Spring及SpringMVC初始化源码分析

首先SpringServletContainerInitializer会执行onStartup()方法（SpringServletContainerInitializer是实现ServletContainerInitializer接口且实现了Servlet3.0的@HandlesTypes(WebApplicationInitializer.class)）。

执行onStartup时，会把实现WebApplicationInitializer的接口按Set集合传入方法中。方法中会把符合判断的类加入到集合中。如果集合不为空，会执行WebApplicationInitializer.onStartup()方法，这里执行的是WebApplicationInitializer的实现类AbstractDispatcherServletInitializer的onStartup()，AbstractDispatcherServletInitializer中执行父类onStartup()方法。代码如下:

SpringServletContainerInitializer类

public void onStartup(Set> webAppInitializerClasses, ServletContext servletContext)
			throws ServletException {

		List initializers = new LinkedList();

		if (webAppInitializerClasses != null) {
			for (Class waiClass : webAppInitializerClasses) {
				// Be defensive: Some servlet containers provide us with invalid classes,
				// no matter what @HandlesTypes says...
				if (!waiClass.isInterface() && !Modifier.isAbstract(waiClass.getModifiers()) &&
						WebApplicationInitializer.class.isAssignableFrom(waiClass)) {
					try {
						initializers.add((WebApplicationInitializer) waiClass.newInstance());
					}
					catch (Throwable ex) {
						throw new ServletException("Failed to instantiate WebApplicationInitializer class", ex);
					}
				}
			}
		}

		if (initializers.isEmpty()) {
			servletContext.log("No Spring WebApplicationInitializer types detected on classpath");
			return;
		}

		servletContext.log(initializers.size() + " Spring WebApplicationInitializers detected on classpath");
		AnnotationAwareOrderComparator.sort(initializers);
		for (WebApplicationInitializer initializer : initializers) {
			//执行AbstractDispatcherServletInitializer的onStartup
			initializer.onStartup(servletContext);
		}
	}

AbstractDispatcherServletInitializer类

public void onStartup(ServletContext servletContext) throws ServletException {
		//调用父类方法
		super.onStartup(servletContext);
		//初始化SpringMVC容器并封装到FrameworkServlet中。
		registerDispatcherServlet(servletContext);
	}
protected void registerDispatcherServlet(ServletContext servletContext) {
		String servletName = getServletName();
		Assert.hasLength(servletName, "getServletName() must not return empty or null");

		WebApplicationContext servletAppContext = createServletApplicationContext();
		Assert.notNull(servletAppContext,
				"createServletApplicationContext() did not return an application " +
				"context for servlet [" + servletName + "]");
		//封装到WebApplicationContext	
		FrameworkServlet dispatcherServlet = createDispatcherServlet(servletAppContext);
		dispatcherServlet.setContextInitializers(getServletApplicationContextInitializers());

		ServletRegistration.Dynamic registration = servletContext.addServlet(servletName, dispatcherServlet);
		Assert.notNull(registration,
				"Failed to register servlet with name '" + servletName + "'." +
				"Check if there is another servlet registered under the same name.");

		registration.setLoadOnStartup(1);
		registration.addMapping(getServletMappings());
		registration.setAsyncSupported(isAsyncSupported());

		Filter[] filters = getServletFilters();
		if (!ObjectUtils.isEmpty(filters)) {
			for (Filter filter : filters) {
				registerServletFilter(servletContext, filter);
			}
		}

		customizeRegistration(registration);
	}

AbstractContextLoaderInitializer类

public void onStartup(ServletContext servletContext) throws ServletException {
		//初始化Spring容器
		registerContextLoaderListener(servletContext);
	}

在AbstractContextLoaderInitializer中，方法registerContextLoaderListener(ServletContext servletContext)会创建Spring的容器WebApplicationContext并封装到ContextLoader中。然后在ServletContext中注册监听器ContextLoaderListener。

AbstractContextLoaderInitializer类

protected void registerContextLoaderListener(ServletContext servletContext) {
		//初始化Spring的容器
		WebApplicationContext rootAppContext = createRootApplicationContext();
		if (rootAppContext != null) {
			//把Spring的容器封装到ContextLoader中。
			ContextLoaderListener listener = new ContextLoaderListener(rootAppContext);
			listener.setContextInitializers(getRootApplicationContextInitializers());
			在ServletContext中注册ContextLoaderListener监听器
			servletContext.addListener(listener);
		}
		else {
			logger.debug("No ContextLoaderListener registered, as " +
					"createRootApplicationContext() did not return an application context");
		}
	}

然后继续执行AbstractDispatcherServletInitializer中的registerDispatcherServlet(ServletContext servletContext)方法。代码如下：

AbstractDispatcherServletInitializer类

public void onStartup(ServletContext servletContext) throws ServletException {
		super.onStartup(servletContext);
		//初始化SpringMVC容器并封装到FrameworkServlet中。
		registerDispatcherServlet(servletContext);
	}
protected void registerDispatcherServlet(ServletContext servletContext) {
		String servletName = getServletName();
		Assert.hasLength(servletName, "getServletName() must not return empty or null");
		//初始化SpringMVC容器并
		WebApplicationContext servletAppContext = createServletApplicationContext();
		Assert.notNull(servletAppContext,
				"createServletApplicationContext() did not return an application " +
				"context for servlet [" + servletName + "]");
		//封装WebApplicationContext到FrameworkServlet中。
		FrameworkServlet dispatcherServlet = createDispatcherServlet(servletAppContext);
		dispatcherServlet.setContextInitializers(getServletApplicationContextInitializers());

