Spring MVC Servlet容器与Spring容器相关联

AbstractDispatcherServletInitializer的registerDispatcherServlet方法中（此方法被调用的时机在之前《Servlet 3.0集成Spring MVC》中有记录）

AbstractDispatcherServletInitializer
    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 + "]");

        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);
    }

其中创建了DispatcherServlet，添加到Servlet上下文中，并且setLoadOnStartup(1)，含义为在Tomcat容器启动时，会调用此DispatcherServlet的init方法。

DispatcherServlet关系图

DispatcherServlet的init方法从HttpServletBean继承而来，内部调用了FrameworkServlet实现的initServletBean方法中

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

        // Set bean properties from init parameters.
        try {
            PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
            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.
        initServletBean();

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

FrameworkServlet
    @Override
    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 {
            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");
        }
    }

关注其中的initWebApplicationContext方法

FrameworkServlet
    protected WebApplicationContext initWebApplicationContext() {
        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;
    }

关注其中的configureAndRefreshWebApplicationContext方法

FrameworkServlet
    protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac) {
        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
            if (this.contextId != null) {
                wac.setId(this.contextId);
            }
            else {
                // Generate default id...
                wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +
                        ObjectUtils.getDisplayString(getServletContext().getContextPath()) + '/' + getServletName());
            }
        }

        wac.setServletContext(getServletContext());
        wac.setServletConfig(getServletConfig());
        wac.setNamespace(getNamespace());
        wac.addApplicationListener(new SourceFilteringListener(wac, new ContextRefreshListener()));

        // 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(getServletContext(), getServletConfig());
        }

        postProcessWebApplicationContext(wac);
        applyInitializers(wac);
        wac.refresh();
    }

此处的wac为承载servlet的ioc容器（不是根ioc容器），其中wac.addApplicationListener(new SourceFilteringListener(wac, new ContextRefreshListener()))

AbstractApplicationContext
    protected void initApplicationEventMulticaster() {
        ConfigurableListableBeanFactory beanFactory = getBeanFactory();
        if (beanFactory.containsLocalBean(APPLICATION_EVENT_MULTICASTER_BEAN_NAME)) {
            this.applicationEventMulticaster =
                    beanFactory.getBean(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, ApplicationEventMulticaster.class);
            if (logger.isDebugEnabled()) {
                logger.debug("Using ApplicationEventMulticaster [" + this.applicationEventMulticaster + "]");
            }
        }
        else {
            this.applicationEventMulticaster = new SimpleApplicationEventMulticaster(beanFactory);
            beanFactory.registerSingleton(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, this.applicationEventMulticaster);
            if (logger.isDebugEnabled()) {
                logger.debug("Unable to locate ApplicationEventMulticaster with name '" +
                        APPLICATION_EVENT_MULTICASTER_BEAN_NAME +
                        "': using default [" + this.applicationEventMulticaster + "]");
            }
        }
    }
...
    @Override
    public void addApplicationListener(ApplicationListener<?> listener) {
        Assert.notNull(listener, "ApplicationListener must not be null");
        if (this.applicationEventMulticaster != null) {
            this.applicationEventMulticaster.addApplicationListener(listener);
        }
        else {
            this.applicationListeners.add(listener);
        }
    }

为容器添加监听器SourceFilteringListener，保存在事件监听器applicationEventMulticaster中。这个事件监听器是在Tomcat容器启动，ContextLoaderListener工作，根IOC容器refresh，方法initApplicationEventMulticaster执行时，创建的SimpleApplicationEventMulticaster。

SourceFilteringListener
    public SourceFilteringListener(Object source, ApplicationListener<?> delegate) {
        this.source = source;
        this.delegate = (delegate instanceof GenericApplicationListener ?
                (GenericApplicationListener) delegate : new GenericApplicationListenerAdapter(delegate));
    }

这里的委派就是传入的ContextRefreshListener，实际上又包装了一层变为GenericApplicationListenerAdapter，这是一个ApplicationListener ，在ioc容器refresh过程中会被调用到。
接着执行wac.refresh()，经典的refresh方法，属于Spring核心，这里只记录与SpringMvc相关的部分。
在refresh的finishRefresh方法中

