DispatcherServlet请求流程解析-initWebApplicationContext(二)

上面一篇文章提到，在Servlet初始化的时候，获取属性后调用initServletBean，这个方法会initWebApplicationContext，这是DispatcherServlet对后面的处理做了很多的预先准备工作，我们一起来看看它到底做了什么事情。

流程

1. 先上代码，初始化和发布WebApplicationContext到servlet中，对于真正的创建操作是交给子类的createWebApplicationContext来实现的。

/**
     * Initialize and publish the WebApplicationContext for this servlet.
     * <p>Delegates to {@link #createWebApplicationContext} for actual creation
     * of the context. Can be overridden in subclasses.
     * @return the WebApplicationContext instance
     * @see #FrameworkServlet(WebApplicationContext)
     * @see #setContextClass
     * @see #setContextConfigLocation
     */
    protected WebApplicationContext initWebApplicationContext() {
// 获取WebApplicationContext，但是首次取得的值一般为null
        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
            // 在当前的servlet中，没有对应的context实例时候，创建一个。
            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;
    }

• 首先获取WebApplicationContext，一般获取的值为null
• 然后试着findWebApplicationContext()，这一步主要看其父类的是否已经注册了对应的context. 父类的context首次也为null
• 如果WebApplicationContext还未找到，那么尝试创建一个WebApplicationContext

1. 上一步对于最初的操作，获得的WebapplicationContext都是null的，因此需要创建一个WebapplicationContext.

   为当前的Servlet实例化WebApplicationContext

/**
     * Instantiate the WebApplicationContext for this servlet, either a default
     * {@link org.springframework.web.context.support.XmlWebApplicationContext}
     * or a {@link #setContextClass custom context class}, if set.
     * <p>This implementation expects custom contexts to implement the
     * {@link org.springframework.web.context.ConfigurableWebApplicationContext}
     * interface. Can be overridden in subclasses.
     * <p>Do not forget to register this servlet instance as application listener on the
     * created context (for triggering its {@link #onRefresh callback}, and to call
     * {@link org.springframework.context.ConfigurableApplicationContext#refresh()}
     * before returning the context instance.
     * @param parent the parent ApplicationContext to use, or {@code null} if none
     * @return the WebApplicationContext for this servlet
     * @see org.springframework.web.context.support.XmlWebApplicationContext
     */
    protected WebApplicationContext createWebApplicationContext(ApplicationContext parent) {

       //获取默认的getContextClass，默认的contextClass为XmlWebApplicationContext.class，可以从源码中看到
        Class<?> contextClass = getContextClass();
        if (this.logger.isDebugEnabled()) {
            this.logger.debug("Servlet with name '" + getServletName() +
                    "' will try to create custom WebApplicationContext context of class '" +
                    contextClass.getName() + "'" + ", using parent context [" + parent + "]");
        }
        if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
            throw new ApplicationContextException(
                    "Fatal initialization error in servlet with name '" + getServletName() +
                    "': custom WebApplicationContext class [" + contextClass.getName() +
                    "] is not of type ConfigurableWebApplicationContext");
        }

        // 使用BeanUtils实例化ApplicationContext类。
        ConfigurableWebApplicationContext wac =
                (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);

        wac.setEnvironment(getEnvironment());
        wac.setParent(parent);
        wac.setConfigLocation(getContextConfigLocation());

        configureAndRefreshWebApplicationContext(wac);

        return wac;
    }

• 首先获取默认的要实例化的ApplicationContxt类，默认的为XmlWebApplicationContext
• 使用BeanUtil实例化ApplicationContxt的类，BeanUtils是个很有用的方法，可以多看下
• ApplicationContxt被实例化之后，做一些配置，设置当前的Environment,设置父类(如果有的话)，获取contextLocation的位置(web.xml中可以设置，当然也有默认值)
• 配置然后刷新WebApplicationContex类

1. 对WebApplicationContext类做配置和刷新。

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();
    }

• 首先生成WebApplicationContext的id，用于后面加载Spring-MVC的配置文件
• 在WebApplicationContext中设置容器的ServletContext,ServiletConfig，并设置默认的命名空间
• 添加ApplicationContext的监听器，监听ContextRefresh事件
• 初始化环境属性
• postProcessWebApplicationContext，初始化WebApplicationContext之后做的事情，默认为空，后面自行扩展的时候可以覆盖
• applyInitializers, 后面可以多了解下，初始器
• 刷新WebApplicationContext.

