读Spring源代码之按图索骥（一）Context创建与配置文件加载
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
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