初始化事件监听多路广播器

/**
 * 初始化事件监听多路广播器
 */
initApplicationEventMulticaster();

注册SimpleApplicationEventMulticaster

内容非常简单，先判断有没有自定义的ApplicationEventMulticaster，没有的话就注册一个。
SimpleApplicationEventMulticaster就是用来发布事件用的。具体可看另一篇文章Spring Framework之监听器的使用，其中有讲到如何发布事件以及发布事件的源码跟踪

protected void initApplicationEventMulticaster() {
	ConfigurableListableBeanFactory beanFactory = getBeanFactory();
	/**
	 * 判断容器中是否存在bdName为applicationEventMulticaster的bd
	 * 也就是自定义的事件监听多路广播器，必须实现ApplicationEventMulticaster接口
	 */
	if (beanFactory.containsLocalBean(APPLICATION_EVENT_MULTICASTER_BEAN_NAME)) {
		this.applicationEventMulticaster =
				beanFactory.getBean(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, ApplicationEventMulticaster.class);
		if (logger.isDebugEnabled()) {
			logger.debug("Using ApplicationEventMulticaster [" + this.applicationEventMulticaster + "]");
		}
	}
	else {
		/**
		 * 如果没有，则默认采用SimpleApplicationEventMulticaster
		 */
		this.applicationEventMulticaster = new SimpleApplicationEventMulticaster(beanFactory);
		beanFactory.registerSingleton(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, this.applicationEventMulticaster);
		if (logger.isDebugEnabled()) {
			logger.debug("Unable to locate ApplicationEventMulticaster with name '" +
					APPLICATION_EVENT_MULTICASTER_BEAN_NAME +
					"': using default [" + this.applicationEventMulticaster + "]");
		}
	}
}