第一步是IOC容器的启动过程：

入口在ContextLoaderListener，我们知道contextLoaderListener实现了ServletContextListener,它有一个初始化方法：

/**
 * Initialize the root web application context.
 */
public void contextInitialized(ServletContextEvent event) {
   this.contextLoader = createContextLoader();
   if (this.contextLoader == null) {
      this.contextLoader = this;
   }
   this.contextLoader.initWebApplicationContext(event.getServletContext());
}

进入initWeb初始化方法：

/**
 * 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);//核心方法 创建webApplicationContext
      }
      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;
   }
}

在这个方法中 我们看到一个createWebApplicationContext方法该方法就是创建根上下文的方法，进入该方法可以看到：

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

在这个方法中会创建我们需要的上下文。进入该方法

protected Class<?> determineContextClass(ServletContext servletContext) {
   String contextClassName = servletContext.getInitParameter(CONTEXT_CLASS_PARAM);//如果你在web.xml配置了相应的contextClass参数的话 那就按照你配置的来创建根上下文
   if (contextClassName != null) {
      try {
         return ClassUtils.forName(contextClassName, ClassUtils.getDefaultClassLoader());
      }
      catch (ClassNotFoundException ex) {
         throw new ApplicationContextException(
               "Failed to load custom context class [" + contextClassName + "]", ex);
      }
   }
   else {
      contextClassName = defaultStrategies.getProperty(WebApplicationContext.class.getName());//如果没有配置contextClass，那么就使用一个默认的上下文,这也就是我们经常使用的XmlWebApplicationContext
      try {
         return ClassUtils.forName(contextClassName, ContextLoader.class.getClassLoader());
      }
      catch (ClassNotFoundException ex) {
         throw new ApplicationContextException(
               "Failed to load default context class [" + contextClassName + "]", ex);
      }
   }
}

至此，上下文创建完成，我们在回到initWeb方法中的另一个主要的方法：

 configureAndRefreshWebApplicationContext(cwac, servletContext);//该方法的作用就是完成配置文件的读取，解析定义的bean，注册bean，设置bean的依赖关系，完成bean的初始化过程

此时整个加载过程已经完成！springmvc的启动过程下一篇再说吧！