getValue();
if (originalValue == AutowiredPropertyMarker.INSTANCE) {
	Method writeMethod = bw.getPropertyDescriptor(propertyName).getWriteMethod();
	if (writeMethod == null) {
		throw new IllegalArgumentException("Autowire marker for property without write method: " + pv);
	}
	originalValue = new DependencyDescriptor(new MethodParameter(writeMethod, 0), true);
}
// 真正去处理属性值
Object resolvedValue = valueResolver.resolveValueIfNecessary(pv, originalValue);
Object convertedValue = resolvedValue;
boolean convertible = bw.isWritableProperty(propertyName) &&
		!PropertyAccessorUtils.isNestedOrIndexedProperty(propertyName);
if (convertible) {
	convertedValue = convertForProperty(resolvedValue, propertyName, bw, converter);
}

// 检查属性是否需要运行时引用另外一个 Bean
// We must check each value to see whether it requires a runtime reference
// to another bean to be resolved.
if (value instanceof RuntimeBeanReference) {
	RuntimeBeanReference ref = (RuntimeBeanReference) value;
	return resolveReference(argName, ref);
}

// 获取 Bean
resolvedName = String.valueOf(doEvaluate(ref.getBeanName()));
bean = this.beanFactory.getBean(resolvedName);

protected Object getSingleton(String beanName, boolean allowEarlyReference) {
	// Quick check for existing instance without full singleton lock
	// 从一级缓存 singletonObjects 获取
	Object singletonObject = this.singletonObjects.get(beanName);
	if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
		// 从二级缓存 earlySingletonObjects 获取
		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) {
						// 从三级缓存 singletonFactories 获取
						ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
						if (singletonFactory != null) {
							singletonObject = singletonFactory.getObject();
							// 从三级缓存 singletonFactories 转移至 二级缓存 earlySingletonObjects
							this.earlySingletonObjects.put(beanName, singletonObject);
							this.singletonFactories.remove(beanName);
						}
					}
				}
			}
		}
	}
	return singletonObject;
}

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);
	}
}

ApplicationContext context = new ClassPathXmlApplicationContext("beans.xml")
ApplicationContext context = new FileSystemXmlApplicationContext("C:/beans.xml")

ApplicationContext context = new AnnotationConfigApplicationContext(SpringConfig.class)

   // 创建 ClassPathXmlApplicationContext，加载 XML 中的定义信息，并且自动 refresh 容器（Context）
public ClassPathXmlApplicationContext(String configLocation) throws BeansException {
	// 调用重载方法
	this(new String[] {configLocation}, true, null);
}

public ClassPathXmlApplicationContext(
		String[] configLocations, boolean refresh, @Nullable ApplicationContext parent)
		throws BeansException {

	// 初始化父类
	super(parent);
	// 设置配置文件
	setConfigLocations(configLocations);
	// refresh context
	if (refresh) {
		refresh();
	}
}

@Override
public void refresh() throws BeansException, IllegalStateException {
	// 对象锁
	synchronized (this.startupShutdownMonitor) {
		StartupStep contextRefresh = this.applicationStartup.start("spring.context.refresh");

		// 刷新前的预处理
		// Prepare this context for refreshing.
		prepareRefresh();

		// 获取 BeanFactory: 默认实现是 DefaultListableBeanFactory
		// 加载 BeanDefinition 并注册到 BeanDefinitionRegistry
		// Tell the subclass to refresh the internal bean factory.
		ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

		// BeanFactory 预准备工作
		// Prepare the bean factory for use in this context.
		prepareBeanFactory(beanFactory);

		try {
			// BeanFactory 准备工作完成后的后置处理，留给子类实现
			// Allows post-processing of the bean factory in context subclasses.
			postProcessBeanFactory(beanFactory);

			StartupStep beanPostProcess = this.applicationStartup.start("spring.context.beans.post-process");
			// 实例化实现了 BeanFactoryPostProcessor 接口的 Bean，并调用该接口方法
			// Invoke factory processors registered as beans in the context.
			invokeBeanFactoryPostProcessors(beanFactory);
			// 注册 BeanPostProcessor （Bean 的后置处理器），在创建 Bean 的前后执行
			// Register bean processors that intercept bean creation.
			registerBeanPostProcessors(beanFactory);
			beanPostProcess.end();

