那么源码的入口就是AbstractApplicationContext.refresh() 的整个方法了。

后面的的阅读顺序refresh()的方法内进行深挖。

	@Override
	public void refresh() throws BeansException, IllegalStateException {
		synchronized (this.startupShutdownMonitor) {
			// Prepare this context for refreshing.
			/**
			 * 做容器刷新前的准备工作
			 * 1、设置容器的启动时间
			 * 2、设置活跃状态为true
			 * 3、设置关闭状态为false
			 * 4、获取Environment对象，并加载当前系统的属性值到Environment对象中
			 * 5、准备监听器和事件的集合对象，默认为空的集合
			 */

			prepareRefresh();

			// Tell the subclass to refresh the internal bean factory.
			// 创建容器对象：DefaultListableBeanFactory
			// 加载xml配置文件的属性值到当前工厂中，最重要的就是BeanDefinition
			ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

			// Prepare the bean factory for use in this context.
			// beanFactory的准备工作，对各种属性进行填充
			prepareBeanFactory(beanFactory);

			try {
				// Allows post-processing of the bean factory in context subclasses.
				// 子类覆盖方法做额外的处理，此处我们自己一般不做任何扩展工作，但是可以查看web中的代码，是有具体实现的
				postProcessBeanFactory(beanFactory);

				// Invoke factory processors registered as beans in the context.
				// 调用各种beanFactory处理器
				invokeBeanFactoryPostProcessors(beanFactory);

				// Register bean processors that intercept bean creation.
				// 注册bean处理器，这里只是注册功能，真正调用的是getBean方法
				registerBeanPostProcessors(beanFactory);

				// Initialize message source for this context.
				// 为上下文初始化message源，即不同语言的消息体，国际化处理,在springmvc的时候通过国际化的代码重点讲
				initMessageSource();

				// Initialize event multicaster for this context.
				// 初始化事件监听多路广播器
				initApplicationEventMulticaster();

				// Initialize other special beans in specific context subclasses.
				// 留给子类来初始化其他的bean
				onRefresh();

				// Check for listener beans and register them.
				// 在所有注册的bean中查找listener bean,注册到消息广播器中
				registerListeners();

				// Instantiate all remaining (non-lazy-init) singletons.
				// 初始化剩下的单实例（非懒加载的）
				finishBeanFactoryInitialization(beanFactory);

				// Last step: publish corresponding event.
				// 完成刷新过程，通知生命周期处理器lifecycleProcessor刷新过程，同时发出ContextRefreshEvent通知别人
				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.
				// 为防止bean资源占用，在异常处理中，销毁已经在前面过程中生成的单件bean
				destroyBeans();

				// Reset 'active' flag.
				// 重置active标志
				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();
			}
		}
	}

步骤解读

1. prepareRefresh()

/**
 * 前戏，做容器刷新前的准备工作
 * 1、设置容器的启动时间
 * 2、设置活跃状态为true
 * 3、设置关闭状态为false
 * 4、获取Environment对象，并加载当前系统的属性值到Environment对象中
 * 5、准备监听器和事件的集合对象，默认为空的集合
 */

	protected void prepareRefresh() {
		// Switch to active.
		// 设置容器启动的时间
		this.startupDate = System.currentTimeMillis();
		// 容器的关闭标志位
		this.closed.set(false);
		// 容器的激活标志位
		this.active.set(true);

		// 记录日志
		if (logger.isDebugEnabled()) {
			if (logger.isTraceEnabled()) {
				logger.trace("Refreshing " + this);
			}
			else {
				logger.debug("Refreshing " + getDisplayName());
			}
		}

		// Initialize any placeholder property sources in the context environment.
		// 留给子类覆盖，初始化属性资源
		initPropertySources();

		// Validate that all properties marked as required are resolvable:
		// see ConfigurablePropertyResolver#setRequiredProperties
		// 创建并获取环境对象，验证需要的属性文件是否都已经放入环境中
		getEnvironment().validateRequiredProperties();

		// Store pre-refresh ApplicationListeners...
		// 判断刷新前的应用程序监听器集合是否为空，如果为空，则将监听器添加到此集合中
		if (this.earlyApplicationListeners == null) {
			this.earlyApplicationListeners = new LinkedHashSet<>(this.applicationListeners);
		}
		else {
			// Reset local application listeners to pre-refresh state.
			// 如果不等于空，则清空集合元素对象
			this.applicationListeners.clear();
			this.applicationListeners.addAll(this.earlyApplicationListeners);
		}

		// Allow for the collection of early ApplicationEvents,
		// to be published once the multicaster is available...
		// 创建刷新前的监听事件集合
		this.earlyApplicationEvents = new LinkedHashSet<>();
	}

2.

