Spring源码情操陶冶-ContextLoader

前言-阅读源码有利于陶冶情操，本文承接前文Spring源码情操陶冶-ContextLoaderListener

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<context-param>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<context-param>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<C extends ConfigurableApplicationContext>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