interface21 - web - ContextLoaderListener（Spring Web Application Context加载流程）

前言

最近打算花点时间好好看看spring的源码，然而现在Spring的源码经过迭代的版本太多了，比较庞大，看起来比较累，所以准备从最初的版本（interface21）开始入手，仅用于学习，理解其设计思想，后续慢慢研究其每次版本变更的内容。。。

先从interface21的一个典型web工程例子看起，宠物诊所 - petclinic，因为该工程基本涵盖了Spring的APO、IOC、JDBC、Web MVC、事务、国际化、主题切换、参数校验等主要功能。。。

继上一篇，了解完Log4jConfigListener（加载Log4j日志）的流程后，看看ContextLoaderListener（加载Spring Web Application Context）流程是如何的~~~~~~~

对应的web.xml配置

    <listener>
        <listener-class>com.interface21.web.context.ContextLoaderListener</listener-class>
    </listener>

执行时序图（看不清的话可以点击查看原图）

时序图中的各个步骤简要分析

执行的入口在ContextLoaderListener类的contextInitialized方法，由于ContextLoaderListener类实现了ServletContextListener接口，所以在Servlet容器（tomcat）启动时，会自动调用contextInitialized方法。

步骤描述：

1. 进入ContextLoaderListener类的contextInitialized方法，该类只有一句代码，执行ContextLoader.initContext(event.getServletContext())方法；

   public void contextInitialized(ServletContextEvent event) {
           ContextLoader.initContext(event.getServletContext());
       }

2. 进入ContextLoader类的initContext方法，首先，从servletContext中获取contextClass参数，如果配置了该参数，则创建该实例对象，否则创建默认的XmlWebApplicationContext实例对象，接下来调用XmlWebApplicationContext的setServletContext方法；

   public static WebApplicationContext initContext(ServletContext servletContext) throws ApplicationContextException {
           servletContext.log("Loading root WebApplicationContext");
           String contextClass = servletContext.getInitParameter(CONTEXT_CLASS_PARAM);
   
           // Now we must load the WebApplicationContext.
           // It configures itself: all we need to do is construct the class with a no-arg
           // constructor, and invoke setServletContext.
           try {
               Class clazz = (contextClass != null ? Class.forName(contextClass) : DEFAULT_CONTEXT_CLASS);
               logger.info("Loading root WebApplicationContext: using context class '" + clazz.getName() + "'");
   
               if (!WebApplicationContext.class.isAssignableFrom(clazz)) {
                   throw new ApplicationContextException("Context class is no WebApplicationContext: " + contextClass);
               }
   
               WebApplicationContext webApplicationContext = (WebApplicationContext) clazz.newInstance();
               webApplicationContext.setServletContext(servletContext);
               return webApplicationContext;
   
           } catch (ApplicationContextException ex) {
               handleException("Failed to initialize application context", ex);
   
           } catch (BeansException ex) {
               handleException("Failed to initialize beans in application context", ex);
   
           } catch (ClassNotFoundException ex) {
               handleException("Failed to load config class '" + contextClass + "'", ex);
   
           } catch (InstantiationException ex) {
               handleException("Failed to instantiate config class '" + contextClass + "': does it have a public no arg constructor?", ex);
   
           } catch (IllegalAccessException ex) {
               handleException("Illegal access while finding or instantiating config class '" + contextClass + "': does it have a public no arg constructor?", ex);
   
           } catch (Throwable ex) {
               handleException("Unexpected error loading context configuration", ex);
           }
   
           return null;
       }

3. 进入XmlWebApplicationContext类的setServletContext方法，首先，调用initConfigLocation方法从servletContext中获取contextConfigLocation参数（Spring Application配置文件），如果没配置该参数，则默认获取/WEB-INF/applicationContext.xml该文件；

       public void setServletContext(ServletContext servletContext) throws ApplicationContextException {
           this.servletContext = servletContext;
           this.configLocation = initConfigLocation();
           logger.info("Using config location '" + this.configLocation + "'");
           refresh();
   
           if (this.namespace == null) {
               // We're the root context
               WebApplicationContextUtils.publishConfigObjects(this);
               // Expose as a ServletContext object
               WebApplicationContextUtils.publishWebApplicationContext(this);
           }
       }

