引出问题？
为什么在web.xml里面配置Spring，里面的context参数一定是contextConfigLocation,如下图：
分析流程
tomcat启动时，里面有配置监听程序的声明周期的监听器，如上图（ContextLoaderListner类）。
点击进入ContextLoaderListener类，如下图：
可以看出，ContextLoaderListner实现了ServletContextListener，实现了两个方法，分别是：
- contextInitialized：web程序初始化的时候回调（web服务器开启）
- contextDestroyed：web程序关闭的时候回调（web服务器关闭）
我们打开 contextInitialized方法
再进入initWebApplicationContext方法查看，如下图：
复制下整个方法的代码，代码如下：
/** * 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!"); } 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; } }
上面的流程只是一些加载操作，会发现里面调用了一个方法，大致是初始化相关的配置参数的，如下图：
点击进入此方法，会发现方法里面需要调用一个全局的变量，通过KEY值获取：
这个KEY是什么呢？没错，猜对了，正是我们web.xml里面配的KEY值，如下：
再看下第一张图：
整个流程就这样了！！！
