读Spring源代码之按图索骥（一）Context创建与配置文件加载

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Spring 和 Struts在web.xml中增加的配置：

    <!-- spring的配置 -->    <context-param>        <param-name>contextConfigLocation</param-name>        <param-value>classpath:/SpringContext/applicationContext-web.xml</param-value>    </context-param>             <listener>        <listener-class>org.springframework.web.context.ContextLoaderListener       </listener-class>   </listener>       <filter>       <filter-name>struts2</filter-name>       <filter-class>org.apache.struts2.dispatcher.FilterDispatcher</filter-class>       <init-param>           <param-name>config</param-name>           <param-value>struts-default.xml,struts-plugin.xml,struts/struts.xml</param-value>       </init-param>   </filter>       <filter-mapping>       <filter-name>struts2</filter-name>       <url-pattern>*.do</url-pattern>   </filter-mapping

第一个tag定义的是spring的配置文件地址到环境参数(context parameter)

第二个tag定义一个listener为org.springframework.web.context.ContextLoaderListener,这里相当于j2ee容器给我们提供的main函数的切入点，可以让我们做一些系统初始化的工作，需要实现的类是：javax.servlet.ServletContextListener

第三个tag则定义了struts2的一个filter。Filter则是对每次请求（可以通过filter-mapping指定）做过滤处理，请求首先请过filter链的处理，然后再到HttpServlet的init方法。对应的类是：javax.servlet.Filter。上面先配置了一个filter,对应的类是org.apache.struts2.dispatcher.FilterDispatcher,参数则是struts的配置文件位置

第四个参数定义了filter怎样行为，显然它对.do为后缀的请求应用struts2这个名称的filter

 

这里需要首先搞清楚servlet规范中什么是listener? 详细请参见 Servlet Listener和Filter

查看ContextLoaderListener可知，它正好继承了javax.servlet.ServletContextListener，用于监听javax.servlet.ServletContextEvent事件查看ContextLoaderListener可知，它正好继承了javax.servlet.ServletContextListener，用于监听javax.servlet.ServletContextEvent事件

 

/* * Copyright 2002-2007 the original author or authors. * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * *      http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */ package org.springframework.web.context; import javax.servlet.ServletContextEvent;import javax.servlet.ServletContextListener; /** * Bootstrap listener to start up Spring's root {@link WebApplicationContext}. * Simply delegates to {@link ContextLoader}. * * <p>This listener should be registered after * {@link org.springframework.web.util.Log4jConfigListener} * in <code>web.xml</code>, if the latter is used. * * @author Juergen Hoeller * @since 17.02.2003 * @see ContextLoaderServlet * @see org.springframework.web.util.Log4jConfigListener */public class ContextLoaderListener implements ServletContextListener {     private ContextLoader contextLoader;      /**     * Initialize the root web application context.     */    public void contextInitialized(ServletContextEvent event) {        this.contextLoader = createContextLoader();        this.contextLoader.initWebApplicationContext(event.getServletContext());    }     /**     * Create the ContextLoader to use. Can be overridden in subclasses.     * @return the new ContextLoader     */    protected ContextLoader createContextLoader() {        return new ContextLoader();    }     /**     * Return the ContextLoader used by this listener.     * @return the current ContextLoader     */    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());        }    } }

此类implement了ServletContextListener的两个方法：

public void contextInitialized(ServletContextEvent event);public void contextDestroyed(ServletContextEvent event);

 

分别做context的初始化和销毁

另外提供了一个protected方法：protected ContextLoader createContextLoader()  用于创建真正做事情的代理类CotextLoader

和一个public方法：public ContextLoader getContextLoader()；

可见，这个Listener类直接将工作代理给了ContextLoader类了

我们按图索骥，下面再分析org.springframework.web.context.ContextLoader

ContextLoader里面有个私有成员：private WebApplicationContext context

此变量提供了create方法：

createWebApplicationContext(ServletContext servletContext, ApplicationContext parent)

