public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {
  //判斷servletContext是否已有根上下文
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) {
//這裡建立根web上下文
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);
}
  //這裡開始配置、初始化上下文
            configureAndRefreshWebApplicationContext(cwac, servletContext);
}
      }
  // 將建立的上下文放入servletContext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.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;
}
}