Web容器启动时加载Spring

在应用程序web.xml中做了以下配置信息时，当启动Web容器时就会自动加载Spring容器。

<listener> 

    <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class> 

</listener> 

ContextLoaderListener类实现了javax.servlet.ServletContextListener接口并且继承了org.springframework.web.context.ContextLoader类。ServletContextListener事件类是Web容器的一部分，处理Web应用的Servlet上下文(context)的监听。实现ServletContextListener接口中的contextInitialized和contextDestroyed方法。当Web容器启动时会自动调用contextInitialized方法，进行初始化Spring Web应用程序上下文，主要加载web.xml中contextConfigLocation的配置文件；当Web容器关闭之前会调用contextDestroyed方法，进行销毁Spring Web应用程序上下文。ContextLoader类实现了Spring上下文初始化的工作，执行initWebApplicationContext方法返回WebApplicationContext。Spring实现的contextInitialized和contextDestroyed代码如下：

public void contextInitialized(ServletContextEvent event) { 

       this.contextLoader = createContextLoader(); 

       if (this.contextLoader ==null) { 

           this.contextLoader = this; 

       } 

    this.contextLoader.initWebApplicationContext(event.getServletContext()); 

    } 

public void contextDestroyed(ServletContextEvent event) { 

       if (this.contextLoader !=null) {       

this.contextLoader.closeWebApplicationContext(event.getServletContext()); 

} 

 ContextCleanupListener.cleanupAttributes(event.getServletContext()); 

} 

Spring执行实现Spring上下文的方法

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(); 

&nb