1. WebApplicationContext的刷新操作。

• 使用synchronized保证线程安全
• 下面每一个方法都包含一些具体的操作

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

            // Tell the subclass to refresh the internal bean factory.
            // 告诉子类刷新内部的bean factory
            ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

            // Prepare the bean factory for use in this context.
            // 准备beanfactory来使用
            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();
            }
        }
    }

1. 仔细看一下refresh中的每一个大方法，首先prepareRefresh，可以看到prepareRefresh，确实只做一些准备性的工作，没有实质性的处理。

protected void prepareRefresh() {
        this.startupDate = System.currentTimeMillis();
        this.closed.set(false);
        this.active.set(true);

        if (logger.isInfoEnabled()) {
            logger.info("Refreshing " + this);
        }

        // Initialize any placeholder property sources in the context environment
        initPropertySources();

        // Validate that all properties marked as required are resolvable
        // see ConfigurablePropertyResolver#setRequiredProperties
        getEnvironment().validateRequiredProperties();

        // Allow for the collection of early ApplicationEvents,
        // to be published once the multicaster is available...
        this.earlyApplicationEvents = new LinkedHashSet<ApplicationEvent>();
    }

1. obtainFreshBeanFactory()告诉子类刷新内部的beanfactory, 默认的beanFactory是DefaultListableBeanFactory。

obtainFreshBeanFactory中刷新BeanFactory的操作是交给其子类来实现的。AbstractRefreshableApplicationContext是真正做了刷新的操作，并且加载了beanDefinitions.

关于如何loadBeandefenition，后面可以再深入去看

/**
     * Tell the subclass to refresh the internal bean factory.
     * @return the fresh BeanFactory instance
     * @see #refreshBeanFactory()
     * @see #getBeanFactory()
     */
    protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
        refreshBeanFactory();
        ConfigurableListableBeanFactory beanFactory = getBeanFactory();
        if (logger.isDebugEnabled()) {
            logger.debug("Bean factory for " + getDisplayName() + ": " + beanFactory);
        }
        return beanFactory;
    }

/**
     * This implementation performs an actual refresh of this context's underlying
     * bean factory, shutting down the previous bean factory (if any) and
     * initializing a fresh bean factory for the next phase of the context's lifecycle.
     */
    @Override
    protected final void refreshBeanFactory() throws BeansException {
        if (hasBeanFactory()) {
            destroyBeans();
            closeBeanFactory();
        }
        try {
            DefaultListableBeanFactory beanFactory = createBeanFactory();
            beanFactory.setSerializationId(getId());
            customizeBeanFactory(beanFactory);

            //加载bean的定义
            loadBeanDefinitions(beanFactory);
            synchronized (this.beanFactoryMonitor) {
                this.beanFactory = beanFactory;
            }
        }
        catch (IOException ex) {
            throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
        }
    }

1. 让beanFactory准备好使用context. 可以看到beanFactory做了很多的配置

• 设置ClasLoader，SPEL,PropertyEditorRegistrar
• 添加ApplicationContextAware，用于让bean设置ApplicationContextde .等等
• 解析依赖关系ResolvableDependency相关的内容
• 一些需要早期的bean处理
• 注册一些系统默认的bean,如Environment,SystemProperties等等

/**
     * Configure the factory's standard context characteristics,
     * such as the context's ClassLoader and post-processors.
     * @param beanFactory the BeanFactory to configure
     */
    protected void prepareBeanFactory(ConfigurableListableBeanFactory beanFactory) {
        // Tell the internal bean factory to use the context's class loader etc.
        beanFactory.setBeanClassLoader(getClassLoader());
        beanFactory.setBeanExpressionResolver(new StandardBeanExpressionResolver(beanFactory.getBeanClassLoader()));
        beanFactory.addPropertyEditorRegistrar(new ResourceEditorRegistrar(this, getEnvironment()));

        // Configure the bean factory with context callbacks.
        beanFactory.addBeanPostProcessor(new ApplicationContextAwareProcessor(this));
        beanFactory.ignoreDependencyInterface(EnvironmentAware.class);
        beanFactory.ignoreDependencyInterface(EmbeddedValueResolverAware.class);
        beanFactory.ignoreDependencyInterface(ResourceLoaderAware.class);
        beanFactory.ignoreDependencyInterface(ApplicationEventPublisherAware.class);
        beanFactory.ignoreDependencyInterface(MessageSourceAware.class);
        beanFactory.ignoreDependencyInterface(ApplicationContextAware.class);

        // BeanFactory interface not registered as resolvable type in a plain factory.
        // MessageSource registered (and found for autowiring) as a bean.
        beanFactory.registerResolvableDependency(BeanFactory.class, beanFactory);
        beanFactory.registerResolvableDependency(ResourceLoader.class, this);
        beanFactory.registerResolvableDependency(ApplicationEventPublisher.class, this);
        beanFactory.registerResolvableDependency(ApplicationContext.class, this);

        // Register early post-processor for detecting inner beans as ApplicationListeners.
        beanFactory.addBeanPostProcessor(new ApplicationListenerDetector(this));

        // Detect a LoadTimeWeaver and prepare for weaving, if found.
        if (beanFactory.containsBean(LOAD_TIME_WEAVER_BEAN_NAME)) {
            beanFactory.addBeanPostProcessor(new LoadTimeWeaverAwareProcessor(beanFactory));
            // Set a temporary ClassLoader for type matching.
            beanFactory.setTempClassLoader(new ContextTypeMatchClassLoader(beanFactory.getBeanClassLoader()));
        }

