Spring容器启动流程

        ContextLoaderListener：启动时，应用户服务器调用Listener.contextInitialized(ServletContextEvent event)进行初始化整个容器。其初始化大体流程如图：

    启动源码：

/**
 * 初始化context入口
 * Initialize the root web application context.
 */
@Override
public void contextInitialized(ServletContextEvent event) {
   initWebApplicationContext(event.getServletContext());
}

        ContextLoader：真正实现ApplicationContext。

public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {
   if (servletContext.getAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE) != null) {
      //......
   }

   ......   

   long startTime = System.currentTimeMillis();

   try {
      // Store context in local instance variable, to guarantee that
      // it is available on ServletContext shutdown.
      if (this.context == null) {
         this.context = createWebApplicationContext(servletContext);
      }
      if (this.context instanceof ConfigurableWebApplicationContext) {
         ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) this.context;
         //判断context是否已经初始化
         if (!cwac.isActive()) {
            // The context has not yet been refreshed -> provide services such as
            // setting the parent context, setting the application context id, etc
            //经过web.xml配置的参数locatorFactorySelector等初始化初始化父容器环境，
            if (cwac.getParent() == null) {
               // The context instance was injected without an explicit parent ->
               // determine parent for root web application context, if any.
               ApplicationContext parent = loadParentContext(servletContext);
               cwac.setParent(parent);
            }
            //Context容器[加载resource资源文件、建立bean等]
            configureAndRefreshWebApplicationContext(cwac, servletContext);‍‍
         }
      }
      servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);

      ClassLoader ccl = Thread.currentThread().getContextClassLoader();
      if (ccl == ContextLoader.class.getClassLoader()) {
         currentContext = this.context;
      }
      else if (ccl != null) {
         currentContextPerThread.put(ccl, this.context);
      }

      if (logger.isDebugEnabled()) {
         logger.debug("Published root WebApplicationContext as ServletContext attribute with name [" +
               WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE + "]");
      }
      if (logger.isInfoEnabled()) {
         long elapsedTime = System.currentTimeMillis() - startTime;
         logger.info("Root WebApplicationContext: initialization completed in " + elapsedTime + " ms");
      }

      return this.context;
   }
   catch (RuntimeException ex) {
      logger.error("Context initialization failed", ex);
      servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, ex);
      throw ex;
   }
   catch (Error err) {
      logger.error("Context initialization failed", err);
      servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, err);
      throw err;
   }
}

/**
 * 一、配置web.xml中的参数
 * 二、初始化
 * 实例化beanFactory、读取resource资源、建立BeanDefinition、加载相关组件
 */
protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac, ServletContext sc) {
   if (ObjectUtils.identityToString(wac).equals(wac.getId())) {
      // The application context id is still set to its original default value
      // -> assign a more useful id based on available information
      String idParam = sc.getInitParameter(CONTEXT_ID_PARAM);
      if (idParam != null) {
         wac.setId(idParam);
      }
      else {
         // Generate default id...
         wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +
               ObjectUtils.getDisplayString(sc.getContextPath()));
      }
   }

   wac.setServletContext(sc);
   String configLocationParam = sc.getInitParameter(CONFIG_LOCATION_PARAM);
   if (configLocationParam != null) {
      wac.setConfigLocation(configLocationParam);
   }

   // The wac environment's #initPropertySources will be called in any case when the context
   // is refreshed; do it eagerly here to ensure servlet property sources are in place for
   // use in any post-processing or initialization that occurs below prior to #refresh
   ConfigurableEnvironment env = wac.getEnvironment();
   if (env instanceof ConfigurableWebEnvironment) {
      ((ConfigurableWebEnvironment) env).initPropertySources(sc, null);
   }

   //初始化web.xml中配置的自定义contextInitializerClasses等
   customizeContext(sc, wac);
   //加载spring中的配置文件xml、properties等，并建立beanFactory、实例化bean等
   wac.refresh();
}

        AbstractApplicationContext：容器refresh()建立beanFactory、读取resource资源文件、建立bean；

@Override
public void refresh() throws BeansException, IllegalStateException {
   synchronized (this.startupShutdownMonitor) {
      // Prepare this context for refreshing.
      prepareRefresh();

