我们平日采用spring 框架 开发项目时，配置文件web.xml 中总是有这样一些配置，比如：

<!-- 加载spring配置文件-->
<context-param><param-name>contextConfigLocation</param-name><param-value>classpath:config/spring-config.xmlclasspath:config/spring-cxf.xml    </param-value>
</context-param>

<!-- web启动监听配置-->

<listener><listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>

而这些配置到底有什么作用呢？我们从源码角度分析一下：

1、首先看ContextLoaderListener 类，它继承ContextLoader 实现 ServletContextListener

public class ContextLoaderListener extends ContextLoader implements ServletContextListener

ContextLoaderListener的作用就是启动Web容器时，读取在contextConfigLocation中定义的xml文件，自动装配ApplicationContext的配置信息，并产生WebApplicationContext对象，然后将这个对象放置在ServletContext的属性里，这样我们只要得到Servlet就可以得到WebApplicationContext对象，并利用这个对象访问spring容器管理的bean。 
总之，就是上面这段配置为项目提供了spring支持，初始化了Ioc容器。

ContextLoader 主要作用作为父类指定Web应用程序启动时载入相应的Ioc容器，完成实际的WebApplicationContext，即Ioc容器的初始化工作在这里完成，后面会源码展示。

ServletContextListener 接口继承EventListener

public interface ServletContextListener extends EventListener

ServletContextListener是ServletContext的监听者，接口里的方法会结合Web容器的生命周期被调用，ServletContext发生某些变化，会触发相应的事件，web服务器启动时、ServletContext创建时、web服务关闭时、ServletContext销毁时等等，都会触发相应事件，做出预先设计好的相应动作，比如此类中：监听器的响应动作就是在服务器启动时contextInitialized会被调用，关闭的时候contextDestroyed被调用。

/*** Initialize the root web application context.启动时在这里开始调用，初始化根上下文应用程序*/
@Override
public void contextInitialized(ServletContextEvent event) {initWebApplicationContext(event.getServletContext());//点击进入
}

2、进入 ContextLoader 类

public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {//如果servletContext对象中存放了spring context，则抛出异常if (servletContext.getAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE) != null) {throw new IllegalStateException("Cannot initialize context because there is already a root application context present - " +"check whether you have multiple ContextLoader* definitions in your web.xml!");}servletContext.log("Initializing Spring root WebApplicationContext");Log logger = LogFactory.getLog(ContextLoader.class);if (logger.isInfoEnabled()) {logger.info("Root WebApplicationContext: initialization started");}long startTime = System.currentTimeMillis();try {// Store context in local instance variable, to guarantee that// it is available on ServletContext shutdown.if (this.context == null) {//创建WebApplicationContext容器并被强制转换为ConfigurableWebApplicationContext类型this.context = createWebApplicationContext(servletContext);}if (this.context instanceof ConfigurableWebApplicationContext) {ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) this.context;if (!cwac.isActive()) {// The context has not yet been refreshed -> provide services such as// setting the parent context, setting the application context id, etcif (cwac.getParent() == null) {// The context instance was injected without an explicit parent ->// determine parent for root web application context, if any.//加载父容器ApplicationContext parent = loadParentContext(servletContext);cwac.setParent(parent);}//加载spring配置文件,点击进入configureAndRefreshWebApplicationContext(cwac, servletContext);}}//把web容器放到servlet容器中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.isInfoEnabled()) {long elapsedTime = System.currentTimeMillis() - startTime;logger.info("Root WebApplicationContext initialized in " + elapsedTime + " ms");}return this.context;}catch (RuntimeException | Error ex) {logger.error("Context initialization failed", ex);servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, ex);throw ex;}
}

来到configureAndRefreshWebApplicationContext方法中

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//获取 web.xml 中定义的 contextId 初始化值，未定义String idParam = sc.getInitParameter(CONTEXT_ID_PARAM);if (idParam != null) {wac.setId(idParam);}else {// Generate default id... 设置一个 全局的idwac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +ObjectUtils.getDisplayString(sc.getContextPath()));}}wac.setServletContext(sc);//配置了 servletContext 属性//获取 web.xml 中定义的 contextConfiLocation 初始化参数值 ，已定义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//初始化 应用上下文 中 和 Servlet 相关的 占位符 属性资源ConfigurableEnvironment env = wac.getEnvironment();if (env instanceof ConfigurableWebEnvironment) {((ConfigurableWebEnvironment) env).initPropertySources(sc, null);}/// 加载web.xml 中 自定义的 其他初始化内容customizeContext(sc, wac);wac.refresh();//WebApplicationContext 的刷新,核心方法，点击进入后豁然开朗，ctr+t 进入
}

3、进入这个方法，非常熟悉，前面几篇已经详细解析过，是spring 源码的重点方法，这里就不介绍了。

public abstract class AbstractApplicationContext extends DefaultResourceLoaderimplements ConfigurableApplicationContext {

@Override
public void refresh() throws BeansException, IllegalStateException {synchronized (this.startupShutdownMonitor) {// Prepare this context for refreshing.prepareRefresh();// Tell the subclass to refresh the internal bean factory.ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();// Prepare the bean factory for use in this context.prepareBeanFactory(beanFactory);try {// Allows post-processing of the bean factory in context subclasses.postProcessBeanFactory(beanFactory);// Invoke factory processors registered as beans in the context.invokeBeanFactoryPostProcessors(beanFactory);// 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();// Instantiate all remaining (non-lazy-init) singletons.finishBeanFactoryInitialization(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 {// Reset common introspection caches in Spring's core, since we// might not ever need metadata for singleton beans anymore...resetCommonCaches();}}
}

4、重点这是spring 的容器初始化，下篇介绍springMVC 容器初始化；他俩是父子容器关系。敬请期待！