		ServletRegistration.Dynamic registration = servletContext.addServlet(servletName, dispatcherServlet);
		Assert.notNull(registration,
				"Failed to register servlet with name '" + servletName + "'." +
				"Check if there is another servlet registered under the same name.");

		registration.setLoadOnStartup(1);
		registration.addMapping(getServletMappings());
		registration.setAsyncSupported(isAsyncSupported());

		Filter[] filters = getServletFilters();
		if (!ObjectUtils.isEmpty(filters)) {
			for (Filter filter : filters) {
				registerServletFilter(servletContext, filter);
			}
		}

		customizeRegistration(registration);
	}

在registerDispatcherServlet(ServletContext servletContext)方法中创建SpringMVC的容器servletAppContext并把些容器封装到FrameworkServlet类中。方便后继在SpringMVC中使用。

Servlet容器启动时，会初始化ContextLoaderListener监听器的contextInitialized()方法。contextInitialized()方法中会取得ServletContextEvent事件并调用父类ContextLoader中的initWebApplicationContext()方法。在类中configureAndRefreshWebApplicationContext()方法会刷新容器。最后把Spring的容器WebApplicationContext保存到ServletContext及本地缓存中（currentContextPerThread）。代码如下:

ContextLoaderListener类

public void contextInitialized(ServletContextEvent event) {
		//执行父类中的initWebApplicationContext
		initWebApplicationContext(event.getServletContext());
	}

ContextLoader类

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);
					}
					//刷新容器，初始化Spring中的所有的Bean
					configureAndRefreshWebApplicationContext(cwac, servletContext);
				}
			}
			//ServletContext保存Spring容器WebApplicationContext.
			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) {
				//保存到本地缓存
				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;
		}
	}

初始化SpringMVC
Servlet容器会执行HttpServletBean(DispatcherServlet的父类，HttpServlet的子类)的初始化init()方法。在init()方法中会调用initServletBean()（在HttpServletBean子类FrameworkServlet类中实现）方法。在initServletBean()方法中有一个重要的方法initWebApplicationContext(),此方法是设置Spring容器和SpringMVC容器父子关系的重要方法。在此方法中，会在ServletContext中取出Spring容器把它设置为SpringMVC的父容器。最后刷新SpringMVC容器。代码如下:

HttpServletBean类

public final void init() throws ServletException {
		if (logger.isDebugEnabled()) {
			logger.debug("Initializing servlet '" + getServletName() + "'");
		}

		// Set bean properties from init parameters.
		PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
		if (!pvs.isEmpty()) {
			try {
				BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
				ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());
				bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment()));
				initBeanWrapper(bw);
				bw.setPropertyValues(pvs, true);
			}
			catch (BeansException ex) {
				if (logger.isErrorEnabled()) {
					logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex);
				}
				throw ex;
			}
		}

		// Let subclasses do whatever initialization they like.
		//调用子类FrameworkServlet中的initServletBean()方法
		initServletBean();

		if (logger.isDebugEnabled()) {
			logger.debug("Servlet '" + getServletName() + "' configured successfully");
		}
	}

FrameworkServlet类

protected final void initServletBean() throws ServletException {
		getServletContext().log("Initializing Spring FrameworkServlet '" + getServletName() + "'");
		if (this.logger.isInfoEnabled()) {
			this.logger.info("FrameworkServlet '" + getServletName() + "': initialization started");
		}
		long startTime = System.currentTimeMillis();

		try {
			//设置Spring容器为SpringMVC的父容器
			this.webApplicationContext = initWebApplicationContext();
			initFrameworkServlet();
		}
		catch (ServletException ex) {
			this.logger.error("Context initialization failed", ex);
			throw ex;
		}
		catch (RuntimeException ex) {
			this.logger.error("Context initialization failed", ex);
			throw ex;
		}

		if (this.logger.isInfoEnabled()) {
			long elapsedTime = System.currentTimeMillis() - startTime;
			this.logger.info("FrameworkServlet '" + getServletName() + "': initialization completed in " +
					elapsedTime + " ms");
		}
	}

protected WebApplicationContext initWebApplicationContext() {
		//在ServletContext中取出Spring容器
		WebApplicationContext rootContext =
				WebApplicationContextUtils.getWebApplicationContext(getServletContext());
		WebApplicationContext wac = null;

		if (this.webApplicationContext != null) {
			// A context instance was injected at construction time -> use it
			//SpringMVC容器，
			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
						//设置Spring容器为SpringMVC容器的父容器
						cwac.setParent(rootContext);
					}
					//刷新SpringMVC容器，初始化SpringMVC容器的Bean
					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.
			//初始化DispatcherServlet中的各种Bean，设置各种配置，为后继的请求所用。
			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;
	}

DispatcherServlet类

protected void onRefresh(ApplicationContext context) {
		//初始化DispatcherServlet中的各种Bean，设置各种配置，为后继的请求所用。
		initStrategies(context);
	}

protected void initStrategies(ApplicationContext context) {
		initMultipartResolver(context);
		initLocaleResolver(context);
		initThemeResolver(context);
		initHandlerMappings(context);
		initHandlerAdapters(context);
		initHandlerExceptionResolvers(context);
		initRequestToViewNameTranslator(context);
		initViewResolvers(context);
		initFlashMapManager(context);
	}

以上为Spring,SpringMVC初始化的流程及相关代码。