http://blog.csdn.net/hz_blog/article/details/7688921

当我们要自动装配ApplicationContext配置信息时候，首先在web.xml配置ContextLoaderListener，下面是部分源代码：

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1. public class ContextLoaderListener implements ServletContextListener {  
2.   
3.     private ContextLoader contextLoader;  
4.   
5.   
6.     /** 
7.      * Initialize the root web application context. 
8.      */  
9.     public void contextInitialized(ServletContextEvent event) {  
10.         this.contextLoader = createContextLoader();  
11.         this.contextLoader.initWebApplicationContext(event.getServletContext());//注意此处   
12.     }  
13.   
14.     protected ContextLoader createContextLoader() {  
15.         return new ContextLoader();  
16.     }  
17.   
18.     public ContextLoader getContextLoader() {  
19.         return this.contextLoader;  
20.     }  
21.   
22.     public void contextDestroyed(ServletContextEvent event) {  
23.         if (this.contextLoader != null) {  
24.             this.contextLoader.closeWebApplicationContext(event.getServletContext());  
25.         }  
26.     }  
27.   
28. }  

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());//注意此处 } protected ContextLoader createContextLoader() { return new ContextLoader(); } public ContextLoader getContextLoader() { return this.contextLoader; } public void contextDestroyed(ServletContextEvent event) { if (this.contextLoader != null) { this.contextLoader.closeWebApplicationContext(event.getServletContext()); } } }

我们主要了解下WebApplicationContext是怎么初始化出来的，首先contextInitialized(ServletContextEvent event)方法的this.contextLoader.initWebApplicationContext(event.getServletContext());进去之后我们看到

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1. public WebApplicationContext initWebApplicationContext(ServletContext servletContext)  
2.         throws IllegalStateException, BeansException {  
3.   
4.     if (servletContext.getAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE) != null) {  
5.         throw new IllegalStateException(  
6.                 “Cannot initialize context because there is already a root application context present – “ +  
7.                 “check whether you have multiple ContextLoader* definitions in your web.xml!”);  
8.     }  
9.   
10.     servletContext.log(“Initializing Spring root WebApplicationContext”);  
11.     if (logger.isInfoEnabled()) {  
12.         logger.info(“Root WebApplicationContext: initialization started”);  
13.     }  
14.     long startTime = System.currentTimeMillis();  
15.   
16.     try {  
17.         // Determine parent for root web application context, if any.   
18.         ApplicationContext parent = loadParentContext(servletContext);  
19.   
20.         // Store context in local instance variable, to guarantee that   
21.         // it is available on ServletContext shutdown.   
22.         this.context = createWebApplicationContext(servletContext, parent);  
23.         servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);//注意此处   
24.         currentContextPerThread.put(Thread.currentThread().getContextClassLoader(), this.context);  
25.   
26.         if (logger.isDebugEnabled()) {  
27.             logger.debug(“Published root WebApplicationContext as ServletContext attribute with name [“ +  
28.                     WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE + “]”);  
29.         }  
30.         if (logger.isInfoEnabled()) {  
31.             long elapsedTime = System.currentTimeMillis() – startTime;  
32.             logger.info(“Root WebApplicationContext: initialization completed in “ + elapsedTime + ” ms”);  
33.         }  
34.   
35.         return this.context;  
36.     }  
37.     catch (RuntimeException ex) {  
38.         logger.error(“Context initialization failed”, ex);  
39.         servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, ex);  
40.         throw ex;  
41.     }  
42.     catch (Error err) {  
43.         logger.error(“Context initialization failed”, err);  
44.         servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, err);  
45.         throw err;  
46.     }  
47. }  

 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);//注意此处 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; } }

注意此处部分，说明当我们要得到一个WebApplicationContext只需要在ServletContext中获取属性名为WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE的常量值就可以得到一个ApplicationContext，最终可以获取spring配置文件中任意一个Bean对象

eg:

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1. public String execute() throws Exception {  
2.     // TODO Auto-generated method stub   
3.     ApplicationContext conn=(ApplicationContext)ServletActionContext.getServletContext().getAttribute(WebApplicationContext.class.getName() + “.ROOT”);  
4.     System.out.println(conn.getBean(“biz”));  
5.     return SUCCESS;  
6. }  

 public String execute() throws Exception { // TODO Auto-generated method stub ApplicationContext conn=(ApplicationContext)ServletActionContext.getServletContext().getAttribute(WebApplicationContext.class.getName() + ".ROOT"); System.out.println(conn.getBean("biz")); return SUCCESS; }

或者直接使用org.springframework.web.context.support.WebApplicationContextUtils这个抽象的工具类来获得WebApplicationContext,该类的方法

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1. public static WebApplicationContext getWebApplicationContext(ServletContext sc) {  
2.     return getWebApplicationContext(sc, WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE);  
3. }