        // Register default environment beans.
        if (!beanFactory.containsLocalBean(ENVIRONMENT_BEAN_NAME)) {
            beanFactory.registerSingleton(ENVIRONMENT_BEAN_NAME, getEnvironment());
        }
        if (!beanFactory.containsLocalBean(SYSTEM_PROPERTIES_BEAN_NAME)) {
            beanFactory.registerSingleton(SYSTEM_PROPERTIES_BEAN_NAME, getEnvironment().getSystemProperties());
        }
        if (!beanFactory.containsLocalBean(SYSTEM_ENVIRONMENT_BEAN_NAME)) {
            beanFactory.registerSingleton(SYSTEM_ENVIRONMENT_BEAN_NAME, getEnvironment().getSystemEnvironment());
        }
    }

1. beanFactory处理之后的处理，这一步可以理解为上一步的后续处理，和上一步类似都是对beanFactory的处理

/**
     * Register request/session scopes, a {@link ServletContextAwareProcessor}, etc.
     */
    @Override
    protected void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {
        beanFactory.addBeanPostProcessor(new ServletContextAwareProcessor(this.servletContext, this.servletConfig));
        beanFactory.ignoreDependencyInterface(ServletContextAware.class);
        beanFactory.ignoreDependencyInterface(ServletConfigAware.class);

        WebApplicationContextUtils.registerWebApplicationScopes(beanFactory, this.servletContext);
        WebApplicationContextUtils.registerEnvironmentBeans(beanFactory, this.servletContext, this.servletConfig);
    }

1. 激发上下文中创建bean的后续操作，这个内部包含了很多内容，大部分的操作在PostProcessorRegistrationDelegate类里面的invokeBeanFactoryPostProcessors中，一个方法写了100多行，内容很多，这里就粘贴出来了，东西太多。记得PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors

包括下一步的registerBeanPostProcessors操作也是由PostProcessorRegistrationDelegate来完成的。这个类看来也挺重要，不过它的访问权限是默认的，看来Spring不想暴露出来给我们用。

PostProcessorRegistrationDelegate

1. initMessageSource，操作，进入里面看到这些操作都是围绕着BeanFactory在进行，beanFactory设置DelegatingMessageSource。
   

2. 设置Application广播发出器，可以看到内部仍然是对beanFactory设置一个默认的类
   

3. onRefresh方法其实现类基本上都是在初始化主题，GenericWebApplicationContext与AbstractRefreshableWebApplicationContext都是如此
   

4. 注册监听器，看截图可以看出其主要在上面的事件caster中添加应用事件的bean

1. 看一下最后的操作吧，结束刷新操作，发布已经刷新后的事件。

    /**
     * Finish the refresh of this context, invoking the LifecycleProcessor's
     * onRefresh() method and publishing the
     * {@link org.springframework.context.event.ContextRefreshedEvent}.
     */
    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);
    }

1. 重置Spring中的cache

    /**
     * Reset Spring's common core caches, in particular the {@link ReflectionUtils},
     * {@link ResolvableType} and {@link CachedIntrospectionResults} caches.
     * @since 4.2
     * @see ReflectionUtils#clearCache()
     * @see ResolvableType#clearCache()
     * @see CachedIntrospectionResults#clearClassLoader(ClassLoader)
     */
    protected void resetCommonCaches() {
        ReflectionUtils.clearCache();
        ResolvableType.clearCache();
        CachedIntrospectionResults.clearClassLoader(getClassLoader());
    }

回顾

感觉Spring-mvc中的initWebApplicationContext做了很多的事情，先抓住大方向，细节很多，回顾一下这次看到了那些内容。

从initServletBean开始，然后initWebApplicationContext，初始情况下ApplicationContext是为null的，我们需要创建默认的ApplicationContext，Spring-mvc中默认的ApplicationContext是XmlWebApplicationContext。

使用反射创建XmlWebApplicationContext之后，将XmlWebApplicationContext作为参数传递到配置和刷新XmlWebApplicationContext的方法中，接下来的操作，集中在对XmlWebApplicationContext的配置中。

配置Enviriment,ServletConfig,初始属性，命名空间，过滤器等等。最后调用XmlWebApplicationContext的refresh操作。

refresh操作中主要围绕bean相关的内容在操作，创建beanFactory，然后对beanFactory做配置，同时加载bean，bean的生命周期等等操作。然后注册bean以外的内容，配置主题，事件，消息等等，最后结束刷新，完成整个bean的生命周期。

最后

这里只是initWebApplicationContext，感觉bean的生命周期处理也是在这一步内部配置的，后面有关bean的生命周期，可以在内部仔细看一下。

initWebApplicationContext中，涉及到很多的类，很多默认的类，多读几遍，也许将来自己造轮子的某一天可以参考Spring的设计。

本人才疏学浅，阅读Spring源码的过程，做了一些记录，供参考作用，如果感兴趣，想后续交流欢迎一起探讨。