Spring源码（五）Spring 事件监听器的两种注册方式

了解专栏完整的内容，请点击跳转：
Spring framework专栏导航页

gitee注释版源码

作为一款优先的框架，怎么能少得了监听器呢？虽然说现基于web开发的项目很少用到监听器（大多使用消息队列），但是有些时候又需要用到。以及我们想监听Spring容器的初始化相关事件时候，就可以用到Spring自带的的监听器实现方式。

重要组件：事件多播器

监听器思维导图

@Lazy
@Component
public class ContextRefreshedInterfaceListener implements ApplicationListener {

	@Override
	public void onApplicationEvent(ContextRefreshedEvent event) {
		System.out.println("接口监听");
	}
}

@Lazy // 加了懒加载 这个Bean不会被创建 但是 里边用注解标记的监听事件会被通过后置处理器创建出来
@Component
public class ContextRefreshedAnnotationListener {

	@EventListener(ContextRefreshedEvent.class)
	public void onAppllicationEvent(ContextRefreshedEvent event) {
		System.out.println("注解监听");
	}

}

// refresh().initApplicationEventMulticaster();
protected void initApplicationEventMulticaster() {
		ConfigurableListableBeanFactory beanFactory = getBeanFactory();
		//判断容器中是没有有我们的applicationEventMulticaster 应用多播器组件
		if (beanFactory.containsLocalBean(APPLICATION_EVENT_MULTICASTER_BEAN_NAME)) {
			//直接显示的调用我们的getBean获取出来赋值给我们的applicationContext对象
			this.applicationEventMulticaster =
					beanFactory.getBean(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, ApplicationEventMulticaster.class);
			if (logger.isTraceEnabled()) {
				logger.trace("Using ApplicationEventMulticaster [" + this.applicationEventMulticaster + "]");
			}
		}
		else {
			//spring ioc显示的new 一个SimpleApplicationEventMulticaster对象保存在applicatoinContext对象中
			this.applicationEventMulticaster = new SimpleApplicationEventMulticaster(beanFactory);
			//并且注入到容器中
			beanFactory.registerSingleton(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, this.applicationEventMulticaster);
			if (logger.isTraceEnabled()) {
				logger.trace("No '" + APPLICATION_EVENT_MULTICASTER_BEAN_NAME + "' bean, using " +
						"[" + this.applicationEventMulticaster.getClass().getSimpleName() + "]");
			}
		}
	}

// refresh().prepareBeanFactory(ConfigurableListableBeanFactory beanFactory);

......

// 用于处理实现了ApplicationListener接口的bean,bean初始化后将其添加到监听器集合中 来监听事件
// Register early post-processor for detecting inner beans as ApplicationListeners.
beanFactory.addBeanPostProcessor(new ApplicationListenerDetector(this));

......

// 注册接口方式的监听器 其实只是注册监听器名字
// 防止懒加载监听器漏掉
// 事件发布时候 会再根据这里注册的名字 把懒加载的bean进行实例化 然后再发布事件

// refresh().registerListeners();
protected void registerListeners() {
		// 获取容器中所有的监听器对象
		// 这个时候正常流程是不会有监听器的
		// Register statically specified listeners first.
		for (ApplicationListener listener : getApplicationListeners()) {
			getApplicationEventMulticaster().addApplicationListener(listener);
		}

		//获取bean定义中的监听器对象
		// Do not initialize FactoryBeans here: We need to leave all regular beans
		// uninitialized to let post-processors apply to them!
		String[] listenerBeanNames = getBeanNamesForType(ApplicationListener.class, true, false);
		for (String listenerBeanName : listenerBeanNames) {
			// 把监听器名字注册进多播器中
			getApplicationEventMulticaster().addApplicationListenerBean(listenerBeanName);
		}

		//在这里获取我们的早期事件
		// Publish early application events now that we finally have a multicaster...
		Set earlyEventsToProcess = this.earlyApplicationEvents;
		// 在这里赋null。  也就是值此之后都将没有早期事件了
		this.earlyApplicationEvents = null;
		if (!CollectionUtils.isEmpty(earlyEventsToProcess)) {
			//通过多播器进行播发早期事件
			for (ApplicationEvent earlyEvent : earlyEventsToProcess) {
				getApplicationEventMulticaster().multicastEvent(earlyEvent);
			}
		}
	}

@Override
	public Object postProcessAfterInitialization(Object bean, String beanName) {
		if (bean instanceof ApplicationListener) {
			// potentially not detected as a listener by getBeanNamesForType retrieval
			Boolean flag = this.singletonNames.get(beanName);
			if (Boolean.TRUE.equals(flag)) {
				// singleton bean (top-level or inner): register on the fly
				this.applicationContext.addApplicationListener((ApplicationListener) bean);
			}
			else if (Boolean.FALSE.equals(flag)) {
				if (logger.isWarnEnabled() && !this.applicationContext.containsBean(beanName)) {
					// inner bean with other scope - can't reliably process events
					logger.warn("Inner bean '" + beanName + "' implements ApplicationListener interface " +
							"but is not reachable for event multicasting by its containing ApplicationContext " +
							"because it does not have singleton scope. only top-level listener beans are allowed " +
							"to be of non-singleton scope.");
				}
				this.singletonNames.remove(beanName);
			}
		}
		return bean;
	}

// DefaultListableBeanFactory#preInstantiateSingletons
// 所有bean都创建完了 可以实现SmartInitializingSingleton来做扩展
// 解析注解方式监听器就是在这里进行的
// Trigger post-initialization callback for all applicable beans...
		for (String beanName : beanNames) {
			Object singletonInstance = getSingleton(beanName);
			if (singletonInstance instanceof SmartInitializingSingleton) {
				StartupStep smartInitialize = this.getApplicationStartup().start("spring.beans.smart-initialize")
						.tag("beanName", beanName);
				SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
				if (System.getSecurityManager() != null) {
					AccessController.doPrivileged((PrivilegedAction