随着spring功能的扩充，以及spring封装了很多底层实现的细节，使得我们在学习spring的源码的时候经常会比较困惑，本节开始研究spring容器启动的相关过程，暂时先从整体大流程上分析，比较细节的部分后续再补充说明，spring的版本为spring-framework-5.1.15.RELEASE，我们先从后台spring应用开始说明，web相关后续补充。
以ClassPathXmlApplicationContext启动说明，其类继承结构如下：

当我们实例化一个 ClassPathXmlApplicationContext的时候，实际上最终会调用这个方法：

public ClassPathXmlApplicationContext(String[] configLocations, boolean refresh, @Nullable ApplicationContext parent)throws BeansException {super(parent);setConfigLocations(configLocations);if (refresh) {refresh();}}

而这里面refresh方法就是启动spring容器的方法，最终是通过：

AbstractAppliationContext.refresh实现

public void refresh() throws BeansException, IllegalStateException {synchronized (this.startupShutdownMonitor) {// Prepare this context for refreshing.prepareRefresh();// Tell the subclass to refresh the internal bean factory.//获取新的beanFactory，销毁原来的beanFactory，// 通过AbstractRefreshableApplicationContext获取到 DefaultListableBeanFactory，// 在这一步获取到beanFactory的时候回调用 loadBeanDefinitions，// AbstractRefreshableApplicationContext并没有实现它，模板方法，子类自己实现逻辑ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();// Prepare the bean factory for use in this context.// 对beanFactory进行一些配置prepareBeanFactory(beanFactory);try {// Allows post-processing of the bean factory in context subclasses.//模板方法，子类继承实现,对beanFactory的后置处理postProcessBeanFactory(beanFactory);// Invoke factory processors registered as beans in the context.// 调用注册实现了BeanFactoryPostProcessor接口类的postProcessBeanDefinitionRegistry方法，// 主要有是两个类 BeanFactoryPostProcessor和BeanDefinitionRegistryPostProcessor，// 会通过beanFactory.getBean方法实例化BeanFactoryPostProcessor，然后执行postProcessBeanDefinitionRegistry// 这一步处理的时候实际上BeanFactory已经准备好了，因此如果想往beanFactory中增加bean可以在这个地方进行处理，// 如ConfigurationClassPostProcessor就是实现了BeanDefinitionRegistryPostProcessor接口，在这里将相关bean加入到了IOC容器中invokeBeanFactoryPostProcessors(beanFactory);// Register bean processors that intercept bean creation.//向beanFactory中注册实现了BeanPostProcessor的beanregisterBeanPostProcessors(beanFactory);// Initialize message source for this context.//初始化国际化工具类initMessageSource();// Initialize event multicaster for this context.//初始化事件广播器initApplicationEventMulticaster();// Initialize other special beans in specific context subclasses.//模板方法，子类自己去实现相应逻辑onRefresh();// Check for listener beans and register them.// 注册监听器registerListeners();// Instantiate all remaining (non-lazy-init) singletons.//初始化所有的单例bean，不包含lazy-initfinishBeanFactoryInitialization(beanFactory);// 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 {resetCommonCaches();}}}

如上是refresh的实现逻辑步骤，我们一项一项分析
首先是：

（1）刷新容器前的准备工作prepareRefresh

AbstractApplicationContext.prepareRefresh启动容器前的一些准备工作，包含设置状态位，验证一些必要的属性是否存在等
具体方法如下：

/*** Prepare this context for refreshing, setting its startup date and* active flag as well as performing any initialization of property sources.*/protected void prepareRefresh() {// Switch to active.this.startupDate = System.currentTimeMillis();this.closed.set(false);this.active.set(true);if (logger.isDebugEnabled()) {if (logger.isTraceEnabled()) {logger.trace("Refreshing " + this);}else {logger.debug("Refreshing " + getDisplayName());}}// Initialize any placeholder property sources in the context environment.// 初始化任意属性，子类可以实现该方法initPropertySources();// Validate that all properties marked as required are resolvable:// see ConfigurablePropertyResolver#setRequiredPropertiesgetEnvironment().validateRequiredProperties();// Store pre-refresh ApplicationListeners...if (this.earlyApplicationListeners == null) {this.earlyApplicationListeners = new LinkedHashSet<>(this.applicationListeners);}else {// Reset local application listeners to pre-refresh state.this.applicationListeners.clear();this.applicationListeners.addAll(this.earlyApplicationListeners);}// Allow for the collection of early ApplicationEvents,// to be published once the multicaster is available...this.earlyApplicationEvents = new LinkedHashSet<>();}