4. 迎来了非常关键的一步操作，调用AbstractApplicationContext类的refresh()方法，该方法具体如下，每个阶段的英文注释已经比较清晰了，下面步骤也会做个详细描述：

       public final void refresh() throws ApplicationContextException {
           if (this.contextOptions != null && !this.contextOptions.isReloadable())
               throw new ApplicationContextException("Forbidden to reload config");
   
           this.startupTime = System.currentTimeMillis();
   
           refreshBeanFactory();
   
           if (getBeanDefinitionCount() == 0)
               logger.warn("No beans defined in ApplicationContext [" + getDisplayName() + "]");
           else
               logger.info(getBeanDefinitionCount() + " beans defined in ApplicationContext [" + getDisplayName() + "]");
   
           // invoke configurers that can override values in the bean definitions
           invokeContextConfigurers();
   
           // load options bean for this context
           loadOptions();
   
           // initialize message source for this context
           initMessageSource();
   
           // initialize other special beans in specific context subclasses
           onRefresh();
   
           // check for listener beans and register them
           refreshListeners();
   
           // instantiate singletons this late to allow them to access the message source
           preInstantiateSingletons();
   
           // last step: publish respective event
           publishEvent(new ContextRefreshedEvent(this));
       }

5. 首先，调用AbstractXmlApplicationContext类的refreshBeanFactory方法，该方法如下，具体完成的操作内容下面步骤会详细描述：

     protected void refreshBeanFactory() throws ApplicationContextException {
           String identifier = "application context with display name [" + getDisplayName() + "]";
           InputStream is = null;
           try {
               // Supports remote as well as local URLs
               is = getInputStreamForBeanFactory();
               this.xmlBeanFactory = new XmlBeanFactory(getParent());
               this.xmlBeanFactory.setEntityResolver(new ResourceBaseEntityResolver(this));
               this.xmlBeanFactory.loadBeanDefinitions(is);
               if (logger.isInfoEnabled()) {
                   logger.info("BeanFactory for application context: " + this.xmlBeanFactory);
               }
           } catch (IOException ex) {
               throw new ApplicationContextException("IOException parsing XML document for " + identifier, ex);
           } catch (NoSuchBeanDefinitionException ex) {
               throw new ApplicationContextException("Cannot load configuration: missing bean definition [" + ex.getBeanName() + "]", ex);
           } catch (BeansException ex) {
               throw new ApplicationContextException("Cannot load configuration: problem instantiating or initializing beans", ex);
           } finally {
               try {
                   if (is != null)
                       is.close();
               } catch (IOException ex) {
                   throw new ApplicationContextException("IOException closing stream for XML document for " + identifier, ex);
               }
           }
       }

6. 调用XmlWebApplicationContext类的getInputStreamForBeanFactory方法，读取阶段3获取到的配置文件为输入流InputStream

       protected InputStream getInputStreamForBeanFactory() throws IOException {
           InputStream in = getResourceAsStream(this.configLocation);
           if (in == null) {
               throw new FileNotFoundException("Config location not found: " + this.configLocation);
           }
           return in;
       }

7. 返回配置文件输入流InputStream
8. 回到AbstractXmlApplicationContext的refreshBeanFactory方法，new出一个XmlBeanFactory对象
9. 设置xmlBeanFactory.setEntityResolver，这里的EntityResolver主要用于寻找DTD声明
10. 调用xmlBeanFactory的loadBeanDefinitions方法加载bean定义声明
11. 进入xmlBeanFactory类的loadBeanDefinitions方法，解析读取的配置文件流InputStream为org.w3c.dom.Document对象，然后调用loadBeanDefinitions方法依次解析各个bean元素节点信息

        public void loadBeanDefinitions(InputStream is) throws BeansException {
            if (is == null)
                throw new BeanDefinitionStoreException("InputStream cannot be null: expected an XML file", null);
    
            try {
                logger.info("Loading XmlBeanFactory from InputStream [" + is + "]");
                DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
                logger.debug("Using JAXP implementation [" + factory + "]");
                factory.setValidating(true);
                DocumentBuilder db = factory.newDocumentBuilder();
                db.setErrorHandler(new BeansErrorHandler());
                db.setEntityResolver(this.entityResolver != null ? this.entityResolver : new BeansDtdResolver());
                Document doc = db.parse(is);
                loadBeanDefinitions(doc);
            } catch (ParserConfigurationException ex) {
                throw new BeanDefinitionStoreException("ParserConfiguration exception parsing XML", ex);
            } catch (SAXException ex) {
                throw new BeanDefinitionStoreException("XML document is invalid", ex);
            } catch (IOException ex) {
                throw new BeanDefinitionStoreException("IOException parsing XML document", ex);
            } finally {
                try {
                    if (is != null)
                        is.close();
                } catch (IOException ex) {
                    throw new FatalBeanException("IOException closing stream for XML document", ex);
                }
            }
        }

