这篇文章主要介绍了Spring事件监听器ApplicationListener源码详解,ApplicationEvent以及Listener是Spring为我们提供的一个事件监听、订阅的实现，内部实现原理是观察者设计模式,需要的朋友可以参考下
ApplicationEvent以及Listener是Spring为我们提供的一个事件监听、订阅的实现，内部实现原理是观察者设计模式，设计初衷也是为了系统业务逻辑之间的解耦，提高可扩展性以及可维护性。事件发布者并不需要考虑谁去监听，监听具体的实现内容是什么，发布者的工作只是为了发布事件而已。
Spring提供的内置事件：
- ContextRefreshedEvent：容器刷新事件
- ContextStartedEvent：容器启动事件
- ContextStoppedEvent：容器停止事件
- ContextClosedEvent：容器关闭事件
使用方式
监听容器的刷新事件
自定义一个ApplicationListener，指定监听的事件类型ContextRefreshedEvent：
package com.morris.spring.listener; import org.springframework.context.ApplicationListener; import org.springframework.context.event.ContextRefreshedEvent; public class ContextRefreshedListener implements ApplicationListener{ @Override public void onApplicationEvent(ContextRefreshedEvent event) { System.out.println("context refresh"); } }
注入到容器中：
AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext(); applicationContext.register(ContextRefreshedListener.class); applicationContext.refresh();
applicationContext.refresh()内部会发送容器刷新的事件。
自定义事件
自定义的事件需要继承ApplicationEvent：
package com.morris.spring.event; import org.springframework.context.ApplicationEvent; public class CustomEvent extends ApplicationEvent { public CustomEvent(Object source) { super(source); } }监听的时候使用ApplicationEvent的子类CustomEvent：
package com.morris.spring.listener; import com.morris.spring.event.CustomEvent; import org.springframework.context.ApplicationListener; public class CustomEventListener implements ApplicationListener{ @Override public void onApplicationEvent(CustomEvent event) { System.out.println("custom event: " + event.getSource()); } }
可以使用AnnotationConfigApplicationContext发布事件：
AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext(); applicationContext.register(CustomEventListener.class); applicationContext.refresh(); applicationContext.publishEvent(new CustomEvent("custom event"));可以向bean中注入一个ApplicationEventPublisher来发布事件：
package com.morris.spring.service; import com.morris.spring.event.CustomEvent; import org.springframework.beans.factory.annotation.Autowired; import org.springframework.context.ApplicationEventPublisher; public class CustomEventService { @Autowired private ApplicationEventPublisher applicationEventPublisher; public void publishEvent() { applicationEventPublisher.publishEvent(new CustomEvent("自定义事件")); } }可以通过实现ApplicationEventPublisherAware接口注入ApplicationEventPublisher来发布事件：
package com.morris.spring.service; import com.morris.spring.event.CustomEvent; import org.springframework.context.ApplicationEventPublisher; import org.springframework.context.ApplicationEventPublisherAware; public class CustomEventService2 implements ApplicationEventPublisherAware { private ApplicationEventPublisher applicationEventPublisher; public void publishEvent() { applicationEventPublisher.publishEvent(new CustomEvent("自定义事件")); } @Override public void setApplicationEventPublisher(ApplicationEventPublisher applicationEventPublisher) { this.applicationEventPublisher = applicationEventPublisher; } }由于ApplicationContext实现了ApplicationEventPublisher接口，也可以直接注入ApplicationContext来发布事件。
使用@EventListener监听事件
在监听事件时，由于类需要实现ApplicationListener接口，对代码有很大的侵入性，可以使用@EventListener注解随时随地监听事件，这样一个Service中可以监听多个事件：
package com.morris.spring.listener; import com.morris.spring.event.CustomEvent; import org.springframework.context.ApplicationListener; import org.springframework.context.event.ContextRefreshedEvent; import org.springframework.context.event.EventListener; public class CustomEventListener2 { @EventListener public void listenContextRefreshedEvent(ContextRefreshedEvent event) { System.out.println("context refresh"); } @EventListener public void listenCustomEvent(CustomEvent event) { System.out.println("custom event: " + event.getSource()); } }还可以在@EventListener注解上指定监听的事件类型：
package com.morris.spring.listener; import com.morris.spring.event.CustomEvent; import org.springframework.context.ApplicationEvent; import org.springframework.context.event.ContextRefreshedEvent; import org.springframework.context.event.EventListener; public class CustomEventListener3 { @EventListener({ContextRefreshedEvent.class, CustomEvent.class}) public void listenEvent(ApplicationEvent event) { System.out.println(event); } }异步发送消息
spring消息的发送默认都是同步的，如果要异步发送消息，首先要在配置类上开启异步功能@EnableAsync：
package com.morris.spring.config; import org.springframework.context.annotation.Configuration; import org.springframework.scheduling.annotation.EnableAsync; @Configuration @EnableAsync // 开启异步 public class EventListenerConfig { }在监听的方法上加上@Async：
