Spring 源碼之 ContextLoaderListener 解析

我們平日采用spring 架構開發項目時，配置檔案web.xml 中總是有這樣一些配置，比如：

<!-- 加載spring配置檔案-->
<context-param>
   <param-name>contextConfigLocation</param-name>
   <param-value>
      classpath:config/spring-config.xml
      classpath:config/spring-cxf.xml    
   </param-value>
</context-param>

<listener>
   <listener-class>
      org.springframework.web.context.ContextLoaderListener
   </listener-class>
</listener>

而這些配置到底有什麼作用呢？我們從源碼角度分析一下：

1、首先看ContextLoaderListener 類，它繼承ContextLoader 實作 ServletContextListener

public class ContextLoaderListener extends ContextLoader implements ServletContextListener

ContextLoaderListener的作用就是啟動Web容器時，讀取在contextConfigLocation中定義的xml檔案，自動裝配ApplicationContext的配置資訊，并産生WebApplicationContext對象，然後将這個對象放置在ServletContext的屬性裡，這樣我們隻要得到Servlet就可以得到WebApplicationContext對象，并利用這個對象通路spring容器管理的bean。

總之，就是上面這段配置為項目提供了spring支援，初始化了Ioc容器。

ContextLoader 主要作用作為父類指定Web應用程式啟動時載入相應的Ioc容器，完成實際的WebApplicationContext，即Ioc容器的初始化工作在這裡完成，後面會源碼展示。

ServletContextListener 接口繼承EventListener

public interface ServletContextListener extends EventListener

ServletContextListener是ServletContext的監聽者，接口裡的方法會結合Web容器的生命周期被調用，ServletContext發生某些變化，會觸發相應的事件，web伺服器啟動時、ServletContext建立時、web服務關閉時、ServletContext銷毀時等等，都會觸發相應事件，做出預先設計好的相應動作，比如此類中：監聽器的響應動作就是在伺服器啟動時contextInitialized會被調用，關閉的時候contextDestroyed被調用。

/**
 * Initialize the root web application context.
   啟動時在這裡開始調用，初始化根上下文應用程式
 */
@Override
public void contextInitialized(ServletContextEvent event) {
   initWebApplicationContext(event.getServletContext());//點選進入
}

2、進入 ContextLoader 類

public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {
  //如果servletContext對象中存放了spring context，則抛出異常
   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!");
   }

   servletContext.log("Initializing Spring root WebApplicationContext");
   Log logger = LogFactory.getLog(ContextLoader.class);
   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) {
         //建立WebApplicationContext容器并被強制轉換為ConfigurableWebApplicationContext類型
         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.
               //加載父容器
               ApplicationContext parent = loadParentContext(servletContext);
               cwac.setParent(parent);
            }
            //加載spring配置檔案,點選進入
            configureAndRefreshWebApplicationContext(cwac, servletContext);
         }
      }//把web容器放到servlet容器中
      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);
      }

      if (logger.isInfoEnabled()) {
         long elapsedTime = System.currentTimeMillis() - startTime;
         logger.info("Root WebApplicationContext initialized in " + elapsedTime + " ms");
      }

      return this.context;
   }
   catch (RuntimeException | Error ex) {
      logger.error("Context initialization failed", ex);
      servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, ex);
      throw ex;
   }
}

來到configureAndRefreshWebApplicationContext方法中

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
      //擷取 web.xml 中定義的 contextId 初始化值，未定義
      String idParam = sc.getInitParameter(CONTEXT_ID_PARAM);
      if (idParam != null) {
         wac.setId(idParam);
      }
      else {
         // Generate default id... 設定一個 全局的id
         wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +
               ObjectUtils.getDisplayString(sc.getContextPath()));
      }
   }

   wac.setServletContext(sc);//配置了 servletContext 屬性
   //擷取 web.xml 中定義的 contextConfiLocation 初始化參數值 ，已定義
   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
   //初始化 應用上下文 中 和 Servlet 相關的 占位符 屬性資源
   ConfigurableEnvironment env = wac.getEnvironment();
   if (env instanceof ConfigurableWebEnvironment) {
      ((ConfigurableWebEnvironment) env).initPropertySources(sc, null);
   }
   /// 加載web.xml 中 自定義的 其他初始化内容
   customizeContext(sc, wac);
   wac.refresh();//WebApplicationContext 的重新整理,核心方法，點選進入後豁然開朗，ctr+t 進入
}

3、進入這個方法，非常熟悉，前面幾篇已經詳細解析過，是spring 源碼的重點方法，這裡就不介紹了。

public abstract class AbstractApplicationContext extends DefaultResourceLoader
      implements ConfigurableApplicationContext {

@Override
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();
      }
   }
}

4、重點這是spring 的容器初始化，下篇介紹springMVC 容器初始化；他倆是父子容器關系。敬請期待！

Spring 源碼之 ContextLoaderListener 解析

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