// 创建容器对象：DefaultListableBeanFactory
// 加载xml配置文件的属性值到当前工厂中，最重要的就是BeanDefinition

	protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
		// 初始化BeanFactory,并进行XML文件读取，并将得到的BeanFactory记录在当前实体的属性中
		refreshBeanFactory();
		// 返回当前实体的beanFactory属性
		return getBeanFactory();
	}

#

	/**
	 * Configure the factory's standard context characteristics,
	 * such as the context's ClassLoader and post-processors.
	 * @param beanFactory the BeanFactory to configure
	 */
	protected void prepareBeanFactory(ConfigurableListableBeanFactory beanFactory) {
		// Tell the internal bean factory to use the context's class loader etc.
		// 设置beanFactory的classloader为当前context的classloader
		beanFactory.setBeanClassLoader(getClassLoader());
		// 设置beanfactory的表达式语言处理器
		beanFactory.setBeanExpressionResolver(new StandardBeanExpressionResolver(beanFactory.getBeanClassLoader()));
		// 为beanFactory增加一个默认的propertyEditor，这个主要是对bean的属性等设置管理的一个工具类
		beanFactory.addPropertyEditorRegistrar(new ResourceEditorRegistrar(this, getEnvironment()));

		// Configure the bean factory with context callbacks.
		// 添加beanPostProcessor,ApplicationContextAwareProcessor此类用来完成某些Aware对象的注入
		beanFactory.addBeanPostProcessor(new ApplicationContextAwareProcessor(this));
		// 设置要忽略自动装配的接口，很多同学理解不了为什么此处要对这些接口进行忽略，原因非常简单，这些接口的实现是由容器通过set方法进行注入的，
		// 所以在使用autowire进行注入的时候需要将这些接口进行忽略
		beanFactory.ignoreDependencyInterface(EnvironmentAware.class);
		beanFactory.ignoreDependencyInterface(EmbeddedValueResolverAware.class);
		beanFactory.ignoreDependencyInterface(ResourceLoaderAware.class);
		beanFactory.ignoreDependencyInterface(ApplicationEventPublisherAware.class);
		beanFactory.ignoreDependencyInterface(MessageSourceAware.class);
		beanFactory.ignoreDependencyInterface(ApplicationContextAware.class);

		// BeanFactory interface not registered as resolvable type in a plain factory.
		// MessageSource registered (and found for autowiring) as a bean.
		// 设置几个自动装配的特殊规则,当在进行ioc初始化的如果有多个实现，那么就使用指定的对象进行注入
		beanFactory.registerResolvableDependency(BeanFactory.class, beanFactory);
		beanFactory.registerResolvableDependency(ResourceLoader.class, this);
		beanFactory.registerResolvableDependency(ApplicationEventPublisher.class, this);
		beanFactory.registerResolvableDependency(ApplicationContext.class, this);

		// Register early post-processor for detecting inner beans as ApplicationListeners.
		// 注册BPP
		beanFactory.addBeanPostProcessor(new ApplicationListenerDetector(this));

		// Detect a LoadTimeWeaver and prepare for weaving, if found.
		// 增加对AspectJ的支持，在java中织入分为三种方式，分为编译器织入，类加载器织入，运行期织入，编译器织入是指在java编译器，采用特殊的编译器，将切面织入到java类中，
		// 而类加载期织入则指通过特殊的类加载器，在类字节码加载到JVM时，织入切面，运行期织入则是采用cglib和jdk进行切面的织入
		// aspectj提供了两种织入方式，第一种是通过特殊编译器，在编译器，将aspectj语言编写的切面类织入到java类中，第二种是类加载期织入，就是下面的load time weaving，此处后续讲
		if (beanFactory.containsBean(LOAD_TIME_WEAVER_BEAN_NAME)) {
			beanFactory.addBeanPostProcessor(new LoadTimeWeaverAwareProcessor(beanFactory));
			// Set a temporary ClassLoader for type matching.
			beanFactory.setTempClassLoader(new ContextTypeMatchClassLoader(beanFactory.getBeanClassLoader()));
		}

		// Register default environment beans.
		// 注册默认的系统环境bean到一级缓存中
		if (!beanFactory.containsLocalBean(ENVIRONMENT_BEAN_NAME)) {
			beanFactory.registerSingleton(ENVIRONMENT_BEAN_NAME, getEnvironment());
		}
		if (!beanFactory.containsLocalBean(SYSTEM_PROPERTIES_BEAN_NAME)) {
			beanFactory.registerSingleton(SYSTEM_PROPERTIES_BEAN_NAME, getEnvironment().getSystemProperties());
		}
		if (!beanFactory.containsLocalBean(SYSTEM_ENVIRONMENT_BEAN_NAME)) {
			beanFactory.registerSingleton(SYSTEM_ENVIRONMENT_BEAN_NAME, getEnvironment().getSystemEnvironment());
		}
	}

4.