package com.morris.spring.listener; import com.morris.spring.event.CustomEvent; import lombok.extern.slf4j.Slf4j; import org.springframework.context.ApplicationEvent; import org.springframework.context.event.ContextRefreshedEvent; import org.springframework.context.event.EventListener; import org.springframework.scheduling.annotation.Async; @Slf4j public class AsyncCustomEventListener { @EventListener({ContextRefreshedEvent.class, CustomEvent.class}) @Async // 异步 public void listenEvent(ApplicationEvent event) { log.info("receive event: {}", event); } }也可以自定义执行异步消息的线程池（默认就是SimpleAsyncTaskExecutor）：
@Bean public TaskExecutor executor() { return new SimpleAsyncTaskExecutor("eventListen-"); }异步消息只是借用spring的异步执行机制，在方法上加上@Async注解，方法都会异步执行。
ApplicationListener原理分析
发布消息的入口
发布消息入口： org.springframework.context.support.AbstractApplicationContext#publishEvent(org.springframework.context.ApplicationEvent)
public void publishEvent(ApplicationEvent event) { publishEvent(event, null); } protected void publishEvent(Object event, @Nullable ResolvableType eventType) { ... /** * @see SimpleApplicationEventMulticaster#multicastEvent(org.springframework.context.ApplicationEvent, org.springframework.core.ResolvableType) */ // 发布消息 getApplicationEventMulticaster().multicastEvent(applicationEvent, eventType); } ... ... }然后调用SimpleApplicationEventMulticaster来进行广播消息：
// org.springframework.context.event.SimpleApplicationEventMulticaster#multicastEvent(org.springframework.context.ApplicationEvent, org.springframework.core.ResolvableType) public void multicastEvent(final ApplicationEvent event, @Nullable ResolvableType eventType) { ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event)); // 如果有线程池，将会异步执行 Executor executor = getTaskExecutor(); for (ApplicationListener listener : getApplicationListeners(event, type)) { if (executor != null) { executor.execute(() -> invokeListener(listener, event)); } else { invokeListener(listener, event); } } } protected void invokeListener(ApplicationListener listener, ApplicationEvent event) { ErrorHandler errorHandler = getErrorHandler(); if (errorHandler != null) { try { doInvokeListener(listener, event); } catch (Throwable err) { errorHandler.handleError(err); } } else { // ApplicationListener.调用onApplicationEvent doInvokeListener(listener, event); } } private void doInvokeListener(ApplicationListener listener, ApplicationEvent event) { try { listener.onApplicationEvent(event); } catch (ClassCastException ex) { String msg = ex.getMessage(); if (msg == null || matchesClassCastMessage(msg, event.getClass())) { // Possibly a lambda-defined listener which we could not resolve the generic event type for // -> let's suppress the exception and just log a debug message. Log logger = LogFactory.getLog(getClass()); if (logger.isTraceEnabled()) { logger.trace("Non-matching event type for listener: " + listener, ex); } } else { throw ex; } } }何时注入SimpleApplicationEventMulticaster
从上面的源码可以发现spring是通过SimpleApplicationEventMulticaster事件多播器来发布消息的，那么这个类是何时注入的呢？容器refresh()时。
org.springframework.context.support.AbstractApplicationContext#initApplicationEventMulticaster
protected void initApplicationEventMulticaster() { ConfigurableListableBeanFactory beanFactory = getBeanFactory(); 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 { // 直接new，然后放入到spring一级缓存中 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() + "]"); } } }何时注入ApplicationListener
spring在发布消息时，会从SimpleApplicationEventMulticaster中拿出所有的ApplicationListener，那么这些ApplicationListener何时被注入的呢？容器refresh()时。
org.springframework.context.support.AbstractApplicationContext#registerListeners
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) { // 添加到SimpleApplicationEventMulticaster中 getApplicationEventMulticaster().addApplicationListenerBean(listenerBeanName); } // Publish early application events now that we finally have a multicaster... Set earlyEventsToProcess = this.earlyApplicationEvents; this.earlyApplicationEvents = null; if (!CollectionUtils.isEmpty(earlyEventsToProcess)) { for (ApplicationEvent earlyEvent : earlyEventsToProcess) { getApplicationEventMulticaster().multicastEvent(earlyEvent); } } } @EventListener的原理
@EventListener注解的功能是通过EventListenerMethodProcessor来实现的，EventListenerMethodProcessor这个类在AnnotationConfigApplicationContext的构造方法中被注入。
EventListenerMethodProcessor主要实现了两个接口：SmartInitializingSingleton和BeanFactoryPostProcessor。
先来看看BeanFactoryPostProcessor的postProcessBeanFactory()，这个方法主要是保存beanFactory和eventListenerFactories，后面的方法将会使用到：
