在應用程序web.xml中做瞭以下配置信息時，當啟動Web容器時就會自動加載Spring容器。
<listener>
<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>
ContextLoaderListener類實現瞭javax.servlet.ServletContextListener接口並且繼承瞭org.springframework.web.context.ContextLoader類。ServletContextListener事件類是Web容器的一部分，處理Web應用的Servlet上下文(context)的監聽。實現ServletContextListener接口中的contextInitialized和contextDestroyed方法。當Web容器啟動時會自動調用contextInitialized方法，進行初始化Spring Web應用程序上下文，主要加載web.xml中contextConfigLocation的配置文件；當Web容器關閉之前會調用contextDestroyed方法，進行銷毀Spring Web應用程序上下文。ContextLoader類實現瞭Spring上下文初始化的工作，執行initWebApplicationContext方法返回WebApplicationContext。Spring實現的contextInitialized和contextDestroyed代碼如下：
public void contextInitialized(ServletContextEvent event) {
this.contextLoader = createContextLoader();
if (this.contextLoader ==null) {
this.contextLoader = this;
}
this.contextLoader.initWebApplicationContext(event.getServletContext());
}
public void contextDestroyed(ServletContextEvent event) {
if (this.contextLoader !=null) {
this.contextLoader.closeWebApplicationContext(event.getServletContext());
}
ContextCleanupListener.cleanupAttributes(event.getServletContext());
}
Spring執行實現Spring上下文的方法
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!");
}
Log logger = LogFactory.getLog(ContextLoader.class);
servletContext.log("Initializing Spring root WebApplicationContext");
if (logger.isInfoEnabled()) {
logger.info("Root WebApplicationContext: initialization started");
}
long startTime = System.currentTimeMillis();
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) {
configureAndRefreshWebApplicationContext((ConfigurableWebApplicationContext)this.context, 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;
}
}
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