传统的SpringMVC项目中，需要在web.xml中配置Contextlistener。ContextLoaderListener是负责引导启动和关闭Spring的Root上下文的监听器。主要将处理委托给ContextLoader和ContextCleanupListener。
类的继承关系。

ContextLoaderListener实现了ServletContextListener接口。该接口主要定义了两个行为：监听上下文创建（contextInitialized）和监听上下文销毁（contextDestroyed）。
ContextLoaderListener只是将方法的处理委托给ContextLoader。

package org.springframework.web.context;import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;public class ContextLoaderListener extends ContextLoader implements ServletContextListener {public ContextLoaderListener() {}public ContextLoaderListener(WebApplicationContext context) {super(context);}//初始化Root WebApplication@Overridepublic void contextInitialized(ServletContextEvent event) {//委托给ContextLoaderinitWebApplicationContext(event.getServletContext());}//销毁Root WebApplication@Overridepublic void contextDestroyed(ServletContextEvent event) {   //委托给ContextLoadercloseWebApplicationContext(event.getServletContext());//委托给ContextCleanupListenerContextCleanupListener.cleanupAttributes(event.getServletContext());}}

ContextLoader的initWebApplicationContext方法负责创建root web application context。

//初始化root web application context
public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {//校验是否已经存在root application，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!");}Log logger = LogFactory.getLog(ContextLoader.class);servletContext.log("Initializing Spring root WebApplicationContext");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.//创建WebApplicationContext，并保存到变量中，等关闭时使用if (this.context == null) {this.context = createWebApplicationContext(servletContext);}//如果是ConfigurableWebApplicationContext，则配置上下文并刷新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.ApplicationContext parent = loadParentContext(servletContext);cwac.setParent(parent);}configureAndRefreshWebApplicationContext(cwac, servletContext);}}//在ServletContext中设置属性，表明已经配置了root web application contextservletContext.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;}}

该方法主要分为四步：

1. 校验是否已经存在root WebApplicationContext
2. 创建root WebApplicationContext
3. 配置上下文并刷新
4. 绑定root WebApplicationContext到servlet context上

这四步具体分析如下：

1.校验

校验的过程比较简单，主要是判断Servlet Context是否已经绑定过WebApplicationContext

2.创建对象

createWebApplicationContext()方法负责创建WebApplicationContext对象，主要过程是确定WebApplicationContext类，并实例化：

    protected WebApplicationContext createWebApplicationContext(ServletContext sc) {//查找WebApplicationContext的类Class<?> contextClass = determineContextClass(sc);if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {throw new ApplicationContextException("Custom context class [" + contextClass.getName() +"] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]");}//实例化对象return(ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);}

决定WebApplicationContext类的主要策略是先判断ServletContext是否配置了contextClass属性，如果是，则用该属性指定的类作为WebApplicationContext类，否则则是否默认策略。默认策略是根绝classpath下的ContextLoader.properties中配置的类作为WebApplicatioNCOntext类：

protected Class<?> determineContextClass(ServletContext servletContext) {//优先根据servletContext配置的contextClass属性String contextClassName = servletContext.getInitParameter(CONTEXT_CLASS_PARAM);if (contextClassName != null) {try {return ClassUtils.forName(contextClassName, ClassUtils.getDefaultClassLoader());}catch (ClassNotFoundException ex) {throw new ApplicationContextException("Failed to load custom context class [" + contextClassName + "]", ex);}}//在通过classpath下的`ContextLoader.properties`文件制定的类作为WebApplicationContext类//默认是XmlWebApplicationContextelse {contextClassName = defaultStrategies.getProperty(WebApplicationContext.class.getName());try {return ClassUtils.forName(contextClassName, ContextLoader.class.getClassLoader());}catch (ClassNotFoundException ex) {throw new ApplicationContextException("Failed to load default context class [" + contextClassName + "]", ex);}}}

配置与刷新

创建WebApplicationContext对象后，ContextLoader会判断是否是ConfigurableWebApplicationContext。
ConfigurableWebApplicationContext拓展了WebApplicationContext的能力。其不仅扩展了自身方法，还继承了ConfigurableApplicationContext接口。

配置与刷新的步骤也正是利用了这些接口提供的能力。

    //配置并刷新上下文protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac, ServletContext sc) {//设置IDif (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 informationString 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()));}}//WebApplicationContext绑定Servlet Contextwac.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 #refreshConfigurableEnvironment env = wac.getEnvironment();if (env instanceof ConfigurableWebEnvironment) {((ConfigurableWebEnvironment) env).initPropertySources(sc, null);}//自定义上下文初始过程，留给用户的拓展机制customizeContext(sc, wac);//刷新上下文，加载beanwac.refresh();}

创建流程图如下：