public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) { // 保存beanFactory this.beanFactory = beanFactory; /** * EventListenerFactory[DefaultEventListenerFactory]在何处被注入？ * @see AnnotationConfigUtils#registerAnnotationConfigProcessors(org.springframework.beans.factory.support.BeanDefinitionRegistry, java.lang.Object) */ Map beans = beanFactory.getBeansOfType(EventListenerFactory.class, false, false); List factories = new ArrayList<>(beans.values()); AnnotationAwareOrderComparator.sort(factories); // 保存eventListenerFactories this.eventListenerFactories = factories; } 再来看看SmartInitializingSingleton的afterSingletonsInstantiated()方法，这个方法会在所有的bean初始化完后执行。
public void afterSingletonsInstantiated() { ConfigurableListableBeanFactory beanFactory = this.beanFactory; Assert.state(this.beanFactory != null, "No ConfigurableListableBeanFactory set"); String[] beanNames = beanFactory.getBeanNamesForType(Object.class); for (String beanName : beanNames) { if (!ScopedProxyUtils.isScopedTarget(beanName)) { // 目标类的类型 Class type = null; try { type = AutoProxyUtils.determineTargetClass(beanFactory, beanName); } catch (Throwable ex) { // An unresolvable bean type, probably from a lazy bean - let's ignore it. if (logger.isDebugEnabled()) { logger.debug("Could not resolve target class for bean with name '" + beanName + "'", ex); } } if (type != null) { if (ScopedObject.class.isAssignableFrom(type)) { try { Class targetClass = AutoProxyUtils.determineTargetClass( beanFactory, ScopedProxyUtils.getTargetBeanName(beanName)); if (targetClass != null) { type = targetClass; } } catch (Throwable ex) { // An invalid scoped proxy arrangement - let's ignore it. if (logger.isDebugEnabled()) { logger.debug("Could not resolve target bean for scoped proxy '" + beanName + "'", ex); } } } try { // 处理目标对象 processBean(beanName, type); } catch (Throwable ex) { throw new BeanInitializationException("Failed to process @EventListener " + "annotation on bean with name '" + beanName + "'", ex); } } } } } private void processBean(final String beanName, final Class targetType) { if (!this.nonAnnotatedClasses.contains(targetType) && AnnotationUtils.isCandidateClass(targetType, EventListener.class) && !isSpringContainerClass(targetType)) { Map annotatedMethods = null; try { // 获得类中所有的带有@EventListener注解的方法 annotatedMethods = MethodIntrospector.selectMethods(targetType, (MethodIntrospector.MetadataLookup) method -> AnnotatedElementUtils.findMergedAnnotation(method, EventListener.class)); } catch (Throwable ex) { // An unresolvable type in a method signature, probably from a lazy bean - let's ignore it. if (logger.isDebugEnabled()) { logger.debug("Could not resolve methods for bean with name '" + beanName + "'", ex); } } if (CollectionUtils.isEmpty(annotatedMethods)) { this.nonAnnotatedClasses.add(targetType); if (logger.isTraceEnabled()) { logger.trace("No @EventListener annotations found on bean class: " + targetType.getName()); } } else { // Non-empty set of methods ConfigurableApplicationContext context = this.applicationContext; Assert.state(context != null, "No ApplicationContext set"); List factories = this.eventListenerFactories; Assert.state(factories != null, "EventListenerFactory List not initialized"); for (Method method : annotatedMethods.keySet()) { for (EventListenerFactory factory : factories) { if (factory.supportsMethod(method)) { Method methodToUse = AopUtils.selectInvocableMethod(method, context.getType(beanName)); // 使用factory创建一个ApplicationListener ApplicationListener applicationListener = factory.createApplicationListener(beanName, targetType, methodToUse); if (applicationListener instanceof ApplicationListenerMethodAdapter) { ((ApplicationListenerMethodAdapter) applicationListener).init(context, this.evaluator); } // 将ApplicationListener添加到Spring容器中 context.addApplicationListener(applicationListener); break; } } } if (logger.isDebugEnabled()) { logger.debug(annotatedMethods.size() + " @EventListener methods processed on bean '" + beanName + "': " + annotatedMethods); } } } } SpringBoot中的事件
SpringBoot在启动时会按以下顺序发送消息：
- ApplicationStartingEvent：在运行开始时发送 ，但在进行任何处理之前（侦听器和初始化程序的注册除外）发送
- ApplicationEnvironmentPreparedEvent：当被发送Environment到中已知的上下文中使用，但是在创建上下文之前
- ApplicationContextInitializedEvent：在ApplicationContext准备好且已调用ApplicationContextInitializers之后但任何bean定义未加载之前发送
- ApplicationPreparedEvent：在刷新开始之前但在加载bean定义之后发送
- ApplicationStartedEvent：上下文已被刷新后发送，但是任何应用程序和命令行都被调用前
- ApplicationReadyEvent：在所有的命令行应用启动后发送此事件，可以处理请求
- ApplicationFailedEvent：在启动时异常发送
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