12. 寻找Document中声明为bean的Element节点，依次解析

        public void loadBeanDefinitions(Document doc) throws BeansException {
            Element root = doc.getDocumentElement();
            logger.debug("Loading bean definitions");
            NodeList nl = root.getElementsByTagName(BEAN_ELEMENT);
            logger.debug("Found " + nl.getLength() + " <" + BEAN_ELEMENT + "> elements defining beans");
            for (int i = 0; i < nl.getLength(); i++) {
                Node n = nl.item(i);
                loadBeanDefinition((Element) n);
            }
        }

13. 解析Element节点内容，获取class声明信息、PropertyValues等信息，封装成AbstractBeanDefinition对象，添加到beanDefinitionMap中

        private void loadBeanDefinition(Element el) throws BeansException {
            // The DTD guarantees an id attribute is present
            String id = el.getAttribute(ID_ATTRIBUTE);
            logger.debug("Parsing bean definition with id '" + id + "'");
    
            // Create BeanDefinition now: we'll build up PropertyValues later
            AbstractBeanDefinition beanDefinition;
    
            PropertyValues pvs = getPropertyValueSubElements(el);
            beanDefinition = parseBeanDefinition(el, id, pvs);
            registerBeanDefinition(id, beanDefinition);
    
            String name = el.getAttribute(NAME_ATTRIBUTE);
            if (name != null && !"".equals(name)) {
                // Automatically create this alias. Used for
                // names that aren't legal in id attributes
                registerAlias(id, name);
            }
        }

14. 判断是否需要注册alias，放到aliasMap中，实际上就是维护了bean的name和id关系
15. 返回到AbstractXmlApplicationContext类refreshBeanFactory方法中
16. 返回到AbstractApplicationContext类refresh方法中
17. 执行AbstractApplicationContext的invokeContextConfigurers方法，实际上内部是执行所有实现了BeanFactoryPostProcessor接口的bean的postProcessBeanFactory方法

        private void invokeContextConfigurers() {
            String[] beanNames = getBeanDefinitionNames(BeanFactoryPostProcessor.class);
            for (int i = 0; i < beanNames.length; i++) {
                String beanName = beanNames[i];
                BeanFactoryPostProcessor configurer = (BeanFactoryPostProcessor) getBean(beanName);
                configurer.postProcessBeanFactory(getBeanFactory());
            }
        }

18. 执行AbstractApplicationContext的loadOptions方法，获取contextOptions bean，首先，查看配置文件是否已经配置contextOptions bean,没有则自己创建一个new ContextOptions()对象，主要用于当应用运行时，是否可以重新加载该配置，如果配置成false的话，会在调用refresh方法时，抛出一个ApplicationContextException("Forbidden to reload config")异常；

        private void loadOptions() {
            try {
                this.contextOptions = (ContextOptions) getBean(OPTIONS_BEAN_NAME);
            } catch (NoSuchBeanDefinitionException ex) {
                logger.info("No options bean (\"" + OPTIONS_BEAN_NAME + "\") found: using default");
                this.contextOptions = new ContextOptions();
            }
        }

19. 执行AbstractApplicationContext的initMessageSource方法，获取messageSource bean,首先，查看配置文件是否已经配置messageSource bean,没有则自己创建一个StaticMessageSource对象，注意如果Parent context不为null的话，需要设置Parent MessageSource

        private void initMessageSource() {
            try {
                this.messageSource = (MessageSource) getBean(MESSAGE_SOURCE_BEAN_NAME);
                // set parent message source if applicable,
                // and if the message source is defined in this context, not in a parent
                if (this.parent != null && (this.messageSource instanceof NestingMessageSource) &&
                        Arrays.asList(getBeanDefinitionNames()).contains(MESSAGE_SOURCE_BEAN_NAME)) {
                    ((NestingMessageSource) this.messageSource).setParent(this.parent);
                }
            } catch (NoSuchBeanDefinitionException ex) {
                logger.info("No MessageSource found for [" + getDisplayName() + "]: using empty StaticMessageSource");
                // use empty message source to be able to accept getMessage calls
                this.messageSource = new StaticMessageSource();
            }
        }