      //一、建立beanFactory（读取resource、加载BeanDefinition并将其注册到beanFactory中）
      // Tell the subclass to refresh the internal bean factory.
      ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
      
      // Prepare the bean factory for use in this context.
      prepareBeanFactory(beanFactory);

      try {
         //beanFactory中添加
         // Allows post-processing of the bean factory in context subclasses.
         postProcessBeanFactory(beanFactory);

         // Invoke factory processors registered as beans in the context.
         invokeBeanFactoryPostProcessors(beanFactory);

         //注册BeanPostProcessor，在实例化bean先后调用，可用于修改bean中信息等
         // Register bean processors that intercept bean creation.
         registerBeanPostProcessors(beanFactory);

         // Initialize message source for this context.
         initMessageSource();

         // Initialize event multicaster for this context.
         initApplicationEventMulticaster();

         // Initialize other special beans in specific context subclasses.
         onRefresh();

         // Check for listener beans and register them.
         registerListeners();
    
         //二、bean实例化
         // Instantiate all remaining (non-lazy-init) singletons.
         finishBeanFactoryInitialization(beanFactory);
      
         // Last step: publish corresponding event.
         finishRefresh();
      }

      catch (BeansException ex) {
         // Destroy already created singletons to avoid dangling resources.
         destroyBeans();

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

         // Propagate exception to caller.
         throw ex;
      }
   }
}

  BeanPostProcessor：自定义BeanPostProcessor的实现能够用户处理bean生成先后相关的逻辑，如修改bean信息，监控bean生成等，可分别调用postProcessBeforeInitialization()、postProcessAfterInitialization()两个方法处理前置和后置逻辑。

 一、经过obtainFreshBeanFactory()建立ConfigurableListableBeanFactory，并经过XmlBeanDefinitionReader读取Resource资源文件，解析为BeanDefinition。

/**
 * 建立beanFactory并加载resource资源文件
 * 解析资源文件、装载BeanDefinition
 * Tell the subclass to refresh the internal bean factory.
 * @return the fresh BeanFactory instance
 * @see #refreshBeanFactory()
 * @see #getBeanFactory()
 */
protected ConfigurableListableBeanFactory() {
   //建立beanFactory并加载resource资源文件
   refreshBeanFactory();
   ConfigurableListableBeanFactory beanFactory = getBeanFactory();
   if (logger.isDebugEnabled()) {
      logger.debug("Bean factory for " + getDisplayName() + ": " + beanFactory);
   }
   return beanFactory;
}

AbstractRefreshableApplicationContext：

@Override
protected final void refreshBeanFactory() throws BeansException {
   if (hasBeanFactory()) {
      destroyBeans();
      closeBeanFactory();
   }
   try {
      //建立beanFactory---DefaultListableBeanFactory
      DefaultListableBeanFactory beanFactory = createBeanFactory();
      beanFactory.setSerializationId(getId());
      customizeBeanFactory(beanFactory);
      //读取resource、加载BeanDefinition
      loadBeanDefinitions(beanFactory);
      synchronized (this.beanFactoryMonitor) {
         this.beanFactory = beanFactory;
      }
   }
   catch (IOException ex) {
      throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
   }
}

        AbstractXmlApplicationContext：loadBeanDefinitions()经过XmlBeanDefinitionReader读取Resource资源文件，解析为BeanDefinition。

/**
 * Loads the bean definitions via an XmlBeanDefinitionReader.
 * @see org.springframework.beans.factory.xml.XmlBeanDefinitionReader
 * @see #initBeanDefinitionReader
 * @see #loadBeanDefinitions
 */
@Override
protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, , IOException {
    //建立XmlBeanDefinitionReader 
   // Create a new XmlBeanDefinitionReader for the given BeanFactory.
   XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);

   // Configure the bean definition reader with this context's
   // resource loading environment.
   beanDefinitionReader.setEnvironment(this.getEnvironment());
   beanDefinitionReader.setResourceLoader(this);
   beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));

   //读取resource、加载BeanDefinition并将其注册到beanFactory的map中
   // Allow a subclass to provide custom initialization of the reader,
   // then proceed with actually loading the bean definitions.
   initBeanDefinitionReader(beanDefinitionReader);
   loadBeanDefinitions(beanDefinitionReader);
}

        二、实例化bean对象：经过finishBeanFactoryInitialization方法实现