spring web的初始化

主要是想探究springMVC中引用ContextLoaderListener中配置的context:property-placeholder，在初始化springMVC的时候会提示“ Could not resolve placeholder ‘jdbc_url’ in string value “${jdbc_url}””的原因

根据servlet的规范可知初始化顺序：context-param -> listener -> filter -> servlet

ContextLoaderListener的初始化

ContextLoaderListener(ContextLoader).initWebApplicationContext(ServletContext) line: 306	 ##下面会贴代码
ContextLoaderListener.contextInitialized(ServletContextEvent) line: 112

WebApplicationContext的初始化

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) {
			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);
				}
				configureAndRefreshWebApplicationContext(cwac, servletContext);
			}
		}
		//放入ServletContext中，等待DispatcherServlet初始化时取出来
		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;
	}
}

DispatcherServlet的初始化

XmlWebApplicationContext.loadBeanDefinitions(XmlBeanDefinitionReader) line: 123	 ##下面会贴代码
XmlWebApplicationContext.loadBeanDefinitions(DefaultListableBeanFactory) line: 94	
XmlWebApplicationContext(AbstractRefreshableApplicationContext).refreshBeanFactory() line: 130	
XmlWebApplicationContext(AbstractApplicationContext).obtainFreshBeanFactory() line: 537	
XmlWebApplicationContext(AbstractApplicationContext).refresh() line: 451	 ##下面会贴代码
DispatcherServlet(FrameworkServlet).configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext) line: 651	
DispatcherServlet(FrameworkServlet).createWebApplicationContext(ApplicationContext) line: 602	
DispatcherServlet(FrameworkServlet).createWebApplicationContext(WebApplicationContext) line: 665	
DispatcherServlet(FrameworkServlet).initWebApplicationContext() line: 521	 ##会使用ContextLoaderListener放入的ApplicationContext
DispatcherServlet(FrameworkServlet).initServletBean() line: 462	
DispatcherServlet(HttpServletBean).init() line: 136	
DispatcherServlet(GenericServlet).init(ServletConfig) line: 244	
ServletHolder.initServlet() line: 532

XmlWebApplicationContext从web.xml中DispatcherServlet的init参数contextConfigLocation得到configLocations。

XmlWebApplicationContext.loadBeanDefinitions方法

protected void loadBeanDefinitions(XmlBeanDefinitionReader reader) throws IOException {
	String[] configLocations = getConfigLocations();
	if (configLocations != null) {
		for (String configLocation : configLocations) {
			reader.loadBeanDefinitions(configLocation);
		}
	}
}

XmlWebApplicationContext的refresh()方法

public void refresh() throws BeansException, IllegalStateException {
	synchronized (this.startupShutdownMonitor) {
		// Prepare this context for refreshing.
		prepareRefresh();

		// Tell the subclass to refresh the internal bean factory.
		ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();//从springMVC配置文件加载beanFactory

		// Prepare the bean factory for use in this context.
		prepareBeanFactory(beanFactory);

		try {
			// Allows post-processing of the bean factory in context subclasses.
			postProcessBeanFactory(beanFactory);

			//BeanFactoryPostProcessor的实现类会在这里被处理，比如PropertyPlaceholderConfigurer，处理完成后就会被抛弃掉，不会放进ApplicationContext中
			// Invoke factory processors registered as beans in the context.
			invokeBeanFactoryPostProcessors(beanFactory); 


			// Register bean processors that intercept bean creation.
			registerBeanPostProcessors(beanFactory);    

			// Initialize message source for this context.
			initMessageSource();

			// Initialize event multicaster for this context.
			initApplicationEventMulticaster();

			// Initialize other special beans in specific context subclasses.
			onRefresh();

			// Check for listener beans and register them.
			registerListeners();

			// Instantiate all remaining (non-lazy-init) singletons.
			finishBeanFactoryInitialization(beanFactory);

			// Last step: publish corresponding event.
			finishRefresh();
		}

		catch (BeansException ex) {
			// Destroy already created singletons to avoid dangling resources.
			destroyBeans();

			// Reset 'active' flag.
			cancelRefresh(ex);

			// Propagate exception to caller.
			throw ex;
		}
	}
}

结论

ApplicationContext初始化完成后，会把其余的配置信息抛弃掉