Spring IOC学习心得之源码级分析ContextLoaderListener的作用(IOC容器初始化入口)

ContextLoaderListener类是负责初始化IOC容器，即在我们的web项目中，这里就是IOC容器初始化的入口，由这个类启动IOC容器的初始化。

它配置在web.xml中，比如如下配置：

<context-param>
<param-name>contextConfigLocation</param-name>
<param-value>classpath:context/applicationContext.xml</param-value>
</context-param>

<!-- Spring监听器 -->
<listener>
<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>
跟踪ContextLoaderListener类，可以看到有一个如下方法

protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac, ServletContext sc) {
String configLocationParam;
if(ObjectUtils.identityToString(wac).equals(wac.getId())) {
configLocationParam = sc.getInitParameter("contextId");
if(configLocationParam != null) {
wac.setId(configLocationParam);
} else {
wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX + ObjectUtils.getDisplayString(sc.getContextPath()));
}
}

wac.setServletContext(sc);
configLocationParam = sc.getInitParameter("contextConfigLocation"); //很显然这里就是读取web.xml中配置的资源文件的路径，也就是上述context-param
标签配置的参数
if(configLocationParam != null) {
wac.setConfigLocation(configLocationParam);
}

ConfigurableEnvironment env = wac.getEnvironment();
if(env instanceof ConfigurableWebEnvironment) {
((ConfigurableWebEnvironment)env).initPropertySources(sc, (ServletConfig)null);
}

this.customizeContext(sc, wac);
wac.refresh(); //有了资源文件后，在这里开启IOC容器的初始化
}

refresh方法的实现是在AbstractApplicationContext类中，代码如下：

public void refresh() throws BeansException, IllegalStateException {
Object var1 = this.startupShutdownMonitor;
synchronized(this.startupShutdownMonitor) {
this.prepareRefresh();
ConfigurableListableBeanFactory beanFactory = this.obtainFreshBeanFactory();
this.prepareBeanFactory(beanFactory);

try {
this.postProcessBeanFactory(beanFactory);
this.invokeBeanFactoryPostProcessors(beanFactory);
this.registerBeanPostProcessors(beanFactory);
this.initMessageSource();
this.initApplicationEventMulticaster();
this.onRefresh();
this.registerListeners();
this.finishBeanFactoryInitialization(beanFactory);
this.finishRefresh();
} catch (BeansException var9) {
if(this.logger.isWarnEnabled()) {
this.logger.warn("Exception encountered during context initialization - cancelling refresh attempt: " + var9);
}

this.destroyBeans();
this.cancelRefresh(var9);
throw var9;
} finally {
this.resetCommonCaches();
}

}
}

上述代码就会进行完整的IOC容器的初始化