			// 初始化 MessageSource 组件：国际化、消息绑定、消息解析等
			// Initialize message source for this context.
			initMessageSource();

			// 初始化事件派发器
			// Initialize event multicaster for this context.
			initApplicationEventMulticaster();

			// 初始化其他特殊的 Bean，在容器刷新的时候子类自定义实现：如创建 Tomcat、Jetty 等 Web 服务器
			// Initialize other special beans in specific context subclasses.
			onRefresh();

			// 注册应用监听器（即实现了 ApplicationListener 接口的 Bean）
			// Check for listener beans and register them.
			registerListeners();

			// 初始化创建非懒加载的单例 Bean、填充属性、调用初始化方法（ afterPropertiesSet，init-method 等）、调用 BeanPostProcessor 后置处理器
			// Instantiate all remaining (non-lazy-init) singletons.
			finishBeanFactoryInitialization(beanFactory);

			// 完成 context 刷新，调用 LifecycleProcessor 的 onRefresh 方法并发布 ContextRefreshedEvent
			// Last step: publish corresponding event.
			finishRefresh();
		}

		catch (BeansException ex) {
			if (logger.isWarnEnabled()) {
				logger.warn("Exception encountered during context initialization - " +
						"cancelling refresh attempt: " + ex);
			}

			// Destroy already created singletons to avoid dangling resources.
			destroyBeans();

			// Reset 'active' flag.
			cancelRefresh(ex);

			// Propagate exception to caller.
			throw ex;
		}

		finally {
			// Reset common introspection caches in Spring's core, since we
			// might not ever need metadata for singleton beans anymore...
			resetCommonCaches();
			contextRefresh.end();
		}
	}
}

protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
	// 对 BeanFactory 进行刷新操作，默认实现 AbstractRefreshableApplicationContext#refreshBeanFactory
	refreshBeanFactory();
	// 返回上一步 beanFactory，默认实现 AbstractRefreshableApplicationContext#getBeanFactory
	return getBeanFactory();
}

@Override
protected final void refreshBeanFactory() throws BeansException {
	// 判断是否已有 BeanFactory
	if (hasBeanFactory()) {
		// 销毁 Bean
		destroyBeans();
		// 关闭 BeanFactory
		closeBeanFactory();
	}
	try {
		// 实例化 DefaultListableBeanFactory
		DefaultListableBeanFactory beanFactory = createBeanFactory();
		// 设置序列化 ID
		beanFactory.setSerializationId(getId());
		// 自定义 BeanFactory 的一些属性：allowBeanDefinitionOverriding、allowCircularReferences
		//   allowBeanDefinitionOverriding 是否允许覆盖
		//   allowCircularReferences  是否允许循环依赖
		customizeBeanFactory(beanFactory);
		// 加载应用中的 BeanFactory
		loadBeanDefinitions(beanFactory);
		// 赋值给当前 beanFactory 属性
		this.beanFactory = beanFactory;
	}
	catch (IOException ex) {
		throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
	}
}

@Override
public final ConfigurableListableBeanFactory getBeanFactory() {
	DefaultListableBeanFactory beanFactory = this.beanFactory;
	if (beanFactory == null) {
		throw new IllegalStateException("BeanFactory not initialized or already closed - " +
				"call 'refresh' before accessing beans via the ApplicationContext");
	}
	return beanFactory;
}

@Override
protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
	// 给指定的 BeanFactory 创建一个 XmlBeanDefinitionReader 进行读取和解析 XML
	// Create a new XmlBeanDefinitionReader for the given BeanFactory.
	XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);

	// 给 XmlBeanDefinitionReader 设置上下文信息
	// Configure the bean definition reader with this context's
	// resource loading environment.
	beanDefinitionReader.setEnvironment(getEnvironment());
	beanDefinitionReader.setResourceLoader(this);
	beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));

