[Spring框架] Spring中的 ContextLoaderListener 实现原理.

前言: 这是关于Spring的第三篇文章, 打算后续还会写入AOP 和Spring 事务管理相关的文章, 这么好的两个周末 都在看code了, 确实是有所收获, 现在就来记录一下.

在上一篇讲解Spring IOC的文章中, 每次产生ApplicationContext工厂的方式是: 
ApplicationContext applicationContext = new ClassPathXmlApplicationContext("applicationContext.xml");

这样产生applicationContext 就有一个弊端, 每次访问加载bean 的时候都会产生这个工厂, 所以 这里需要解决这个问题.
解决问题的方法很简单, 在web 启动的时候将applicationContext转到到servletContext中, 因为在web 应用中的所有servlet都共享一个servletContext对象. 那么我们就可以利用ServletContextListener去监听servletContext事件, 当web 应用启动的是时候, 我们就将applicationContext 装载到servletContext中.

然而Spring容器底层已经为我们想到了这一点, 在spring-web-xxx-release.jar包中有一个 已经实现了ServletContextListener的类, 下面我们就来看一下这个类:

 public class ContextLoaderListener extends ContextLoader implements ServletContextListener {

     private ContextLoader contextLoader;

     /**
      * Create a new {@code ContextLoaderListener} that will create a web application
      * context based on the "contextClass" and "contextConfigLocation" servlet
      * context-params. See {@link ContextLoader} superclass documentation for details on
      * default values for each.
      * <p>This constructor is typically used when declaring {@code ContextLoaderListener}
      * as a {@code <listener>} within {@code web.xml}, where a no-arg constructor is
      * required.
      * <p>The created application context will be registered into the ServletContext under
      * the attribute name {@link WebApplicationContext#ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE}
      * and the Spring application context will be closed when the {@link #contextDestroyed}
      * lifecycle method is invoked on this listener.
      * @see ContextLoader
      * @see #ContextLoaderListener(WebApplicationContext)
      * @see #contextInitialized(ServletContextEvent)
      * @see #contextDestroyed(ServletContextEvent)
      */
     public ContextLoaderListener() {
     }

     /**
      * Create a new {@code ContextLoaderListener} with the given application context. This
      * constructor is useful in Servlet 3.0+ environments where instance-based
      * registration of listeners is possible through the {@link javax.servlet.ServletContext#addListener}
      * API.
      * <p>The context may or may not yet be {@linkplain
      * org.springframework.context.ConfigurableApplicationContext#refresh() refreshed}. If it
      * (a) is an implementation of {@link ConfigurableWebApplicationContext} and
      * (b) has <strong>not</strong> already been refreshed (the recommended approach),
      * then the following will occur:
      * <ul>
      * <li>If the given context has not already been assigned an {@linkplain
      * org.springframework.context.ConfigurableApplicationContext#setId id}, one will be assigned to it</li>
      * <li>{@code ServletContext} and {@code ServletConfig} objects will be delegated to
      * the application context</li>
      * <li>{@link #customizeContext} will be called</li>
      * <li>Any {@link org.springframework.context.ApplicationContextInitializer ApplicationContextInitializer}s
      * specified through the "contextInitializerClasses" init-param will be applied.</li>
      * <li>{@link org.springframework.context.ConfigurableApplicationContext#refresh refresh()} will be called</li>
      * </ul>
      * If the context has already been refreshed or does not implement
      * {@code ConfigurableWebApplicationContext}, none of the above will occur under the
      * assumption that the user has performed these actions (or not) per his or her
      * specific needs.
      * <p>See {@link org.springframework.web.WebApplicationInitializer} for usage examples.
      * <p>In any case, the given application context will be registered into the
      * ServletContext under the attribute name {@link
      * WebApplicationContext#ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE} and the Spring
      * application context will be closed when the {@link #contextDestroyed} lifecycle
      * method is invoked on this listener.
      * @param context the application context to manage
      * @see #contextInitialized(ServletContextEvent)
      * @see #contextDestroyed(ServletContextEvent)
      */
     public ContextLoaderListener(WebApplicationContext context) {
         super(context);
     }