// 之间我们说过，如果有一些的的方法执行为空，那么就由子类实现执行，书写具体的执行逻辑
protected void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {
}

#

// 实例化并且调用所有已经注册了的beanFactoryPostProcessor,遵循指明的顺序
protected void invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory beanFactory) {
		// 获取到当前应用程序上下文的beanFactoryPostProcessors变量的值，并且实例化调用执行所有已经注册的beanFactoryPostProcessor
		// 默认情况下，通过getBeanFactoryPostProcessors()来获取已经注册的BFPP，但是默认是空的，那么问题来了，如果你想扩展，怎么进行扩展工作？
		PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors());

		// Detect a LoadTimeWeaver and prepare for weaving, if found in the meantime
		// (e.g. through an @Bean method registered by ConfigurationClassPostProcessor)
		if (beanFactory.getTempClassLoader() == null && beanFactory.containsBean(LOAD_TIME_WEAVER_BEAN_NAME)) {
			beanFactory.addBeanPostProcessor(new LoadTimeWeaverAwareProcessor(beanFactory));
			beanFactory.setTempClassLoader(new ContextTypeMatchClassLoader(beanFactory.getBeanClassLoader()));
		}
	}

#

// 实例化并且注册所有的beanPostProcessor
protected void registerBeanPostProcessors(ConfigurableListableBeanFactory beanFactory) {
		PostProcessorRegistrationDelegate.registerBeanPostProcessors(beanFactory, this);
	}

#

	protected void initMessageSource() {
		// 获取bean工厂，一般是DefaultListableBeanFactory
		ConfigurableListableBeanFactory beanFactory = getBeanFactory();
		// 首先判断是否已有xml文件定义了id为messageSource的bean对象
		if (beanFactory.containsLocalBean(MESSAGE_SOURCE_BEAN_NAME)) {
			// 如果有，则从BeanFactory中获取这个对象
			this.messageSource = beanFactory.getBean(MESSAGE_SOURCE_BEAN_NAME, MessageSource.class);
			// Make MessageSource aware of parent MessageSource.
			// 当父类bean工厂不为空，并且这个bean对象是HierarchicalMessageSource类型
			if (this.parent != null && this.messageSource instanceof HierarchicalMessageSource) {
				// 类型强制转换，转换为HierarchicalMessageSource的类型
				HierarchicalMessageSource hms = (HierarchicalMessageSource) this.messageSource;
				// 判断父类的messageSource是否为空，如果等于空，则设置父类的messageSource
				if (hms.getParentMessageSource() == null) {
					// Only set parent context as parent MessageSource if no parent MessageSource
					// registered already.
					hms.setParentMessageSource(getInternalParentMessageSource());
				}
			}
			if (logger.isTraceEnabled()) {
				logger.trace("Using MessageSource [" + this.messageSource + "]");
			}
		}
		else {
			// Use empty MessageSource to be able to accept getMessage calls.
			// 如果没有xml文件定义信息源对象，新建DelegatingMessageSource类作为messageSource的bean
			DelegatingMessageSource dms = new DelegatingMessageSource();
			// 给这个DelegatingMessageSource添加父类消息源
			dms.setParentMessageSource(getInternalParentMessageSource());
			this.messageSource = dms;
			// 将这个messageSource实例注册到bean工厂中
			beanFactory.registerSingleton(MESSAGE_SOURCE_BEAN_NAME, this.messageSource);
			if (logger.isTraceEnabled()) {
				logger.trace("No '" + MESSAGE_SOURCE_BEAN_NAME + "' bean, using [" + this.messageSource + "]");
			}
		}
	}

#

protected void initApplicationEventMulticaster() {
		// 获取当前bean工厂,一般是DefaultListableBeanFactory
		ConfigurableListableBeanFactory beanFactory = getBeanFactory();
		// 判断容器中是否存在bdName为applicationEventMulticaster的bd，也就是说自定义的事件监听多路广播器，必须实现ApplicationEventMulticaster接口
		if (beanFactory.containsLocalBean(APPLICATION_EVENT_MULTICASTER_BEAN_NAME)) {
			// 如果有，则从bean工厂得到这个bean对象
			this.applicationEventMulticaster =
					beanFactory.getBean(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, ApplicationEventMulticaster.class);
			if (logger.isTraceEnabled()) {
				logger.trace("Using ApplicationEventMulticaster [" + this.applicationEventMulticaster + "]");
			}
		}
		else {
			// 如果没有，则默认采用SimpleApplicationEventMulticaster
			this.applicationEventMulticaster = new SimpleApplicationEventMulticaster(beanFactory);
			beanFactory.registerSingleton(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, this.applicationEventMulticaster);
			if (logger.isTraceEnabled()) {
				logger.trace("No '" + APPLICATION_EVENT_MULTICASTER_BEAN_NAME + "' bean, using " +
						"[" + this.applicationEventMulticaster.getClass().getSimpleName() + "]");
			}
		}
	}

