SpringMVC源码（一）ContextLoaderListener

在我们启动 springweb 服务时，spring会初始化i
4000
oc容器(WebApplicationContext)，装载我们所需要的bean。

具体配看我们的web.xml中配置。

<context-param>
<param-name>contextConfigLocation</param-name>
<param-value>
classpath:applicationContext*.xml
</param-value>
</context-param>
<listener>
<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>

看一下ContextLoaderListener 类，extends ContextLoader implements ServletContextListener  

父类ContextLoader可以说是一个工具类，主要用来实现ContextLoaderListener中初始化ioc容器需要的方法。

即ContextLoaderListener 的initWebApplicationContext（）方法在其父类中实现；

//初始化方法
public void contextInitialized(ServletContextEvent event) {
initWebApplicationContext(event.getServletContext());
}
//销毁方法
public void contextDestroyed(ServletContextEvent event) {
closeWebApplicationContext(event.getServletContext());
ContextCleanupListener.cleanupAttributes(event.getServletContext());
}

而实现的 ServletContextListener  类可以说是一个servlet 的监听器，当我们启动tomcat的时候，会默认初始化一些servlet，

当监听到servlet初始化时，ContextLoaderListener便会进行我们的初始化ioc容器。

即servlet 在初始化之后我们的ContextLoaderListener也会进行初始化。

//servlet/jsp 容器会在应用程序初始化期间调用 contextInitialized()方法。
public void contextInitialized ( ServletContextEvent sce );
//销毁
public void contextDestroyed ( ServletContextEvent sce );

我们主要看一下父类ContextLoader中的initWebApplicationContext() 方法，它将我们ioc(WebApplicationContext)
进行

初始化，加载spring文件里面的创建的bean实例，并将它放入到servletcontext当中去。看一下它的具体代码。

public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {
//判断是否创建过容器
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);
//打印日志 web容器开始初始化
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.
//创建WebApplicationContext
if (this.context == null) {
this.context = createWebApplicationContext(servletContext);
}
if (this.context instanceof ConfigurableWebApplicationContext) {
ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) this.context;
//判断上下文
if (!cwac.isActive()) {
// The context has not yet been refreshed -> provide services such as
// setting the parent context, setting the application context id, etc
//是否有父容器
if (cwac.getParent() == null) {
// The context instance was injected without an explicit parent ->
// determine parent for root web application context, if any.
//获得父容器
ApplicationContext parent = loadParentContext(servletContext);
//设置父容器
cwac.setParent(parent);
}
//刷新WebApplicationContext
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");
}

//返回这个容器
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;
}
}