	// 提供给子类上西安的模板方法：自定义初始化策略
	// Allow a subclass to provide custom initialization of the reader,
	// then proceed with actually loading the bean definitions.
	initBeanDefinitionReader(beanDefinitionReader);
	// 真正的去加载 BeanDefinitions
	loadBeanDefinitions(beanDefinitionReader);
}

protected void loadBeanDefinitions(XmlBeanDefinitionReader reader) throws BeansException, IOException {
	// 从 Resource 资源对象加载 BeanDefinitions
	Resource[] configResources = getConfigResources();
	if (configResources != null) {
		reader.loadBeanDefinitions(configResources);
	}
	// 从 XML 配置文件加载 BeanDefinition 对象
	String[] configLocations = getConfigLocations();
	if (configLocations != null) {
		reader.loadBeanDefinitions(configLocations);
	}
}

@Override
public int loadBeanDefinitions(String... locations) throws BeanDefinitionStoreException {
	Assert.notNull(locations, "Location array must not be null");
	int count = 0;
	// 如果有多个配置文件，循环读取加载，并统计数量
	for (String location : locations) {
		count += loadBeanDefinitions(location);
	}
	return count;
}

@Override
public int loadBeanDefinitions(String location) throws BeanDefinitionStoreException {
	return loadBeanDefinitions(location, null);
}

public int loadBeanDefinitions(String location, @Nullable Set<Resource> actualResources) throws BeanDefinitionStoreException {
	// 获取上下文的 ResourceLoader （资源加载器）
	ResourceLoader resourceLoader = getResourceLoader();
	if (resourceLoader == null) {
		throw new BeanDefinitionStoreException(
				"Cannot load bean definitions from location [" + location + "]: no ResourceLoader available");
	}

	// 判断资源加载器是否为 ResourcePatternResolver 类型
	if (resourceLoader instanceof ResourcePatternResolver) {
		// Resource pattern matching available.
		try {
			// 统一加载转换为 Resource 对象
			Resource[] resources = ((ResourcePatternResolver) resourceLoader).getResources(location);
			// 通过 Resource 对象加载 BeanDefinitions
			int count = loadBeanDefinitions(resources);
			if (actualResources != null) {
				Collections.addAll(actualResources, resources);
			}
			if (logger.isTraceEnabled()) {
				logger.trace("Loaded " + count + " bean definitions from location pattern [" + location + "]");
			}
			return count;
		}
		catch (IOException ex) {
			throw new BeanDefinitionStoreException(
					"Could not resolve bean definition resource pattern [" + location + "]", ex);
		}
	}
	else {
   		// 否则以绝对路径转换为 Resource 对象
		// Can only load single resources by absolute URL.
		Resource resource = resourceLoader.getResource(location);
		int count = loadBeanDefinitions(resource);
		if (actualResources != null) {
			actualResources.add(resource);
		}
		if (logger.isTraceEnabled()) {
			logger.trace("Loaded " + count + " bean definitions from location [" + location + "]");
		}
		return count;
	}
}

@Override
public int loadBeanDefinitions(Resource... resources) throws BeanDefinitionStoreException {
	Assert.notNull(resources, "Resource array must not be null");
	int count = 0;
	for (Resource resource : resources) {
		count += loadBeanDefinitions(resource);
	}
	return count;
}

@Override
public int loadBeanDefinitions(Resource... resources) throws BeanDefinitionStoreException {
	Assert.notNull(resources, "Resource array must not be null");
	int count = 0;
	for (Resource resource : resources) {
		// 加载
		count += loadBeanDefinitions(resource);
	}
	return count;
}

@Override
public int loadBeanDefinitions(Resource resource) throws BeanDefinitionStoreException {
	return loadBeanDefinitions(new EncodedResource(resource));
}

public int loadBeanDefinitions(EncodedResource encodedResource) throws BeanDefinitionStoreException {
	Assert.notNull(encodedResource, "EncodedResource must not be null");
	if (logger.isTraceEnabled()) {
		logger.trace("Loading XML bean definitions from " + encodedResource);
	}

