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();         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;        }    }