20. 执行AbstractXmlUiApplicationContext的onRefresh方法，获取themeSource bean, 主题相关（如应用可配置暗色主题或亮色主题功能），同样，这里也首先查看配置文件是否已经配置themeSource bean,没有则自己创建一个ResourceBundleThemeSource对象，注意这里还需要根据判断条件设置Parent ThemeSource

        protected void onRefresh() {
            this.themeSource = UiApplicationContextUtils.initThemeSource(this);
        }

        public static ThemeSource initThemeSource(ApplicationContext applicationContext) {
            ThemeSource themeSource;
            try {
                themeSource = (ThemeSource) applicationContext.getBean(THEME_SOURCE_BEAN_NAME);
                // set parent theme source if applicable,
                // and if the theme source is defined in this context, not in a parent
                if (applicationContext.getParent() instanceof ThemeSource && themeSource instanceof NestingThemeSource &&
                        Arrays.asList(applicationContext.getBeanDefinitionNames()).contains(THEME_SOURCE_BEAN_NAME)) {
                    ((NestingThemeSource) themeSource).setParent((ThemeSource) applicationContext.getParent());
                }
            } catch (NoSuchBeanDefinitionException ex) {
                logger.info("No ThemeSource found for [" + applicationContext.getDisplayName() + "]: using ResourceBundleThemeSource");
                themeSource = new ResourceBundleThemeSource();
            }
            return themeSource;
        }

21. 执行AbstractApplicationContext的refreshListeners方法，寻找所有ApplicationListener bean,将其放到ApplicationEventMulticaster对象的Set集合中

        private void refreshListeners() {
            logger.info("Refreshing listeners");
            List listeners = BeanFactoryUtils.beansOfType(ApplicationListener.class, this);
            logger.debug("Found " + listeners.size() + " listeners in bean factory");
            for (int i = 0; i < listeners.size(); i++) {
                ApplicationListener listener = (ApplicationListener) listeners.get(i);
                addListener(listener);
                logger.info("Bean listener added: [" + listener + "]");
            }
        }

22. 执行AbstractApplicationContext的preInstantiateSingletons方法，创建单例的bean实例，创建bean对象是在调用getBean方法时创建的，具体创建逻辑在getSharedInstance方法里；另外，对实现了ApplicationContextAware接口的bean，会调用对应的接口setApplicationContext方法，这里涉及的细节比较多，后续有时间可以具体详细分析；

        private void preInstantiateSingletons() {
            logger.info("Configuring singleton beans in context");
            String[] beanNames = getBeanDefinitionNames();
            logger.debug("Found " + beanNames.length + " listeners in bean factory: names=[" +
                    StringUtils.arrayToDelimitedString(beanNames, ",") + "]");
            for (int i = 0; i < beanNames.length; i++) {
                String beanName = beanNames[i];
                if (isSingleton(beanName)) {
                    getBean(beanName);
                }
            }
        }

        public Object getBean(String name) throws BeansException {
            Object bean = getBeanFactory().getBean(name);
            configureManagedObject(name, bean);
            return bean;
        }

        private final synchronized Object getSharedInstance(String pname, Map newlyCreatedBeans) throws BeansException {
            // Get rid of the dereference prefix if there is one
            String name = transformedBeanName(pname);
    
            Object beanInstance = this.singletonCache.get(name);
            if (beanInstance == null) {
                logger.info("Creating shared instance of singleton bean '" + name + "'");
                beanInstance = createBean(name, newlyCreatedBeans);
                this.singletonCache.put(name, beanInstance);
            } else {
                if (logger.isDebugEnabled())
                    logger.debug("Returning cached instance of singleton bean '" + name + "'");
            }
    
            // Don't let calling code try to dereference the
            // bean factory if the bean isn't a factory
            if (isFactoryDereference(pname) && !(beanInstance instanceof FactoryBean)) {
                throw new BeanIsNotAFactoryException(name, beanInstance);
            }
    
            // Now we have the beanInstance, which may be a normal bean
            // or a FactoryBean. If it's a FactoryBean, we use it to
            // create a bean instance, unless the caller actually wants
            // a reference to the factory.
            if (beanInstance instanceof FactoryBean) {
                if (!isFactoryDereference(pname)) {
                    // Configure and return new bean instance from factory
                    FactoryBean factory = (FactoryBean) beanInstance;
                    logger.debug("Bean with name '" + name + "' is a factory bean");
                    beanInstance = factory.getObject();
    