AbstractApplicationContext
    protected void finishRefresh() {
        // Initialize lifecycle processor for this context.
        initLifecycleProcessor();

        // Propagate refresh to lifecycle processor first.
        getLifecycleProcessor().onRefresh();

        // Publish the final event.
        publishEvent(new ContextRefreshedEvent(this));

        // Participate in LiveBeansView MBean, if active.
        LiveBeansView.registerApplicationContext(this);
    }
...
    @Override
    public void publishEvent(ApplicationEvent event) {
        publishEvent(event, null);
    }
...
    protected void publishEvent(Object event, ResolvableType eventType) {
        Assert.notNull(event, "Event must not be null");
        if (logger.isTraceEnabled()) {
            logger.trace("Publishing event in " + getDisplayName() + ": " + event);
        }

        // Decorate event as an ApplicationEvent if necessary
        ApplicationEvent applicationEvent;
        if (event instanceof ApplicationEvent) {
            applicationEvent = (ApplicationEvent) event;
        }
        else {
            applicationEvent = new PayloadApplicationEvent<Object>(this, event);
            if (eventType == null) {
                eventType = ((PayloadApplicationEvent)applicationEvent).getResolvableType();
            }
        }

        // Multicast right now if possible - or lazily once the multicaster is initialized
        if (this.earlyApplicationEvents != null) {
            this.earlyApplicationEvents.add(applicationEvent);
        }
        else {
            getApplicationEventMulticaster().multicastEvent(applicationEvent, eventType);
        }

        // Publish event via parent context as well...
        if (this.parent != null) {
            if (this.parent instanceof AbstractApplicationContext) {
                ((AbstractApplicationContext) this.parent).publishEvent(event, eventType);
            }
            else {
                this.parent.publishEvent(event);
            }
        }
    }
...
    ApplicationEventMulticaster getApplicationEventMulticaster() throws IllegalStateException {
        if (this.applicationEventMulticaster == null) {
            throw new IllegalStateException("ApplicationEventMulticaster not initialized - " +
                    "call 'refresh' before multicasting events via the context: " + this);
        }
        return this.applicationEventMulticaster;
    }

关注publishEvent方法，最终在getApplicationEventMulticaster中获取到了上述保存了ioc容器监听器的事件广播器SimpleApplicationEventMulticaster，然后执行其multicastEvent方法。

SimpleApplicationEventMulticaster
    @Override
    public void multicastEvent(final ApplicationEvent event, ResolvableType eventType) {
        ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));
        for (final ApplicationListener<?> listener : getApplicationListeners(event, type)) {
            Executor executor = getTaskExecutor();
            if (executor != null) {
                executor.execute(new Runnable() {
                    @Override
                    public void run() {
                        invokeListener(listener, event);
                    }
                });
            }
            else {
                invokeListener(listener, event);
            }
        }
    }
...
    protected void invokeListener(ApplicationListener listener, ApplicationEvent event) {
        ErrorHandler errorHandler = getErrorHandler();
        if (errorHandler != null) {
            try {
                listener.onApplicationEvent(event);
            }
            catch (Throwable err) {
                errorHandler.handleError(err);
            }
        }
        else {
            try {
                listener.onApplicationEvent(event);
            }
            catch (ClassCastException ex) {
                String msg = ex.getMessage();
                if (msg == null || msg.startsWith(event.getClass().getName())) {
                    // Possibly a lambda-defined listener which we could not resolve the generic event type for
                    Log logger = LogFactory.getLog(getClass());
                    if (logger.isDebugEnabled()) {
                        logger.debug("Non-matching event type for listener: " + listener, ex);
                    }
                }
                else {
                    throw ex;
                }
            }
        }
    }

可以清楚的看到，这里调用了保存的监听器SourceFilteringListener的onApplicationEvent方法。

SourceFilteringListener
    @Override
    public void onApplicationEvent(ApplicationEvent event) {
        if (event.getSource() == this.source) {
            onApplicationEventInternal(event);
        }
    }
...
    protected void onApplicationEventInternal(ApplicationEvent event) {
        if (this.delegate == null) {
            throw new IllegalStateException(
                    "Must specify a delegate object or override the onApplicationEventInternal method");
        }
        this.delegate.onApplicationEvent(event);
    }