	Set<EncodedResource> currentResources = this.resourcesCurrentlyBeingLoaded.get();

	if (!currentResources.add(encodedResource)) {
		throw new BeanDefinitionStoreException(
				"Detected cyclic loading of " + encodedResource + " - check your import definitions!");
	}

	try (InputStream inputStream = encodedResource.getResource().getInputStream()) {
		// 把 XML 文件流封装为 InputSource 对象
		InputSource inputSource = new InputSource(inputStream);
		if (encodedResource.getEncoding() != null) {
			inputSource.setEncoding(encodedResource.getEncoding());
		}
		// 执行加载逻辑
		return doLoadBeanDefinitions(inputSource, encodedResource.getResource());
	}
	catch (IOException ex) {
		throw new BeanDefinitionStoreException(
				"IOException parsing XML document from " + encodedResource.getResource(), ex);
	}
	finally {
		currentResources.remove(encodedResource);
		if (currentResources.isEmpty()) {
			this.resourcesCurrentlyBeingLoaded.remove();
		}
	}
}

protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
		throws BeanDefinitionStoreException {

	try {
		// 读取 XML 信息，将 XML 中信息保存到 Document 对象中
		Document doc = doLoadDocument(inputSource, resource);
		// 再解析 Document 对象，封装为 BeanDefinition 对象进行注册
		int count = registerBeanDefinitions(doc, resource);
		if (logger.isDebugEnabled()) {
			logger.debug("Loaded " + count + " bean definitions from " + resource);
		}
		return count;
	}
	catch (BeanDefinitionStoreException ex) {
		throw ex;
	}
	catch (SAXParseException ex) {
		throw new XmlBeanDefinitionStoreException(resource.getDescription(),
				"Line " + ex.getLineNumber() + " in XML document from " + resource + " is invalid", ex);
	}
	catch (SAXException ex) {
		throw new XmlBeanDefinitionStoreException(resource.getDescription(),
				"XML document from " + resource + " is invalid", ex);
	}
	catch (ParserConfigurationException ex) {
		throw new BeanDefinitionStoreException(resource.getDescription(),
				"Parser configuration exception parsing XML from " + resource, ex);
	}
	catch (IOException ex) {
		throw new BeanDefinitionStoreException(resource.getDescription(),
				"IOException parsing XML document from " + resource, ex);
	}
	catch (Throwable ex) {
		throw new BeanDefinitionStoreException(resource.getDescription(),
				"Unexpected exception parsing XML document from " + resource, ex);
	}
}

public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
	BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
	// 获取已有 BeanDefinition 的数量
	int countBefore = getRegistry().getBeanDefinitionCount();
	// 注册 BeanDefinition
	documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
	// 计算出新注册的 BeanDefinition 数量
	return getRegistry().getBeanDefinitionCount() - countBefore;
}

@Override
public void registerBeanDefinitions(Document doc, XmlReaderContext readerContext) {
	this.readerContext = readerContext;
	doRegisterBeanDefinitions(doc.getDocumentElement());
}

protected void doRegisterBeanDefinitions(Element root) {
	// Any nested <beans> elements will cause recursion in this method. In
	// order to propagate and preserve <beans> default-* attributes correctly,
	// keep track of the current (parent) delegate, which may be null. Create
	// the new (child) delegate with a reference to the parent for fallback purposes,
	// then ultimately reset this.delegate back to its original (parent) reference.
	// this behavior emulates a stack of delegates without actually necessitating one.
	BeanDefinitionParserDelegate parent = this.delegate;
	this.delegate = createDelegate(getReaderContext(), root, parent);

	if (this.delegate.isDefaultNamespace(root)) {
		String profileSpec = root.getAttribute(PROFILE_ATTRIBUTE);
		if (StringUtils.hasText(profileSpec)) {
			String[] specifiedProfiles = StringUtils.tokenizeToStringArray(
					profileSpec, BeanDefinitionParserDelegate.MULTI_VALUE_ATTRIBUTE_DELIMITERS);
			// We cannot use Profiles.of(...) since profile expressions are not supported
			// in XML config. See SPR-12458 for details.
			if (!getReaderContext().getEnvironment().acceptsProfiles(specifiedProfiles)) {
				if (logger.isDebugEnabled()) {
					logger.debug("Skipped XML bean definition file due to specified profiles [" + profileSpec +
							"] not matching: " + getReaderContext().getResource());
				}
				return;
			}
		}
	}