因为Spring提供了多种WebApplicationContext类，所以需要一个方法来决定使用哪个WebApplicationContextContext类

protected Class determineContextClass(ServletContext servletContext) throws ApplicationContextException {        String contextClassName = servletContext.getInitParameter(CONTEXT_CLASS_PARAM);        if (contextClassName != null) {            try {                return ClassUtils.forName(contextClassName);            }            catch (ClassNotFoundException ex) {                throw new ApplicationContextException(                        "Failed to load custom context class [" + contextClassName + "]", ex);            }        }        else {            contextClassName = defaultStrategies.getProperty(WebApplicationContext.class.getName());            try {                return ClassUtils.forName(contextClassName, ContextLoader.class.getClassLoader());            }            catch (ClassNotFoundException ex) {                throw new ApplicationContextException(                        "Failed to load default context class [" + contextClassName + "]", ex);            }        }    }

 

public static final String CONTEXT_CLASS_PARAM = "contextClass";

所以如果在web.xml中的<context-param> </context-param>中定义了参数contextClass，那么直接就决定了用此Context类

否则，就应用缺省策略来决定使用哪个Context类。

缺省策略如下：

private static final Properties defaultStrategies; static {    // Load default strategy implementations from properties file.    // This is currently strictly internal and not meant to be customized    // by application developers.    try {        ClassPathResource resource = new ClassPathResource(DEFAULT_STRATEGIES_PATH, ContextLoader.class);        defaultStrategies = PropertiesLoaderUtils.loadProperties(resource);    }    catch (IOException ex) {        throw new IllegalStateException("Could not load 'ContextLoader.properties': " + ex.getMessage());    }}

 

实际上，缺省策略从org\springframework\web\context\ContextLoader.properties 文件中取得属性org.springframework.web.context.WebApplicationContext

我们看看ContextLoader.properties文件的内容：

# Default WebApplicationContext implementation class for ContextLoader.# Used as fallback when no explicit context implementation has been specified as context-param.# Not meant to be customized by application developers. org.springframework.web.context.WebApplicationContext=org.springframework.web.context.support.XmlWebApplicationContext

 

显然，缺省的Context类就是 XmlWebApplicationContext类

！！！真累啊，Spring...

OK,总算知道用哪个Context类了，那么现在到了create这个Context实例的时候了

protected WebApplicationContext createWebApplicationContext(            ServletContext servletContext, ApplicationContext parent) throws BeansException {         Class contextClass = determineContextClass(servletContext);        if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {            throw new ApplicationContextException("Custom context class [" + contextClass.getName() +                    "] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]");        }         ConfigurableWebApplicationContext wac =                (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);        wac.setParent(parent);        wac.setServletContext(servletContext);        wac.setConfigLocation(servletContext.getInitParameter(CONFIG_LOCATION_PARAM));        customizeContext(servletContext, wac);        wac.refresh();         return wac;    }

 

这里很容易理解，只不过它的创建不是直接new，而是封装了一层，调用BeanUtils.instantiateClass()工具方法

接下来设定WebApplicationCcontext实例的parent, servletContext,

其中配置文件位置(web.xml中)的contextConfigLocation参数指定的

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

 

倒数第二行调用的customizeContext()方法目前Spring实现代码是空的，估计留作以后（看来万事要留余地啊^_^）

最后一件事就是调用WebApplicationContext的refresh()方法。（这个方法是个stratup方法，很重要。他干的事情后面会着重涉及）

最后就是真正做事的initWebApplicationContext()方法：

public WebApplicationContext initWebApplicationContext(ServletContext servletContext)            throws IllegalStateException, BeansException {         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");        if (logger.isInfoEnabled()) {            logger.info("Root WebApplicationContext: initialization started");        }        long startTime = System.currentTimeMillis();         try {            // Determine parent for root web application context, if any.            ApplicationContext parent = loadParentContext(servletContext);             // Store context in local instance variable, to guarantee that            // it is available on ServletContext shutdown.            this.context = createWebApplicationContext(servletContext, parent);            servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);            currentContextPerThread.put(Thread.currentThread().getContextClassLoader(), 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;        }    }

 

它其实做得事情很简单，调用loadParentContext()方法获取父context,调用createWebApplicationContext()创建WebApplicationContext实例

一切都似乎完成了，至少我们明白了配置文件是如何加载进去的，至于IoC容器如何帮你注入配置文件中的bean，下次再探索。线索是什么呢？你还记得在createWebApplication()的最后一步做什么了吗？refresh(), 对了，这个就是你作为职业"嘿客"要探索的下一个线索