     /**
      * Initialize the root web application context.
      */
     public void contextInitialized(ServletContextEvent event) {
         this.contextLoader = createContextLoader();
         if (this.contextLoader == null) {
             this.contextLoader = this;
         }
         this.contextLoader.initWebApplicationContext(event.getServletContext());
     }

     /**
      * Create the ContextLoader to use. Can be overridden in subclasses.
      * @return the new ContextLoader
      * @deprecated in favor of simply subclassing ContextLoaderListener itself
      * (which extends ContextLoader, as of Spring 3.0)
      */
     @Deprecated
     protected ContextLoader createContextLoader() {
         return null;
     }

     /**
      * Return the ContextLoader used by this listener.
      * @return the current ContextLoader
      * @deprecated in favor of simply subclassing ContextLoaderListener itself
      * (which extends ContextLoader, as of Spring 3.0)
      */
     @Deprecated
     public ContextLoader getContextLoader() {
         return this.contextLoader;
     }

     /**
      * Close the root web application context.
      */
     public void contextDestroyed(ServletContextEvent event) {
         if (this.contextLoader != null) {
             this.contextLoader.closeWebApplicationContext(event.getServletContext());
         }
         ContextCleanupListener.cleanupAttributes(event.getServletContext());
     }

 }

这里就监听到了servletContext的创建过程, 那么 这个类又是如何将applicationContext装入到serveletContext容器中的呢? 
我们接着来看看 :this.contextLoader.initWebApplicationContext(event.getServletContext())　方法:

 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);
                     }
                     configureAndRefreshWebApplicationContext(cwac, servletContext);
                 }
             }
             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.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;
         }
     }

这里的重点是:servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);
用key:WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE   value: this.context的形式将applicationContext装载到servletContext中了. 
另外从上面的一些注释我们可以看出:  WEB-INF/applicationContext.xml, 如果我们项目中的配置文件不是这么一个路径的话  那么我们使用ContextLoaderListener 就会 出问题, 所以我们还需要在web.xml中配置我们的applicationContext.xml配置文件的路径.

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

 <context-param>
     <param-name>contextConfigLocation</param-name>
     <param-value>classpath:applicationContext.xml</param-value>
 </context-param>

剩下的就是在项目中开始使用 servletContext中装载的applicationContext对象了:
那么这里又有一个问题, 装载时的key 是 WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, 我们在代码中真的要使用这个吗? 其实Spring为我们提供了一个工具类WebApplicationContextUtils, 接着我们先看下如何使用, 然后再去看下这个工具类的源码:

WebApplicationContext applicationContext = WebApplicationContextUtils.getWebApplicationContext(request.getServletContext());

接着来看下这个工具了的源码:

 /**
      * Find the root WebApplicationContext for this web application, which is
      * typically loaded via {@link org.springframework.web.context.ContextLoaderListener}.
      * <p>Will rethrow an exception that happened on root context startup,
      * to differentiate between a failed context startup and no context at all.
      * @param sc ServletContext to find the web application context for
      * @return the root WebApplicationContext for this web app, or {@code null} if none
      * @see org.springframework.web.context.WebApplicationContext#ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE
      */
     public static WebApplicationContext getWebApplicationContext(ServletContext sc) {
         return getWebApplicationContext(sc, WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE);
     }

 /**
      * Find a custom WebApplicationContext for this web application.
      * @param sc ServletContext to find the web application context for
      * @param attrName the name of the ServletContext attribute to look for
      * @return the desired WebApplicationContext for this web app, or {@code null} if none
      */
     public static WebApplicationContext getWebApplicationContext(ServletContext sc, String attrName) {
         Assert.notNull(sc, "ServletContext must not be null");
         Object attr = sc.getAttribute(attrName);
         if (attr == null) {
             return null;
         }
         if (attr instanceof RuntimeException) {
             throw (RuntimeException) attr;
         }
         if (attr instanceof Error) {
             throw (Error) attr;
         }
         if (attr instanceof Exception) {
             throw new IllegalStateException((Exception) attr);
         }
         if (!(attr instanceof WebApplicationContext)) {
             throw new IllegalStateException("Context attribute is not of type WebApplicationContext: " + attr);
         }
         return (WebApplicationContext) attr;
     }

这里就能很直观清晰地看到  通过key值直接获取到装载到servletContext中的 applicationContext对象了.