	preProcessXml(root);
	// 真正解析 XML
	parseBeanDefinitions(root, this.delegate);
	postProcessXml(root);

	this.delegate = parent;
}

protected void parseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate) {
	if (delegate.isDefaultNamespace(root)) {
		NodeList nl = root.getChildNodes();
		for (int i = 0; i < nl.getLength(); i++) {
			Node node = nl.item(i);
			if (node instanceof Element) {
				Element ele = (Element) node;
				if (delegate.isDefaultNamespace(ele)) {
					// 解析默认标签元素："import", "alias", "bean"
					parseDefaultElement(ele, delegate);
				}
				else {
					// 解析自定义标签
					delegate.parseCustomElement(ele);
				}
			}
		}
	}
	else {
		// 解析自定义标签
		delegate.parseCustomElement(root);
	}
}

private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) {
	// import 标签
	if (delegate.nodeNameEquals(ele, IMPORT_ELEMENT)) {
		importBeanDefinitionResource(ele);
	}
	// alias 标签
	else if (delegate.nodeNameEquals(ele, ALIAS_ELEMENT)) {
		processAliasRegistration(ele);
	}
	// bean 标签：着重分析
	else if (delegate.nodeNameEquals(ele, BEAN_ELEMENT)) {
		processBeanDefinition(ele, delegate);
	}
	// 嵌套 bean 标签
	else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {
		// recurse
		doRegisterBeanDefinitions(ele);
	}
}

protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
	// 解析 bean 标签为 BeanDefinitionHolder 里面持有 BeanDefinition
	BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
	if (bdHolder != null) {
		// 若有必要装饰 bdHolder （bean 标签内有自定义标签的情况下）
		bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
		try {
			// 完成 BeanDefinition 的注册
			// Register the final decorated instance.
			BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, getReaderContext().getRegistry());
		}
		catch (BeanDefinitionStoreException ex) {
			getReaderContext().error("Failed to register bean definition with name '" +
					bdHolder.getBeanName() + "'", ele, ex);
		}
		// Send registration event.
		getReaderContext().fireComponentRegistered(new BeanComponentDefinition(bdHolder));
	}
}

public static void registerBeanDefinition(
		BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)
		throws BeanDefinitionStoreException {

       // 注册
	// Register bean definition under primary name.
	String beanName = definitionHolder.getBeanName();
	registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());

	// Register aliases for bean name, if any.
	String[] aliases = definitionHolder.getAliases();
	if (aliases != null) {
		for (String alias : aliases) {
			registry.registerAlias(beanName, alias);
		}
	}
}

@Override
public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)
		throws BeanDefinitionStoreException {

	Assert.hasText(beanName, "Bean name must not be empty");
	Assert.notNull(beanDefinition, "BeanDefinition must not be null");

	if (beanDefinition instanceof AbstractBeanDefinition) {
		try {
			((AbstractBeanDefinition) beanDefinition).validate();
		}
		catch (BeanDefinitionValidationException ex) {
			throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
					"Validation of bean definition failed", ex);
		}
	}