其中initPropertySources如下三个类中有实现，基本上都是web相关

准备工作完成之后，接下来就开始实例化BeanFactory：

(2)实例化BeanFactory，obtainFreshBeanFactory

AbstractApplicationContext.obtainFreshBeanFactory();
其实现在
AbstractRefreshableApplicationContext.refreshBeanFactory
中，其代码逻辑如下：

protected final void refreshBeanFactory() throws BeansException {//如果已经有了BeanFactory先销毁关闭它if (hasBeanFactory()) {destroyBeans();closeBeanFactory();}try {//直接new 了一个 DefaultListableBeanFactory，// 并且在其继承父类中 AbstractAutowireCapableBeanFactory配置忽略自动装配的配置DefaultListableBeanFactory beanFactory = createBeanFactory();beanFactory.setSerializationId(getId());customizeBeanFactory(beanFactory);//这是重要的步骤，加载所有bean的配置，这是一个模板方法，由具体子类去实现loadBeanDefinitions(beanFactory);synchronized (this.beanFactoryMonitor) {this.beanFactory = beanFactory;}}catch (IOException ex) {throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);}}

可以看到，主要做了如下事情：
（1）关闭销毁如果已经存在的BeanFactory
（2）实例化一个新的beanFactory实例（DefaultListableBeanFactory）以及将之前的一些属性赋值到新的beanFactory上
（3）加载bean的配置，这是最重要的一步

loadBeanDefinitions是一个模板方法，子类实现，通过idea，我们发现有如下几种实现方式，

这块后面再详细描述。

这样通过obtainFreshBeanFactory我们获得了一个beanFactory，并且已经加载了所有bean的信息，下面是对beanFactory进行一些前置处理：

（3）beanFactory的前置准备工作，prepareBeanFactory

AbstractApplicationContext.prepareBeanFactory
其代码逻辑如下：

protected void prepareBeanFactory(ConfigurableListableBeanFactory beanFactory) {// Tell the internal bean factory to use the context's class loader etc.// 设置beanFactory的类加载器beanFactory.setBeanClassLoader(getClassLoader());// 设置EL表达式处理类，bean在初始化后填充属性会用到beanFactory.setBeanExpressionResolver(new StandardBeanExpressionResolver(beanFactory.getBeanClassLoader()));//设置属性表达式处理类beanFactory.addPropertyEditorRegistrar(new ResourceEditorRegistrar(this, getEnvironment()));// Configure the bean factory with context callbacks.beanFactory.addBeanPostProcessor(new ApplicationContextAwareProcessor(this));// 设置忽略自动装配的bean，当bean实现这些接口，不要Autowired自动装配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.// 会将下面 beanFactory（DefaultListableBeanFactory）和// 当前类（可以理解为我们常用的ApplicationContext）放入到 DefaultListableBeanFactory的resolvableDependencies 这个map中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.beanFactory.addBeanPostProcessor(new ApplicationListenerDetector(this));// Detect a LoadTimeWeaver and prepare for weaving, if found.// 这里就是对AspectJ的支持，LOAD_TIME_WEAVER 类加载时候编织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.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());}}

可以看到，这块主要是对beanFactory做一些前置处理为了接下来加载实例化做准备。
接下来是beanFactory准备工作的后置处理，

(4)beanFactory准备工作的后置处理，postProcessBeanFactory

AbstractApplicationContext.postProcessBeanFactory

这是一个模板方法，由子类自己去实现

当前ClasspathXmlApplicationContext没有相关实现，可以发现一般web相关容器会实现该方法，后续研究web相关容器我们在讨论。

到这一步，我们的beanFactory基本上就准备好了，当beanFactory准备好了之后，我们需要进行beanFactory准备好之后的后置方法，这样供相关子类在这个阶段：在beanFactory准备好了之后但是还没开始任何bean实例初始化的时候进行一些操作。

