Spring启动流程源码解析

<!-- Spring 启动配置文件 -->
    <context-param>
		<param-name>contextConfigLocation</param-name>
		<param-value>classpath:spring-mybatis.xml</param-value>
	</context-param>
<!-- Spring 启动监听器 -->
	<listener>
		<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
	</listener>

<!-- Spring MVC servlet -->
	<servlet>
		<servlet-name>SpringMVC</servlet-name>
		<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
		<init-param>
			<param-name>contextConfigLocation</param-name>
			<param-value>classpath:spring-mvc.xml</param-value>
		</init-param>
		<load-on-startup>1</load-on-startup>
		<async-supported>true</async-supported>
	</servlet>

public class ContextLoaderListener extends ContextLoader implements ServletContextListener {
 /**
  * Initialize the root web application context.
  */
 @Override
 public void contextInitialized(ServletContextEvent event) {
 	initWebApplicationContext(event.getServletContext());
 }
}

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!");
   }
   
   ······
   
   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);
   		}
   	}
   	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);
   	}

   	······
   	
   	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;
   }
}

protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac, ServletContext sc) {
	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
		String idParam = sc.getInitParameter(CONTEXT_ID_PARAM);
		if (idParam != null) {
			wac.setId(idParam);
		}
		else {
			// Generate default id...
			wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +
					ObjectUtils.getDisplayString(sc.getContextPath()));
		}
	}

	wac.setServletContext(sc);
	// 获取 contextConfigLocation 配置文件
	String configLocationParam = sc.getInitParameter(CONFIG_LOCATION_PARAM);
	if (configLocationParam != null) {
		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
	ConfigurableEnvironment env = wac.getEnvironment();
	if (env instanceof ConfigurableWebEnvironment) {
		((ConfigurableWebEnvironment) env).initPropertySources(sc, null);
	}

	customizeContext(sc, wac);
	wac.refresh(); // Spring IOC 创建 Bean
}

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) {
			if (logger.isWarnEnabled()) {
				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;
		}

		finally {
			// Reset common introspection caches in Spring's core, since we
			// might not ever need metadata for singleton beans anymore...
			resetCommonCaches();
		}
	}
}

GenericServlet#init()->HttpServletBean#init()->FrameworkServlet#initServletBean()->initWebApplicationContext()

protected WebApplicationContext initWebApplicationContext() {
		// 获取 root context
		WebApplicationContext rootContext =
				WebApplicationContextUtils.getWebApplicationContext(getServletContext());
		WebApplicationContext wac = null;

		if (this.webApplicationContext != null) {
			// A context instance was injected at construction time -> use it
			wac = this.webApplicationContext;
			if (wac instanceof ConfigurableWebApplicationContext) {
				ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) wac;
				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 -> set
						// the root application context (if any; may be null) as the parent
						cwac.setParent(rootContext);
					}
					configureAndRefreshWebApplicationContext(cwac);
				}
			}
		}
		if (wac == null) {
			// No context instance was injected at construction time -> see if one
			// has been registered in the servlet context. If one exists, it is assumed
			// that the parent context (if any) has already been set and that the
			// user has performed any initialization such as setting the context id
			wac = findWebApplicationContext();
		}
		if (wac == null) {
			// No context instance is defined for this servlet -> create a local one
			wac = createWebApplicationContext(rootContext);
		}

		if (!this.refreshEventReceived) {
			// Either the context is not a ConfigurableApplicationContext with refresh
			// support or the context injected at construction time had already been
			// refreshed -> trigger initial onRefresh manually here.
			onRefresh(wac);
		}

		if (this.publishContext) {
			// Publish the context as a servlet context attribute.
			String attrName = getServletContextAttributeName();
			getServletContext().setAttribute(attrName, wac);
			if (this.logger.isDebugEnabled()) {
				this.logger.debug("Published WebApplicationContext of servlet '" + getServletName() +
						"' as ServletContext attribute with name [" + attrName + "]");
			}
		}

		return wac;
	}

protected Class<?> determineContextClass(ServletContext servletContext) {
    // 如果在 web.xml 中直接指定了 ContextClass
 	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 { 
 	// 没有直接指定，则读取属性文件 ContextLoader.properties 的配置
 	/** org.springframework.web.context.WebApplicationContext=
     org.springframework.web.context.support.XmlWebApplicationContext
     */
 		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);
 		}
 	}
 }