SourceFilteringListener内调用了上述保存的委派类GenericApplicationListenerAdapter的onApplicationEvent方法，实际还是调用了未包装前ContextRefreshListener的onApplicationEvent方法

GenericApplicationListenerAdapter
    @Override
    public void onApplicationEvent(ApplicationEvent event) {
        this.delegate.onApplicationEvent(event);
    }

    private class ContextRefreshListener implements ApplicationListener<ContextRefreshedEvent> {

        @Override
        public void onApplicationEvent(ContextRefreshedEvent event) {
            FrameworkServlet.this.onApplicationEvent(event);
        }
    }

可以看到，终于到了FrameworkServlet的onApplicationEvent方法

FrameworkServlet
    public void onApplicationEvent(ContextRefreshedEvent event) {
        this.refreshEventReceived = true;
        onRefresh(event.getApplicationContext());
    }

再到DispatcherServlet内的onRefresh方法

DispatcherServlet
    @Override
    protected void onRefresh(ApplicationContext context) {
        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);
    }

上面可以看到初始化SpringMvc的各个部件，挑initHandlerMappings方法来看

    private void initHandlerMappings(ApplicationContext context) {
        this.handlerMappings = null;

        if (this.detectAllHandlerMappings) {
            // Find all HandlerMappings in the ApplicationContext, including ancestor contexts.
            Map<String, HandlerMapping> matchingBeans =
                    BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerMapping.class, true, false);
            if (!matchingBeans.isEmpty()) {
                this.handlerMappings = new ArrayList<HandlerMapping>(matchingBeans.values());
                // We keep HandlerMappings in sorted order.
                AnnotationAwareOrderComparator.sort(this.handlerMappings);
            }
        }
        else {
            try {
                HandlerMapping hm = context.getBean(HANDLER_MAPPING_BEAN_NAME, HandlerMapping.class);
                this.handlerMappings = Collections.singletonList(hm);
            }
            catch (NoSuchBeanDefinitionException ex) {
                // Ignore, we'll add a default HandlerMapping later.
            }
        }

        // Ensure we have at least one HandlerMapping, by registering
        // a default HandlerMapping if no other mappings are found.
        if (this.handlerMappings == null) {
            this.handlerMappings = getDefaultStrategies(context, HandlerMapping.class);
            if (logger.isDebugEnabled()) {
                logger.debug("No HandlerMappings found in servlet '" + getServletName() + "': using default");
            }
        }
    }

这里的detectAllHandlerMappings为true，从容器中取出所有HandlerMapping类型的bean，排序后保存在DispatcherServlet的handlerMappings变量中（在《Spring MVC @EnableWebMvc源码笔记》中提到的RequestMappingHandlerMapping就是该类型的bean，会被保存在handlerMappings变量中）

总结：

• Tomcat启动，AbstractDispatcherServletInitializer的registerDispatcherServlet方法被调用
• 方法内部创建了DispatcherServlet，加入到Servlet上下文中并设置启动顺序1
• 由于设置了启动顺序，Tomcat启动时，会调用DispatcherServlet的init方法
• init方法执行过程中，调用了父类FrameworkServlet的configureAndRefreshWebApplicationContext方法
• 方法内创建监听器SourceFilteringListener，但实际上监听器内保存了ContextRefreshListener，子ioc容器（承载servlet的ioc容器，非根ioc容器）将创建的监听器保存在事件监听器applicationEventMulticaster中（这个事件监听器是在根IOC容器refresh，方法initApplicationEventMulticaster执行时创建的SimpleApplicationEventMulticaster，为AbstractApplicationContext的applicationEventMulticaster变量）
• 子ioc容器执行refresh动作
• refresh的finishRefresh过程中，执行广播事件，通过委派，会执行上面创建的SourceFilteringListener内保存的ContextRefreshListener的onApplicationEvent方法
• 实际上执行了FrameworkServlet的onApplicationEvent方法
• 最后在DispatcherServlet的initHandlerMappings方法内，完成了对SpringMvc各个部件的初始化