SpringApplicationListener用法与原理

SpringApplicationListener的简介与基本用法

SpringApplicationListener是Spring为我们提供的事件监听器，它主要是为我们提供事件监听机制，我们可以创建一个监听器，并向该监听器创建相应的事件，当该事件发生时，我们的监听器就会得到相应的通知，并且触发相应的逻辑。是典型的观察者模式

基本用法

@FunctionalInterface public interface ApplicationListener<E extends ApplicationEvent> extends EventListener { /** * Handle an application event. * @param event the event to respond to */ void onApplicationEvent(E event); }

事件监听器是Spring为我们定义的一个接口，该接口所携带的泛型就是我们要监听的事件，我们来实现一个ApplicationListener接口

@Component public class MyApplicationListener implements ApplicationListener<ApplicationEvent> { //当容器中发布此事件以后，方法触发 @Override public void onApplicationEvent(ApplicationEvent event) { System.out.println("reveice event: " + event); } }

发布一个事件

Spring的ApplicationContext为我们提供了一个方法publishEvent()该方法可以为我们发布事件。

applicationContext.publishEvent(new ApplicationEvent(new Stirng("我发布的事件")));

原理

ServletWebServerInitializedEvent的接收

在Spring refresh容器后，会执行一个finishRefresh()方法。

getLifecycleProcessor().onRefresh();

this.applicationContext.publishEvent(new ServletWebServerInitializedEvent(this.webServer,this.applicationContext));

getApplicationEventMulticaster().multicastEvent(applicationEvent,eventType);

//拿到所有符合条件的ApplicationListener，遍历它 for (ApplicationListener<?> listener : getApplicationListeners(event, type)) { if (executor != null) { executor.execute(() -> invokeListener(listener, event)); } else { //依次调用每个ApplicationListener的onApplicationEvent方法 invokeListener(listener, event); } }

其他Event的发布和上述流程基本相符。

关于applicationEventMulticaster的注入

protected void initApplicationEventMulticaster() { ConfigurableListableBeanFactory beanFactory = getBeanFactory(); //如果容器中有APPLICATION_EVENT_MULTICASTER_BEAN_NAME，则获取该组件，并加入到applicationEventMulticaster中 if (beanFactory.containsLocalBean(APPLICATION_EVENT_MULTICASTER_BEAN_NAME)) { this.applicationEventMulticaster = beanFactory.getBean(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, ApplicationEventMulticaster.class); if (logger.isTraceEnabled()) { logger.trace("Using ApplicationEventMulticaster [" + this.applicationEventMulticaster + "]"); } } else { //如果容器中没有如果容器中有APPLICATION_EVENT_MULTICASTER_BEAN_NAME，就自己创建一个单实例bean，注入到beanFactory中 this.applicationEventMulticaster = new SimpleApplicationEventMulticaster(beanFactory); 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() + "]"); } } }

容器中有哪些监听器？

protected void registerListeners() { // Register statically specified listeners first. for (ApplicationListener<?> listener : getApplicationListeners()) { getApplicationEventMulticaster().addApplicationListener(listener); } // 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) { getApplicationEventMulticaster().addApplicationListenerBean(listenerBeanName); } // Publish early application events now that we finally have a multicaster... Set<ApplicationEvent> earlyEventsToProcess = this.earlyApplicationEvents; this.earlyApplicationEvents = null; if (!CollectionUtils.isEmpty(earlyEventsToProcess)) { for (ApplicationEvent earlyEvent : earlyEventsToProcess) { getApplicationEventMulticaster().multicastEvent(earlyEvent); } } }