                    // Set pass-through properties
                    if (factory.getPropertyValues() != null) {
                        logger.debug("Applying pass-through properties to bean with name '" + name + "'");
                        new BeanWrapperImpl(beanInstance).setPropertyValues(factory.getPropertyValues());
                    }
                    // Initialization is really up to factory
                    //invokeInitializerIfNecessary(beanInstance);
                } else {
                    // The user wants the factory itself
                    logger.debug("Calling code asked for BeanFactory instance for name '" + name + "'");
                }
            }    // if we're dealing with a factory bean
    
            return beanInstance;
        }

        private void configureManagedObject(String name, Object bean) {
            if (bean instanceof ApplicationContextAware &&
                    (!isSingleton(name) || !this.managedSingletons.contains(bean))) {
                logger.debug("Setting application context on ApplicationContextAware object [" + bean + "]");
                ApplicationContextAware aca = (ApplicationContextAware) bean;
                aca.setApplicationContext(this);
                this.managedSingletons.add(bean);
            }
        }

23. 执行AbstractApplicationContext的publishEvent方法，发布ContextRefreshedEvent事件，如果parent不为空，一起发布，内部的逻辑是执行对应eventListeners的onApplicationEvent方法

        public final void publishEvent(ApplicationEvent event) {
            if (logger.isDebugEnabled()) {
                logger.debug("Publishing event in context [" + getDisplayName() + "]: " + event.toString());
            }
            this.eventMulticaster.onApplicationEvent(event);
            if (this.parent != null) {
                parent.publishEvent(event);
            }
        }

24. 回到XmlWebApplicationContext类
25. 执行WebApplicationContextUtils.publishConfigObjects方法，寻找所有config bean，将其设置到ServletContext的属性中

        public static void publishConfigObjects(WebApplicationContext wac) throws ApplicationContextException {
            logger.info("Configuring config objects");
            String[] beanNames = wac.getBeanDefinitionNames();
            for (int i = 0; i < beanNames.length; i++) {
                String name = beanNames[i];
                if (name.startsWith(CONFIG_OBJECT_PREFIX)) {
                    // Strip prefix
                    String strippedName = name.substring(CONFIG_OBJECT_PREFIX.length());
                    try {
                        Object configObject = wac.getBean(name);
                        wac.getServletContext().setAttribute(strippedName, configObject);
                        logger.info("Config object with name [" + name + "] and class [" + configObject.getClass().getName() +
                                "] initialized and added to ServletConfig");
                    } catch (BeansException ex) {
                        throw new ApplicationContextException("Couldn't load config object with name '" + name + "': " + ex, ex);
                    }
                }
            }
        }

26. 执行WebApplicationContextUtils.publishWebApplicationContext,将WebApplicationContext设置到ServletContext属性中

        public static void publishWebApplicationContext(WebApplicationContext wac) {
            // Set WebApplicationContext as an attribute in the ServletContext so
            // other components in this web application can access it
            ServletContext sc = wac.getServletContext();
            if (sc == null)
                throw new IllegalArgumentException("ServletContext can't be null in WebApplicationContext " + wac);
    
            sc.setAttribute(WebApplicationContext.WEB_APPLICATION_CONTEXT_ATTRIBUTE_NAME, wac);
            logger.info(
                    "Loader initialized on server name "
                            + wac.getServletContext().getServerInfo()
                            + "; WebApplicationContext object is available in ServletContext with name '"
                            + WebApplicationContext.WEB_APPLICATION_CONTEXT_ATTRIBUTE_NAME
                            + "'");
        }

27. 返回webApplicationContext到ContextLoader类
28. ContextLoaderListener.contextInitialized方法执行结束

就这样，Spring Web Application Context加载完成了，是不是感觉也挺简单的，主要就是读取xml配置文件中bean的配置信息，创建bean实例放到一个map中维护，当然，中间还穿插了各种逻辑；

另外补充下，当Servlet容器销毁时，会调用ContextLoaderListener的contextDestroyed方法，最终是调用ContextLoader.closeContext(event.getServletContext()，执行一些资源销毁等操作，销毁工厂创建的bean对象，发布ContextClosedEvent事件等；

  public void close() {
        logger.info("Closing application context [" + getDisplayName() + "]");

        // destroy all cached singletons in this context,
        // invoking DisposableBean.destroy and/or "destroy-method"
        getBeanFactory().destroySingletons();

        // publish respective event
        publishEvent(new ContextClosedEvent(this));
    }

interface21代码参考

https://github.com/peterchenhdu/interface21