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();

 

        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) {

                configureAndRefreshWebApplicationContext((ConfigurableWebApplicationContext)this.context, 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;

        }

    }