Spring 和 Struts在web.xml中增加的配置：
1: <!-- spring的配置 -->
2: <context-param>
3: <param-name>contextConfigLocation</param-name>
4: <param-value>classpath:/SpringContext/applicationContext-web.xml</param-value>
5: </context-param>
7:
8: <listener>
9: <listener-class>org.springframework.web.context.ContextLoaderListener
10: </listener-class>
11: </listener>
12:
13: <filter>
14: <filter-name>struts2</filter-name>
15: <filter-class>org.apache.struts2.dispatcher.FilterDispatcher</filter-class>
17: <init-param>
18: <param-name>config</param-name>
19: <param-value>struts-default.xml,struts-plugin.xml,struts/struts.xml</param-value>
20: </init-param>
21: </filter>
22:
23: <filter-mapping>
24: <filter-name>struts2</filter-name>
25: <url-pattern>*.do</url-pattern>
26: </filter-mapping>
第一個tag定義的是spring的配置文件地址到環境參數(context parameter)
第二個tag定義一個listener為org.springframework.web.context.ContextLoaderListener,這里相當于j2ee容器給我們提供的main函數的切入點，可以讓我們做一些系統初始化的工作，需要實現的類是：javax.servlet.ServletContextListener
第三個tag則定義了struts2的一個filter。Filter則是對每次請求（可以通過filter-mapping指定）做過濾處理，請求首先請過filter鏈的處理，然后再到HttpServlet的init方法。對應的類是：javax.servlet.Filter。上面先配置了一個filter,對應的類是org.apache.struts2.dispatcher.FilterDispatcher,參數則是struts的配置文件位置
第四個參數定義了filter怎樣行為，顯然它對.do為后綴的請求應用struts2這個名稱的filter
查看ContextLoaderListener可知，它正好繼承了javax.servlet.ServletContextListener，用于監聽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());
}
}
}
此類implement了ServletContextListener的兩個方法：
public void contextInitialized(ServletContextEvent event);
public void contextDestroyed(ServletContextEvent event);
分別做context的初始化和銷毀
另外提供了一個protected方法：protected ContextLoader createContextLoader() 用于創建真正做事情的代理類CotextLoader
和一個public方法：public ContextLoader getContextLoader()；
可見，這個Listener類直接將工作代理給了ContextLoader類了
____________________________________________________________________________________________________________________________
我們按圖索驥，下面再分析org.springframework.web.context.ContextLoader
ContextLoader里面有個私有成員：private WebApplicationContext context
此變量提供了create方法：
createWebApplicationContext(ServletContext servletContext, ApplicationContext parent)
因為Spring提供了多種WebApplicationContext類，所以需要一個方法來決定使用哪個WebApplicationContextContext類
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>中定義了參數contextClass，那么直接就決定了用此Context類
否則，就應用缺省策略來決定使用哪個Context類。
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()); }
}
實際上，缺省策略從org\springframework\web\context\ContextLoader.properties 文件中取得屬性org.springframework.web.context.WebApplicationContext
我們看看ContextLoader.properties文件的內容：
# 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
顯然，缺省的Context類就是 XmlWebApplicationContext類
！！！真累啊，Spring...
OK,總算知道用哪個Context類了，那么現在到了create這個Context實例的時候了
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;
}
這里很容易理解，只不過它的創建不是直接new，而是封裝了一層，調用BeanUtils.instantiateClass()工具方法
接下來設定WebApplicationCcontext實例的parent, servletContext,
其中配置文件位置(web.xml中)的contextConfigLocation參數指定的
<context-param>
<param-name>contextConfigLocation</param-name>
<param-value>classpath:/SpringContext/applicationContext-web.xml
</param-value>
</context-param>
倒數第二行調用的customizeContext()方法目前Spring實現代碼是空的，估計留作以后（看來萬事要留余地啊^_^）
最后一件事就是調用WebApplicationContext的refresh()方法。（這個方法是個stratup方法，很重要。他干的事情后面會著重涉及）
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;
}
}
它其實做得事情很簡單，調用loadParentContext()方法獲取父context,調用createWebApplicationContext()創建WebApplicationContext實例
一切都似乎完成了，至少我們明白了配置文件是如何加載進去的，至于IoC容器如何幫你注入配置文件中的bean，下次再探索。線索是什么呢？你還記得在createWebApplication()的最后一步做什么了嗎？refresh(), 對了，這個就是你作為職業"嘿客"要探索的下一個線索