	BeanDefinition existingDefinition = this.beanDefinitionMap.get(beanName);
	if (existingDefinition != null) {
		if (!isAllowBeanDefinitionOverriding()) {
			throw new BeanDefinitionOverrideException(beanName, beanDefinition, existingDefinition);
		}
		else if (existingDefinition.getRole() < beanDefinition.getRole()) {
			// e.g. was ROLE_APPLICATION, now overriding with ROLE_SUPPORT or ROLE_INFRASTRUCTURE
			if (logger.isInfoEnabled()) {
				logger.info("Overriding user-defined bean definition for bean '" + beanName +
						"' with a framework-generated bean definition: replacing [" +
						existingDefinition + "] with [" + beanDefinition + "]");
			}
		}
		else if (!beanDefinition.equals(existingDefinition)) {
			if (logger.isDebugEnabled()) {
				logger.debug("Overriding bean definition for bean '" + beanName +
						"' with a different definition: replacing [" + existingDefinition +
						"] with [" + beanDefinition + "]");
			}
		}
		else {
			if (logger.isTraceEnabled()) {
				logger.trace("Overriding bean definition for bean '" + beanName +
						"' with an equivalent definition: replacing [" + existingDefinition +
						"] with [" + beanDefinition + "]");
			}
		}
		this.beanDefinitionMap.put(beanName, beanDefinition);
	}
	else {
		// 注册逻辑
		if (hasBeanCreationStarted()) {
			// Cannot modify startup-time collection elements anymore (for stable iteration)
			synchronized (this.beanDefinitionMap) {
				this.beanDefinitionMap.put(beanName, beanDefinition);
				List<String> updatedDefinitions = new ArrayList<>(this.beanDefinitionNames.size() + 1);
				updatedDefinitions.addAll(this.beanDefinitionNames);
				updatedDefinitions.add(beanName);
				this.beanDefinitionNames = updatedDefinitions;
				removeManualSingletonName(beanName);
			}
		}
		else {
			// Still in startup registration phase
			this.beanDefinitionMap.put(beanName, beanDefinition);
			this.beanDefinitionNames.add(beanName);
			removeManualSingletonName(beanName);
		}
		this.frozenBeanDefinitionNames = null;
	}

	if (existingDefinition != null || containsSingleton(beanName)) {
		resetBeanDefinition(beanName);
	}
	else if (isConfigurationFrozen()) {
		clearByTypeCache();
	}
}

package io.github.linweiwang.bean;

public class Person {
	private String name;
	private int age;

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public int getAge() {
		return age;
	}

	public void setAge(int age) {
		this.age = age;
	}

	@Override
	public String toString() {
		return "Person{" +
				"name='" + name + '\'' +
				", age=" + age +
				'}';
	}
}

package io.github.linweiwang;

import io.github.linweiwang.bean.Person;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class Main {
	public static void main(String[] args) {
		ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("spring-config.xml");
		Person person = context.getBean(Person.class);
		System.out.println(person);
	}
}

public class RCTest {
    static class Test {
        public Test object = null;
    }

    public static void main(String[] args) {
        Test object1 = new Test();
        Test object2 = new Test();

        object1.object = object2;
        object2.object = object1;

        object1 = null;
        object2 = null;
    }
}

public Class<?> loadClass(String name) throws ClassNotFoundException {
    return loadClass(name, false);
}

protected Class<?> loadClass(String name, boolean resolve)
        throws ClassNotFoundException
{
    synchronized (getClassLoadingLock(name)) {
        // First, check if the class has already been loaded  // 1. 首先，检查一下指定名称的类是否已经加载过，如果加载过了，就不需要再加载，直接返回。
        Class<?> c = findLoadedClass(name);
        if (c == null) {
            long t0 = System.nanoTime();
            try {
                if (parent != null) {                         // 2. 判断一下是否有父加载器，若有交给父加载器加，否则调用 BootstrapClassLoader 加载。
                    c = parent.loadClass(name, false);
                } else {
                    c = findBootstrapClassOrNull(name);
                }
            } catch (ClassNotFoundException e) {
                // ClassNotFoundException thrown if class not found
                // from the non-null parent class loader
            }

            if (c == null) {                                  // 3. 如果第二步骤依然没有找到指定的类，那么调用当前类加载器的 findClass 方法来完成类加载。
                // If still not found, then invoke findClass in order
                // to find the class.
                long t1 = System.nanoTime();
                c = findClass(name);

                // this is the defining class loader; record the stats
                sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
                sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
                sun.misc.PerfCounter.getFindClasses().increment();
            }
        }
        if (resolve) {
            resolveClass(c);
        }
        return c;
    }
}

// 自定义类加载器就可以重写 finalClass 方法
protected Class<?> findClass(String name) throws ClassNotFoundException {
    throw new ClassNotFoundException(name);
}