1、对于一个web 应用，其部署在web 容器中，web 容器提供其一个全局的上下文环境，这个上下文就是 ServletContext ，其后面的spring IoC 容器提供宿主环境
通过这行代码就可以看出: servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);
在web.xml 中会提供有 contextLoaderListener。在web 容器启动时，会触发容器初始化事件，此时 contextLoaderListener 会监听到这个事件，其 contextInitialized 方法会被调用，在这个方法中，spring 会初始化一个启动上下文，这个上下文就被称为根上下文，即 WebApplicationContext ，这是一个借口类，确切的说，其实际实现类是 XmlWebApplicaitonContext 。这个就是Spring 的Ioc 容器，其对应的Bean 定义的配置由web.xml 中的 context-param 标签指定。在这个Ioc 容器初始化完毕后，spring 以WebApplicationContext.ROOTWEBAPPLICATIONEXTATTRIBUTE 为属性key，将其存储到 servletContext 中，便于获取.
在web.xml中这个就是web容器的入口：
<listener>
<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>ContextLoaderListener继承了ContextLoader,实现了ServletContextListener接口。我们看看ContextLoader源码：
/**
* Name of servlet context parameter (i.e., {@value}) that can specify the * config location for the root context, falling back to the implementation's * default otherwise. * @see org.springframework.web.context.support.XmlWebApplicationContext#DEFAULT_CONFIG_LOCATION */ public static final String CONFIG_LOCATION_PARAM = "contextConfigLocation"; 这个可以把响应的配置文件加载到web容器中，这个配置十分重要，可以定义相应配置的位置。如下配置：
<!--加载spring上下文对象 -->
<context-param> <param-name>contextConfigLocation</param-name> <param-value>classpath:conf/cpic/root-context.xml</param-value> </context-param>/**
* Config param for the root WebApplicationContext implementation class to use: {@value} * @see #determineContextClass(ServletContext) * @see #createWebApplicationContext(ServletContext, ApplicationContext) */ public static final String CONTEXT_CLASS_PARAM = "contextClass"; //这个配置决定web容器的实现类，默认为XmlWebApplicationContextpublic WebApplicationContext initWebApplicationContext(ServletContext servletContext) ;//这个方法是重点，整个web容器的初始化，会在ContextLoaderListener中调用，分析ContextLoderListener这个类源码的时候时候回调用。
我们来看看 ServletContextListener这个接口源码：
public interface ServletContextListener extends EventListener {
/** ** Notification that the web application initialization ** process is starting. ** All ServletContextListeners are notified of context ** initialization before any filter or servlet in the web ** application is initialized. */public void contextInitialized ( ServletContextEvent sce ); //当容器的启动的时候自动加载这个方法
/**
** Notification that the servlet context is about to be shut down. ** All servlets and filters have been destroy()ed before any ** ServletContextListeners are notified of context ** destruction. */ public void contextDestroyed ( ServletContextEvent sce );//当容器destoryed时候调用该方法}我们来看看ContextLoadListener 源码:
/**
* Initialize the root web application context.实现了容器ServletContextListener的接口contextInitialized方法，初始化web容器
*/ @Override public void contextInitialized(ServletContextEvent event) { initWebApplicationContext(event.getServletContext()); }
/**
* Initialize Spring's web application context for the given servlet context, * using the application context provided at construction time, or creating a new one * according to the "{@link #CONTEXT_CLASS_PARAM contextClass}" and * "{@link #CONFIG_LOCATION_PARAM contextConfigLocation}" context-params. * @param servletContext current servlet context * @return the new WebApplicationContext * @see #ContextLoader(WebApplicationContext) * @see #CONTEXT_CLASS_PARAM * @see #CONFIG_LOCATION_PARAM */ 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!"); } //容器启动的时候发现已经存在web容器会出错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); //创建一个web容器 } 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); //在servletContext上下文中获取父容器 cwac.setParent(parent);//设置父级容器 } configureAndRefreshWebApplicationContext(cwac, servletContext); } }//serletContext 中设置web容器，serletContext提供了宿主环境
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; } }具体web容器容器初始化过程会在springIoc 介绍