ContextLoaderListener初始化了webapp的root context（应用根上下文），过程如下

ContextLoaderListener继承了ContextLoader，主要方法如下

private WebApplicationContext  context;public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {    //判断是否已经有根上下文，如果已经有了，报错，否则，创建根上下文    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!");    } else {        //开始创建根上下文        try {            if(this.context == null) {                //创建根上下文                this.context = this.createWebApplicationContext(servletContext);            }            if(this.context instanceof ConfigurableWebApplicationContext) {                ConfigurableWebApplicationContext err = (ConfigurableWebApplicationContext)this.context;                if(!err.isActive()) {                    if(err.getParent() == null) {                        ApplicationContext elapsedTime = this.loadParentContext(servletContext);                        err.setParent(elapsedTime);                    }                    // 给上下文设置各种属性                    // 给上下文设置id                    // 给上下文设置配置文件的路径（applicationcontext.xml类似）                    // 通过refresh方法设置beanfactory？这里不是太清楚                    this.configureAndRefreshWebApplicationContext(err, servletContext);                }            }            //创建好的根上下文放到servletContext里，整个webapp只有一个servletContext            //servletContext是ServletConfig的一个成员变量            servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);            ClassLoader err1 = Thread.currentThread().getContextClassLoader();            if(err1 == ContextLoader.class.getClassLoader()) {                currentContext = this.context;            } else if(err1 != null) {                currentContextPerThread.put(err1, this.context);            }            return this.context;        } catch (RuntimeException var8) {          }    }}

protected WebApplicationContext createWebApplicationContext(ServletContext sc) {    //获取上下文类型，只能为ConfigurableWebApplicationContext类型的    Class contextClass = this.determineContextClass(sc);    if(!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {        throw new ApplicationContextException("Custom context class [" + contextClass.getName() + "] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]");    } else {        //如果类型正确，实例化并返回        return (ConfigurableWebApplicationContext)BeanUtils.instantiateClass(contextClass);    }}

protected Class
     determineContextClass(ServletContext servletContext) {    //首先，看在web.xml中有没有配置我们自己的上下文类型，很明显，我们没有，此处为null    String contextClassName = servletContext.getInitParameter("contextClass");    if(contextClassName != null) {        try {            return ClassUtils.forName(contextClassName, ClassUtils.getDefaultClassLoader());        } catch (ClassNotFoundException var4) {            throw new ApplicationContextException("Failed to load custom context class [" + contextClassName + "]", var4);        }    } else {        //由于我们自己没有配置，spring加载自己的默认类型（XmlWebApplicaionContext.java）        contextClassName = defaultStrategies.getProperty(WebApplicationContext.class.getName());        try {            return ClassUtils.forName(contextClassName, ContextLoader.class.getClassLoader());        } catch (ClassNotFoundException var5) {            throw new ApplicationContextException("Failed to load default context class [" + contextClassName + "]", var5);        }    }}

//这个是默认类型static {    try {        ClassPathResource ex = new ClassPathResource("ContextLoader.properties", ContextLoader.class);        defaultStrategies = PropertiesLoaderUtils.loadProperties(ex);    } catch (IOException var1) {        throw new IllegalStateException("Could not load \'ContextLoader.properties\': " + var1.getMessage());    }    currentContextPerThread = new ConcurrentHashMap(1);}

ContextLoader.properties文件内容如下

org.springframework.web.context.WebApplicationContext=org.springframework.web.context.support.XmlWebApplicationContext

XmlWebApplicationContext是web应用中定制的getBean的方法

ServletContext servletContext = request.getSession().getServletContext();WebApplicationContext was = WebApplicationContextUtils.getWebApplicationContext(servletContext);User user = (User)was.getBean("user");

其余两种方法为：

ApplicationContext ctx =

new FileSystemXmlApplicationContext( "G:/Test/applicationcontext.xml ");

或

ApplicationContext ctx =

new ClassPathXmlApplicationContext( "/applicationcontext.xml ");

最后一句话：一个请求一个request，一个用户一个session，一个应用一个ServletContext

ServletContext中可以存储共享的数据，因为它的生命周期很长，从应用启动直到消亡。所以里面不要存大的数据，会很占内存

ServletContext对象是由容器创建的，也就是tomcat

和request一样，ServletContext也有setAttribute以及getAttribute和removeAttribute方法

通过ServletContext，我们也可以在servlet之间传递信息，也可以读取web.xml中的初始化参数context-param，如web.xml中context-param如下

        
     
      log4jConfigLocation
         
     
      classpath:${env}-conf/log4j.properties
       
      
        
     
      contextConfigLocation
         
     
      classpath:spring.xml
     
       
      
        
     
      webAppRootKey
         
     
      ott-vms-admin.root
       
    

ServletContext servletContext = request.getSession().getServletContext();String value = (String)servletContext.getInitParameter("contextConfigLocation");