web上下文与SerlvetContext的生命周期应该是相同的，springmvc中的web上下文初始化是由ContextLoaderListener来启动的

在web.xml中配置ContextLoaderListener

<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>

ContextLoaderListener

ContextLoaderListener实现了ServletContextListener接口，ServletContextListener是Servlet定义的，提供了与Servlet生命周期结合的回调，contextInitialized方法和contextDestroyed方法；ContextLoaderListener继承了ContextLoader类，通过ContextLoader来完成实际的WebApplicationContext的初始化工作，并将WebApplicationContext存放至ServletContext中，ContextLoader就像是spring应用在web容器中的启动器

从spring3.x起就不需要配置监听器ContextLoaderListener，只需要配置DispatcherServlet就可以了

public class ContextLoaderListener extends ContextLoader implements ServletContextListener {

   /**
    * Initialize the root web application context.
    */
  // ServletContext被创建时触发，初始化web上下文
   @Override
   public void contextInitialized(ServletContextEvent event) {
      initWebApplicationContext(event.getServletContext());
   }

   /**
    * Close the root web application context.
    */
  // ServletContext被销毁时触发，关闭web上下文
   @Override
   public void contextDestroyed(ServletContextEvent event) {
      closeWebApplicationContext(event.getServletContext());
      ContextCleanupListener.cleanupAttributes(event.getServletContext());
   }

}

在启动时会创建一个WebApplicationContext作为IOC容器，默认的实现类是XmlWebApplicationContext

/** Default config location for the root context */
// 默认的配置位置，如果不进行配置，就会读取这个文件
public static final String DEFAULT_CONFIG_LOCATION = "/WEB-INF/applicationContext.xml";

/** Default prefix for building a config location for a namespace */
// 配置文件默认在/WEB-INF/目录下
public static final String DEFAULT_CONFIG_LOCATION_PREFIX = "/WEB-INF/";

/** Default suffix for building a config location for a namespace */
// 配置文件默认后缀名是.xml
public static final String DEFAULT_CONFIG_LOCATION_SUFFIX = ".xml";

initWebApplicationContext初始化

public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {
  // WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE是WebApplicationContext.class.getName() + ".ROOT"
  // 如果servletContext中已经存在根上下文属性，则说明已经有一个根上下文存在了
   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");
   
   long startTime = System.currentTimeMillis();

   try {
      // Store context in local instance variable, to guarantee that
      // it is available on ServletContext shutdown.
      if (this.context == null) {
        // 初始化context
         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);
         }
      }
     // 记录在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);
      }

      

      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;
   }
}

创建WebApplicationContext上下文

protected WebApplicationContext createWebApplicationContext(ServletContext sc) {
  // 如果在web.xml中有配置contextClass，则使用配置的类
  // 如果没有配置，则使用默认的配置 ContextLoader.properties
  // org.springframework.web.context.WebApplicationContext=org.springframework.web.context.support.XmlWebApplicationContext

   Class<?> contextClass = determineContextClass(sc);
   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);
}