<!-- ServletContext参数，配置Ioc容器的xml文件名 --> <context-param> <param-name>contextConfigLocation</param-name> <param-value>classpath:applicationContext.xml</param-value> </context-param> <!-- 初始化Ioc容器的监听器（重点就是这个监听器）--> <listener> <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class> </listener>
看下这个监听器：
public class ContextLoaderListener extends ContextLoader implements ServletContextListener { public ContextLoaderListener() { } public ContextLoaderListener(WebApplicationContext context) { super(context); } @Override public void contextInitialized(ServletContextEvent event) { initWebApplicationContext(event.getServletContext()); } public void contextDestroyed(ServletContextEvent event) { closeWebApplicationContext(event.getServletContext()); ContextCleanupListener.cleanupAttributes(event.getServletContext()); } }
这个监听器首先实现了ServletContextListener，所以会在Servlet容器初始化和销毁的时候分别调用contextInitialized( )与contextDestroyed( )方法。
注意这里的带参构造方法 ContextLoaderListener（WebApplicationContext context）是调用了父类ContextLoader的构造方法，并传参进去。
而ContextLoader的主要作用就是初始化Ioc容器，下面我们来看看ContextLoader这个类
public class ContextLoader { public static final String CONFIG_LOCATION_PARAM = "contextConfigLocation";//ServletContext上下文参数 private WebApplicationContext context;
public ContextLoader(WebApplicationContext context) { this.context = context; }
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!"); }//不能初始化Ioc容器，因为已经有一个根Application上下文，检查在web.xml中是否有多个ContextLoader Log logger = LogFactory.getLog(ContextLoader.class); servletContext.log("Initializing Spring root WebApplicationContext"); if (logger.isInfoEnabled()) { logger.info("Root WebApplicationContext: initialization started"); }//开始初始化WebApplicationContext long startTime = System.currentTimeMillis();//当前毫秒数 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()) {／／Ｉｏｃ容器并未初始化，所以暂时不能提供服务 // 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) {／／设置父applicationContext // 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; } }
这里是总过程的描述：
protected WebApplicationContext createWebApplicationContext(ServletContext sc) { Class<?> contextClass = determineContextClass(sc);//获取context是由那个类定义的 if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {//判断是否为同一个类，或是子类 throw new ApplicationContextException("Custom context class [" + contextClass.getName() + "] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]"); } return (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass); }
通过determineContextClass( ）知道了实例化IOC容器方式，然后用这个方式，再通过反射机制实例化容器，即实例化webApplicationContext。
protected Class<?> determineContextClass(ServletContext servletContext) { String contextClassName = servletContext.getInitParameter(CONTEXT_CLASS_PARAM);//该属性为"ContextClass" if (contextClassName != null) { try { return ClassUtils.forName(contextClassName, ClassUtils.getDefaultClassLoader()); } 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); } } }
因为实例化Ioc容器的方式有几种，而这个方法是判定需要实例化那种类来实例化当前Web容器的Spring Ioc根容器。要判定需要实例化哪种类来实例化当前web容器的Spring根容器，如果我们设置了名称为“contextClass”的context-param，则取我们设置的类，该类应当实现ConfigurableWebApplicationContext接口或继承自实现了该接口的子类（如XmlWebApplicationContext、GroovyWebApplicationContext和AnnotationConfigWebApplicationContext），通常我们都不会设置，Spring会默认取与ContextLoader同目录下的ContextLoader.properties中记录的类名作为根容器的类型（默认是org.springframework.web.context.support.XmlWebApplicationContext）；
protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac, ServletContext sc) { if (ObjectUtils.identityToString(wac).equals(wac.getId())) { // The application context id is still set to its original default value // -> assign a more useful id based on available information String idParam = sc.getInitParameter(CONTEXT_ID_PARAM); if (idParam != null) { wac.setId(idParam); } else { // Generate default id... wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX + ObjectUtils.getDisplayString(sc.getContextPath())); } } wac.setServletContext(sc); String configLocationParam = sc.getInitParameter(CONFIG_LOCATION_PARAM); if (configLocationParam != null) { wac.setConfigLocation(configLocationParam); } // The wac environment's #initPropertySources will be called in any case when the context // is refreshed; do it eagerly here to ensure servlet property sources are in place for // use in any post-processing or initialization that occurs below prior to #refresh ConfigurableEnvironment env = wac.getEnvironment(); if (env instanceof ConfigurableWebEnvironment) { ((ConfigurableWebEnvironment) env).initPropertySources(sc, null); } customizeContext(sc, wac); wac.refresh(); }
虽然实例化了IOC容器，但是并未完成IOC容器的初始化，还不能提供服务。该方法的逻辑主要有一下几点：设置一个contextId（从contextId这个param获取，如果没有则默认是WebApplicationContext的类名 + “:” + servlet context的路径）；设置配置位置（从contextConfigLocation 这个param获取，如果未配置，则默认是/WEB-INF/applicationContext.xml，在XmlWebApplicationContext中可以看出）；自定义该congtext；调用该Context的refresh()方法。
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上面描述了WebApplicationContext的构造，默认还是使用反射的方式构建XmlWebApplicationContext实例
public void closeWebApplicationContext(ServletContext servletContext) { servletContext.log("Closing Spring root WebApplicationContext"); try { if (this.context instanceof ConfigurableWebApplicationContext) { ((ConfigurableWebApplicationContext) this.context).close(); } } finally { ClassLoader ccl = Thread.currentThread().getContextClassLoader(); if (ccl == ContextLoader.class.getClassLoader()) { currentContext = null; } else if (ccl != null) { currentContextPerThread.remove(ccl); } servletContext.removeAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE); if (this.parentContextRef != null) { this.parentContextRef.release(); } } }
该方法完成了对Ioc容器的销毁。
PS：webApplicationContext实现了ApplicationContext接口的子类。是专门为WEB应用准备的。
1. 它允许从相对于Web根目录的路径中加载配置文件完成初始化工作。从WebApplicationContext中可以获取ServletContext引用，整个Web应用上下文对象将作为属性放置在ServletContext中，以便Web应用环境可以访问Spring上下文。
2.WebApplicationContext还为Bean提供了三个新的作用域，request、session和globalsession。
其中两个参数HttpServletRequest：服务器从客户端拿去数据
HttpServletResponse：服务器向前台传送数据
原文：https://blog.csdn.net/Wayne_y/article/details/79778207