在开发Spring的Web项目中，通常我们都会在web.xml中配置一个Spring的核心监听器，就是把Spring的IOC容器纳入Servlet容器中，配置如下：
<listener> <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class> </listener> <context-param> <param-name>contextConfigLocation</param-name> <param-value>classpath:applicationContext.xml</param-value> </context-param>
查看ContextLoaderListener源码如下：
public class ContextLoaderListener extends ContextLoader implements ServletContextListener { public ContextLoaderListener() {} public ContextLoaderListener(WebApplicationContext context) { super(context); } @Override public void contextInitialized(ServletContextEvent event) { initWebApplicationContext(event.getServletContext()); } @Override public void contextDestroyed(ServletContextEvent event) { closeWebApplicationContext(event.getServletContext()); ContextCleanupListener.cleanupAttributes(event.getServletContext()); } }
ContextLoaderListener官方源码定义是Spring的root(也就是WebApplicationContext)启动和关闭的引导监听器，当调用ContextLoaderListener的无参构造器时，就会创建一个新的ContextLoaderListener,也将会创建一个web应用上下文基于web.xml中的contextLoader和contextConfigLocation的servlet context-param参数，创建的应用上下文会被注册到ServletContext中通过WebApplicationContext里的ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE这个属性。
ContextLoaderListener继承自ContextLoader并且实现了ServletContextListener接口，实现ServletContextListener作用不言而喻，就是要让Spring和Servlet容器关联起来，让Spring容器伴随着Servlet容器的启动而启动，其实这个类中最主要的一个方法就是contextInitialized()，作用就是Spring上下文初始化，里面的initWebApplicationContext()是从父类ContextLoader中继承过来的，查看源码：
public WebApplicationContext initWebApplicationContext(ServletContext servletContext) { 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. if (this.context == null) { 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, etc 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); } 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; } }