(5)执行BeanDefinitionRegistryPostProcessor.postProcessBeanDefinitionRegistry和BeanFactoryPostProcessor.postProcessBeanFactory方法，通过invokeBeanFactoryPostProcessors

AbstractApplicationContext. invokeBeanFactoryPostProcessors

具体代码实现如下：

protected void invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory beanFactory) {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()));}}

主要分为两步：
（1）调用PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors来处理beanFactoryPostProcessor相关逻辑
（2）织入AspectJ相关处理

这里加入的LoadTimeWeaverAwareProcessor后置处理器，只处理LoadTimeWeaverAware类型bean，而实现这个接口的只有如下几个类：

这块后面讲AspectJ我们在细谈。

在PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors());对两类BeanFactoryPostProcessor和BeanDefinitionRegistryPostProcessor进行处理，首先会获取自定义添加的

public List<BeanFactoryPostProcessor> getBeanFactoryPostProcessors() {return this.beanFactoryPostProcessors;}

可以发现，我们可以通过调用:

public void addBeanFactoryPostProcessor(BeanFactoryPostProcessor postProcessor) {Assert.notNull(postProcessor, "BeanFactoryPostProcessor must not be null");this.beanFactoryPostProcessors.add(postProcessor);}

可以手动添加BeanFactoryPostProcessor.
在
PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors());中，首先会处理自定义添加的：
实现了BeanFactoryPostProcessor接口的postProcessBeanDefinitionRegistry方法。然后在从容器中获取实现了这个接口bean并调用postProcessBeanDefinitionRegistry。执行完这个处理后，接下来会处理BeanFactoryPostProcessor.postProcessBeanFactory。这两步处理中，如果对应的bean没有实例化，通过getBean实例化Bean，但是自定义的添加进去的是已经实例化后的bean。

(6)注册BeanPostProcessor，通过registerBeanPostProcessors

AbstractApplicationContext.registerBeanPostProcessors

将容器中BeanPostProcessor类型的bean的信息从beanFactory中获取到并通过getBean(String name, Class<T> requiredType)方法实例化这些Bean，然后将其注册到beanFactory中，这里加入的都是BeanPostProcessor的实例。

private static void registerBeanPostProcessors(ConfigurableListableBeanFactory beanFactory, List<BeanPostProcessor> postProcessors) {for (BeanPostProcessor postProcessor : postProcessors) {beanFactory.addBeanPostProcessor(postProcessor);}}

(7)初始化国际化相关工具类，通过registerBeanPostProcessors实现

AbstractApplicationContext.initMessageSource
初始化国际化相关信息

protected void initMessageSource() {ConfigurableListableBeanFactory beanFactory = getBeanFactory();if (beanFactory.containsLocalBean(MESSAGE_SOURCE_BEAN_NAME)) {this.messageSource = beanFactory.getBean(MESSAGE_SOURCE_BEAN_NAME, MessageSource.class);// Make MessageSource aware of parent MessageSource.if (this.parent != null && this.messageSource instanceof HierarchicalMessageSource) {HierarchicalMessageSource hms = (HierarchicalMessageSource) this.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.DelegatingMessageSource dms = new DelegatingMessageSource();dms.setParentMessageSource(getInternalParentMessageSource());this.messageSource = dms;beanFactory.registerSingleton(MESSAGE_SOURCE_BEAN_NAME, this.messageSource);if (logger.isTraceEnabled()) {logger.trace("No '" + MESSAGE_SOURCE_BEAN_NAME + "' bean, using [" + this.messageSource + "]");}}}

#（8）初始化事件广播器

AbstractApplicationContext.initApplicationEventMulticaster

初始化事件广播器

protected void initApplicationEventMulticaster() {ConfigurableListableBeanFactory beanFactory = getBeanFactory();if (beanFactory.containsLocalBean(APPLICATION_EVENT_MULTICASTER_BEAN_NAME)) {this.applicationEventMulticaster =beanFactory.getBean(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, ApplicationEventMulticaster.class);if (logger.isTraceEnabled()) {logger.trace("Using ApplicationEventMulticaster [" + this.applicationEventMulticaster + "]");}}else {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() + "]");}}}

可以看到如果没有当前beanFactory中不包含名为applicationEventMulticaster的bean，则会默认注入SimpleApplicationEventMulticaster当做当前applicationContext的事件广播器

（9）执行模板方法onRefresh

AbstractApplicationContext.onRefresh
这是一个模板方法，当前类是一个空的实现，子类可以复写这块的逻辑，可以看到实际实现：

按照spring官方给出的注释：

Initialize other special beans in specific context subclasses.

