研究IOC首先创建一个简单的web项目，在web.xml中我们都会加上这么一句
<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>
这代表了web容器启动的时候会首先进入ContextLoaderListener这个类，并且之后会去加载classpath下的applicationContext.xml文件。那么重点就在ContextLoaderListener上，点开源码：
/** * Initialize the root web application context. */ @Override public void contextInitialized(ServletContextEvent event) { initWebApplicationContext(event.getServletContext()); } /** * Close the root web application context. */ @Override public void contextDestroyed(ServletContextEvent event) { closeWebApplicationContext(event.getServletContext()); ContextCleanupListener.cleanupAttributes(event.getServletContext()); }
里面主要为ServletContextListener接口的两个实现方法。web容器会首先调用contextInitialized方法，传入tomcat封装的容器资源，之后调用父类的初始化容器方法。
/** * The root WebApplicationContext instance that this loader manages. */ private WebApplicationContext context; public WebApplicationContext initWebApplicationContext(ServletContext servletContext) { ...........省略// 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); } } 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); } ......省略 return this.context; }
这个方法里主要步骤createWebApplicationContext方法用来创建XmlWebApplicationContext这个root根容器，这个容器就是取自servletContextEvent。
loadParentContext方法用来加载父容器。主要方法configureAndRefreshWebApplicationContext用来配置和刷新root容器，在方法内最主要的就是refresh方法，里面实现了最主要的功能。
@Override 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(); // Prepare the bean factory for use in this context. prepareBeanFactory(beanFactory); try { // Allows post-processing of the bean factory in context subclasses. postProcessBeanFactory(beanFactory); // 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) { logger.warn("Exception encountered during context initialization - cancelling refresh attempt", ex); // Destroy already created singletons to avoid dangling resources. destroyBeans(); // Reset 'active' flag. cancelRefresh(ex); // Propagate exception to caller. throw ex; } } }
prepareRefresh方法用来准备之后需要用到的环境。
obtainFreshBeanFactory方法获取beanFactory