#

	/**
	 * 此方式是模板方法，没有添加任何实现，在springmvc中会有对应的实现
	 *
	 * Template method which can be overridden to add context-specific refresh work.
	 * Called on initialization of special beans, before instantiation of singletons.
	 * <p>This implementation is empty.
	 * @throws BeansException in case of errors
	 * @see #refresh()
	 */
	protected void onRefresh() throws BeansException {
		// For subclasses: do nothing by default.
	}

#

protected void registerListeners() {
		// Register statically specified listeners first.
		// 遍历应用程序中存在的监听器集合，并将对应的监听器添加到监听器的多路广播器中
		for (ApplicationListener<?> listener : getApplicationListeners()) {
			getApplicationEventMulticaster().addApplicationListener(listener);
		}

		// Do not initialize FactoryBeans here: We need to leave all regular beans
		// uninitialized to let post-processors apply to them!
		// 从容器中获取所有实现了ApplicationListener接口的bd的bdName
		// 放入ApplicationListenerBeans集合中
		String[] listenerBeanNames = getBeanNamesForType(ApplicationListener.class, true, false);
		for (String listenerBeanName : listenerBeanNames) {
			getApplicationEventMulticaster().addApplicationListenerBean(listenerBeanName);
//			getApplicationEventMulticaster().addApplicationListener(this.getBean(listenerBeanName,ApplicationListener.class));
		}

		// Publish early application events now that we finally have a multicaster...
		// 此处先发布早期的监听器集合
		Set<ApplicationEvent> earlyEventsToProcess = this.earlyApplicationEvents;
		this.earlyApplicationEvents = null;
		if (!CollectionUtils.isEmpty(earlyEventsToProcess)) {
			for (ApplicationEvent earlyEvent : earlyEventsToProcess) {
				getApplicationEventMulticaster().multicastEvent(earlyEvent);
			}
		}
	}

#

protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
		// Initialize conversion service for this context.
		// 为上下文初始化类型转换器
		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.
		// 如果beanFactory之前没有注册嵌入值解析器，则注册默认的嵌入值解析器，主要用于注解属性值的解析
		if (!beanFactory.hasEmbeddedValueResolver()) {
			beanFactory.addEmbeddedValueResolver(strVal -> getEnvironment().resolvePlaceholders(strVal));
		}

		// Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.
		// 尽早初始化loadTimeWeaverAware bean,以便尽早注册它们的转换器
		String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
		for (String weaverAwareName : weaverAwareNames) {
			getBean(weaverAwareName);
		}

		// Stop using the temporary ClassLoader for type matching.
		// 禁止使用临时类加载器进行类型匹配
		beanFactory.setTempClassLoader(null);

		// Allow for caching all bean definition metadata, not expecting further changes.
		// 冻结所有的bean定义，说明注册的bean定义将不被修改或任何进一步的处理
		beanFactory.freezeConfiguration();

		// Instantiate all remaining (non-lazy-init) singletons.
		// 实例化剩下的单例对象
		beanFactory.preInstantiateSingletons();
	}

#

/**
	 * 完成刷新
	 * Finish the refresh of this context, invoking the LifecycleProcessor's
	 * onRefresh() method and publishing the
	 * {@link org.springframework.context.event.ContextRefreshedEvent}.
	 */
	protected void finishRefresh() {
		// Clear context-level resource caches (such as ASM metadata from scanning).
		// 清除上下文级别的资源缓存(如扫描的ASM元数据)
		// 清空在资源加载器中的所有资源缓存
		clearResourceCaches();

		// Initialize lifecycle processor for this context.
		// 为这个上下文初始化生命周期处理器
		// 初始化LifecycleProcessor.如果上下文中找到'lifecycleProcessor'的LifecycleProcessor Bean对象，
		// 则使用DefaultLifecycleProcessor
		initLifecycleProcessor();

		// Propagate refresh to lifecycle processor first.
		// 首先将刷新传播到生命周期处理器
		// 上下文刷新的通知，例如自动启动的组件
		getLifecycleProcessor().onRefresh();

		// Publish the final event.
		// 发布最终事件
		// 新建ContextRefreshedEvent事件对象，将其发布到所有监听器。
		publishEvent(new ContextRefreshedEvent(this));

		// Participate in LiveBeansView MBean, if active.
		// 参与LiveBeansView MBean，如果是活动的
		// LiveBeansView:Sping用于支持JMX 服务的类
		// 注册当前上下文到LiveBeansView，以支持JMX服务
		LiveBeansView.registerApplicationContext(this);
	}