前言-阅读源码有利于陶冶情操，本文承接前文

静态代码块内容

ContextLoader在被主动调用的时候，会执行其的一个静态块，代码如下

static {        // Load default strategy implementations from properties file.        // This is currently strictly internal and not meant to be customized        // by application developers.        try {                //这里DEFAULT_STRATEGIES_PATH值为ContextLoader.properties            ClassPathResource resource = new ClassPathResource(DEFAULT_STRATEGIES_PATH, ContextLoader.class);            //这里的loadProperties方法其实调用的getClass().getResourceAsStream(path)方法            //其是从当前包路径下查找相应的资源，点开相应的class路径，果不其然ContextLoader.properties与ContextLoader.class在同一目录下            defaultStrategies = PropertiesLoaderUtils.loadProperties(resource);        }        catch (IOException ex) {            throw new IllegalStateException("Could not load 'ContextLoader.properties': " + ex.getMessage());        }    }

具体的解析功能见注释，上面分析到其会读取ContextLoader.properties，这个文件下只有一个属性：

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

由此可见其最有可能是通过XmlWebApplicationContext类来进行相应的web上下文初始化

初始化方法initWebApplicationContext

从前文得知，ContextLoaderListener会调用ContextLoader#initWebApplicationContext(ServletContext context)方法来创建web application上下文环境，代码清单如下

//首先判断有无org.springframework.web.context.WebApplicationContext.ROOT属性，不允许已有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");        }        //用来计算初始化这个过程会耗费多少时间        long startTime = System.currentTimeMillis();        try {            // Store context in local instance variable, to guarantee that            // it is available on ServletContext shutdown.            if (this.context == null) {                //创建方法，一般会创建前点所述的XmlWebApplicationContext                this.context = createWebApplicationContext(servletContext);            }            //XmlWebApplicationContext是ConfigurableWebApplicationContext的实现类，指定的contextClass应该是ConfigurableWebApplicationContext的实现类            if (this.context instanceof ConfigurableWebApplicationContext) {                ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) this.context;                //一般来说刚创建的context并没有处于激活状态                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.                        //在web.xml中配置了
    
     的parentContextKey才会指定父级应用                        ApplicationContext parent = loadParentContext(servletContext);                        cwac.setParent(parent);                    }                    //读取相应的配置并且刷新context对象                    configureAndRefreshWebApplicationContext(cwac, servletContext);                }            }                        //设置属性，避免再被初始化                                  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);                        //即使初始化失败仍不允许有再次的初始化                                    servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, err);            throw err;        }
    

上述的代码片段中主要用到了两个关键方法createWebApplicationContext(ServletContext context)和configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac,ServletContext sc)，本文必须对这两个方法进行相应的解读

ContextLoader#createWebApplicationContext(ServletContext context)

代码清单如下

//决定采用默认的contextClass还是ServletContext中的参数属性contextClass                Class
     contextClass = determineContextClass(sc);        //这里Spring强调指定的contextClass必须是ConfigurableWebApplicationContext的实现类或者子类        if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {            throw new ApplicationContextException("Custom context class [" + contextClass.getName() +                    "] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]");        }        //这里就是实例化指定的contextClass        return (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);

实现的主要功能为创建ConfigurableWebApplicationContext对象，这里可以稍微瞄一眼determineContextClass(ServletContext context)

//优先从ServletContext取contextClass参数对应的值，即查看是否在web.xml中配置            //对应的contextClass
    
     参数值            String contextClassName = servletContext.getInitParameter(CONTEXT_CLASS_PARAM);        if (contextClassName != null) {            try {                return ClassUtils.forName(contextClassName, ClassUtils.getDefaultClassLoader());            }            catch (ClassNotFoundException ex) {                throw new ApplicationContextException(                        "Failed to load custom context class [" + contextClassName + "]", ex);            }        }        else {            //倘若不存在，则加载指定的XmlWebApplicationContext类            contextClassName = defaultStrategies.getProperty(WebApplicationContext.class.getName());            try {                return ClassUtils.forName(contextClassName, ContextLoader.class.getClassLoader());            }            catch (ClassNotFoundException ex) {                throw new ApplicationContextException(                        "Failed to load default context class [" + contextClassName + "]", ex);            }        }
    

ContextLoader#configureAndRefreshWebApplicationContext()

代码清单如下

protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac, ServletContext sc){//一般此处为真if (ObjectUtils.identityToString(wac).equals(wac.getId())) {            // The application context id is still set to its original default value            // -> assign a more useful id based on available information            //获取servletContext中的contextId属性            String idParam = sc.getInitParameter(CONTEXT_ID_PARAM);            if (idParam != null) {                //存在则设为指定的id名                wac.setId(idParam);            }            else {                // Generate default id... 一般为org.springframework.web.context.WebApplicationContext:${contextPath}                wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +                        ObjectUtils.getDisplayString(sc.getContextPath()));            }        }        wac.setServletContext(sc);        //读取contextConfigLocation属性        String configLocationParam = sc.getInitParameter(CONFIG_LOCATION_PARAM);        if (configLocationParam != null) {            //设置指定的spring文件所在地，支持classpath前缀并多文件，以,;为分隔符            wac.setConfigLocation(configLocationParam);        }        // The wac environment's #initPropertySources will be called in any case when the context        // is refreshed; do it eagerly here to ensure servlet property sources are in place for        // use in any post-processing or initialization that occurs below prior to #refresh        //获得真实对象为StandardEnvironment 其非ConfigurableWebEnvironment的实现类        ConfigurableEnvironment env = wac.getEnvironment();        if (env instanceof ConfigurableWebEnvironment) {            ((ConfigurableWebEnvironment) env).initPropertySources(sc, null);        }        //查看是否有ApplicationContextInitializer
    
     启动类，其实是web.xml需要指定globalInitializerClasses参数或者contextInitializerClasses参数，前提是指定的这些类的泛型类必须是wac的父类或者与wac类相同，否则就会有异常抛出        customizeContext(sc, wac);        //调用AbstractApplicationContext的refresh方法实现加载spring文件等操作        wac.refresh();}
    

可见关键处理还是在AbstractApplicationContext#refresh()方法，前面都是refresh()方法的准备工作，包括指定contextConfigLocationSpring配置文件位置、给应用一个id倘若指定了contextId属性、如果指定了globalInitializerClasses或者contextInitializerClasses参数则调用其中的initialize()方法

记录总结

1. ContextLoader在web.xml配置文件中没有指定contextClass的属性下会默认加载XmlWebApplicationContext类，如果指定了contextClass属性，Spring强调指定的contextClass必须是ConfigurableWebApplicationContext的实现类或者子类

2. web.xml配置文件中如果没有指定contextId属性，则WebApplicationContext的id为org.springframework.web.context.WebApplicationContext:${contextPath},其中${contextPath}是项目的上下文路径，例如localhost:8080/test/info/query路径下的contextPath为/test

3. web.xml配置的contextConfigLocation属性支持多文件，以,;为分隔符，不指定默认会加载applicationContext.xml，其在后续章节剖析

4. web.xml倘若指定globalInitializerClasses参数或者contextInitializerClasses参数，具体要求如下
   • 指定的类实现ApplicationContextInitializer<C extends ConfigurableApplicationContext>接口
   • 指定的这些类中的泛型类必须是contextClass(默认为XmlWebApplicationContext)的父类或者一致，否则就会有异常抛出

下节预告

Spring源码情操陶冶-AbstractApplicationContext