http://blog.csdn.net/qq924862077/article/details/52769754

<context-param>  
    <param-name>contextConfigLocation</param-name>  
    <param-value>  
        classpath:applicationContext.xml  
    </param-value>  
</context-param>  
<listener>  
    <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>  
</listener>  

在tomcat启动时，spring通过上述配置会初始化spring容器，注入applicationContext.xml中配置的bean以及其他一些配置。

public class ContextLoaderListener extends ContextLoader implements ServletContextListener {

    public ContextLoaderListener() {
    }
    public ContextLoaderListener(WebApplicationContext context) {
        super(context);
    }
    @Override
    public void contextInitialized(ServletContextEvent event) {
        initWebApplicationContext(event.getServletContext());
    }
    @Override
    public void contextDestroyed(ServletContextEvent event) {
        closeWebApplicationContext(event.getServletContext());
        ContextCleanupListener.cleanupAttributes(event.getServletContext());
    }
}

ContextLoderListener实现ServletContextListener接口，而该接口继承java的监听标记接口EventListener，表示当servlet容器启动时，就会接受通知，进而进行一系列的初始化操作。

public interface ServletContextListener extends EventListener {

    /**
     * Receives notification that the web application initialization
     * process is starting.
     *
     * <p>All ServletContextListeners are notified of context
     * initialization before any filters or servlets in the web
     * application are initialized.
     *
     * @param sce the ServletContextEvent containing the ServletContext
     * that is being initialized
     */
    public void contextInitialized(ServletContextEvent sce);

    /**
     * Receives notification that the ServletContext is about to be
     * shut down.
     *
     * <p>All servlets and filters will have been destroyed before any
     * ServletContextListeners are notified of context
     * destruction.
     *
     * @param sce the ServletContextEvent containing the ServletContext
     * that is being destroyed
     */
    public void contextDestroyed(ServletContextEvent sce);
}

spring的具体初始化工作均在父类ContextLoder中进行，具体初始化步骤如下：

1. 判断是否已经存在WebApplicationContext(spring容器)，如果存在，则抛出异常。

2. 创建WebApplicationContext容器，容web.xml中获取context-param名称为contextClass的value，该类必须实现WebApplicationContext接口，如果没有指定，则默认使用xmlWebApplicationContext。

3. 判断创建的WebApplicationContext容器是否为ConfigurableApplicationContext的实例或子类，如果是，则判断该容器是否至少刷新一次并且还没有关闭（是否有效），如果无效，则重新设置容器的id，并加载对应的contextConfigLocation配置（spring配置)， 加载父容器，spring提供一个模板方法，如果需要配置父容器，可以继承ContextLoder，并覆盖loadParentContext方法。

4. 线程绑定容器

   /** * 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) { //创建容器 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); } //如果容器无效，则重置contextId，且重新加载contextConfigLocation配置 configureAndRefreshWebApplicationContext(cwac, servletContext); } } servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context); //线程绑定容器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; } }