可以在这个方法中实例化某个自定义的applicationContext中特殊的bean的处理，我们也可以看到，这块主要是web相关的application在应用，我们在讲解web相关处理的时候回详细说明。

（10）注册监听器，registerListeners

AbstractApplicationContext.registerListeners
注册监听器

在（8）步的时候，我们初始化了事件广播器，这里我们将注册的ApplicationListener加入到（8）步中注册的ApplicationEventMulticaster中去，这样后续当ApplicationEventMulticaster收到相关的事件的时候，通过调用SimpleApplicationEventMulticaster.multicastEvent继而调用SimpleApplicationEventMulticaster.invokeListener来通知监听器监听相关事件（这是以默认的SimpleApplicationEventMulticaster举例说明）

（11）初始化所有的单例bean（不包含lazy-init)，finishBeanFactoryInitialization

AbstractApplicationContext.finishBeanFactoryInitialization

这一步就开始初始化我们自定义的相关bean，实现如下：

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 bean post-processor// (such as a PropertyPlaceholderConfigurer 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();// Instantiate all remaining (non-lazy-init) singletons.beanFactory.preInstantiateSingletons();}

1. 首先是设置当前applicationContext的ConversionService.ConversionService主要是用来类型转换。

public interface ConversionService {boolean canConvert(@Nullable Class<?> sourceType, Class<?> targetType);boolean canConvert(@Nullable TypeDescriptor sourceType, TypeDescriptor targetType);<T> T convert(@Nullable Object source, Class<T> targetType);Object convert(@Nullable Object source, @Nullable TypeDescriptor sourceType, TypeDescriptor targetType);
}

其默认实现DefaultConversionService继承如下：

2. 注册默认的值替换器，
   比如我们经常在属性上进行值注入：

@Value("${name}")
private String name;

这里注册默认的值解决类。

3. 实例化Aspect相关类
4. 缓存当前bean的配置，并冻结配置不允许更改
5. 实例化剩下的单例，通过DeaultListableBeanFactory.preInstantiateSingletons来预实例化单例
   我们可以看下DeaultListableBeanFactory.preInstantiateSingletons的实现。

public void preInstantiateSingletons() throws BeansException {if (logger.isTraceEnabled()) {logger.trace("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.List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);// Trigger initialization of all non-lazy singleton beans...for (String beanName : beanNames) {RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {if (isFactoryBean(beanName)) {Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);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);}}}else {getBean(beanName);}}}// Trigger post-initialization callback for all applicable beans...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();}}}}

主要是两步，如果bean对应的是一个OjectFactory类型，则通过getBean获取实例，如果ObjectFactory是SmartFactoryBean且isEagerInit为true时，提前实例化对应单例。
如果是普通单例，直接通过getBean获取。
getBean这个方法后面会着重描述

(12)结束容器刷新，执行相关方法，finishRefresh

protected void finishRefresh() {// Clear context-level resource caches (such as ASM metadata from scanning).clearResourceCaches();// Initialize lifecycle processor for this context.initLifecycleProcessor();// Propagate refresh to lifecycle processor first.getLifecycleProcessor().onRefresh();// Publish the final event.publishEvent(new ContextRefreshedEvent(this));// Participate in LiveBeansView MBean, if active.LiveBeansView.registerApplicationContext(this);}

首先是清空相关资源的缓存。

然后初始化运行周期相关处理类，默认使用DefaultLifecycleProcessor

初始化生命周期处理类之后，执行onRefresh生命周期相关方法

发布容器刷新完成事件消息

如果开启了JMX，执行JMX相关处理。

至此，一个spring容器启动成功。