1. 使用Spring 事件

Spring事件体系包括三个组件：事件，事件监听器，事件发布操作

事件

Event	说明
ContextRefreshedsEvent	当容器被实例化或refresh时发布,如调用refresh()方法,此处的实例化是指所有的对象都已被加载,后置处理器都被激活,所有单例bean都被实例化,所有的容器对象都已准备好可使用,如果容器支持热重载,则refresh可以被触发多次(XmlWebApplicationContext支持热刷新,而CenericApplicationContext则不支持)
ContextStartedEvent	当容器启动时发布,即调用start()方法,已启动意味着所有的Lifecycle Bean都已显示接收到了start信号
ContextstoppedEvent	当容器停止时发布,即调用stop()方法,即所有的Lifecycle Bean都已显示接收到了stop信号,关闭的容器可以通过start()方法重启
ContextclosedEvent	当容器关闭时发布,即调用close方法,关闭意味着所有的单例bea都已被销毁,关闭的容器不能被重启或者refresh
ContextHandledEvent	这只在使用Spring的DispatcherServlet时有效,当一个请求被处理完成时发布

自定义事件

事件类需要继承ApplicationEvent，代码如下

public class BigEvent  extends ApplicationEvent {private String name;public BigEvent(Object source, String name) {super(source);this.name = name;}public String getName() {return name;}
}

事件监听器

事件监听器-基于接口

@Component
public class HelloEventListener implements ApplicationListener<OrderEvent> {@Overridepublic void onApplicationEvent(OrderEvent event) {if(event.getName().equals("减库存")){System.out.println("减库存.......");}}
}

• 事件监听器需要实现ApplicationListener接口，这是个泛型接口，泛型类类型就是事件类型，其次需要是spring容器托管的bean，所以这里加了@component，只有一个方法，就是onApplicationEvent。

事件监听器-基于注解

@Component
public class OrderEventListener {@EventListener(OrderEvent.class)public void onApplicationEvent(OrderEvent event) {if(event.getName().equals("减库存")){System.out.println("减库存.......");}}}

事件发布操作

   applicationContext.publishEvent(new HelloEvent(this,"lgb")); 

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疑问

1. 同样的事件能有多个监听器 – 可以的
2. 事件监听器一定要写一个类去实现吗 – 其实是可以不需要的，spring有个注解@EventListener，修饰在方法上，稍后给出使用方法
3. 事件监听操作和发布事件的操作是同步的吗？ – 是的，所以如果有事务，监听操作也在事务内
4. 可以作为异步处理吗？ --可以 看源码有解释。：

@Bean(name = "applicationEventMulticaster")
public ApplicationEventMulticaster simpleApplicationEventMulticaster() {SimpleApplicationEventMulticaster eventMulticaster= new SimpleApplicationEventMulticaster();eventMulticaster.setTaskExecutor(new SimpleAsyncTaskExecutor());return eventMulticaster;
}

2.Spring事件原理

原理：观察者模式

spring的事件监听有三个部分组成：

• • 事件（ApplicationEvent) 负责对应相应监听器 事件源发生某事件是特定事件监听器被触发的原因。
  • 监听器(ApplicationListener) 对应于观察者模式中的观察者。监听器监听特定事件,并在内部定义了事件发生后的响应逻辑。
  • 事件发布器（ApplicationEventMulticaster ）对应于观察者模式中的被观察者/主题， 负责通知观察者 对外提供发布事件和增删事件监听器的接口,维护事件和事件监听器之间的映射关系,并在事件发生时负责通知相关监听器。

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Spring事件机制是观察者模式的一种实现，但是除了发布者和监听者者两个角色之外，还有一个EventMultiCaster的角色负责把事件转发给监听者，工作流程如下：

也就是说上面代码中发布者调用applicationEventPublisher.publishEvent(msg); 是会将事件发送给了EventMultiCaster， 而后由EventMultiCaster注册着所有的Listener，然后根据事件类型决定转发给那个Listener。

AbstractApplicationContext拥有一个applicationEventMulticaster成员变量，applicationEventMulticaster提供了容器监听器的注册表。

AbstractApplicationContext在refresh()这个容器启动方法中搭建了事件的基础设施

1 事件广播器的初始化

//创建事件多播器,负责调用事件对应的监听器,管理所有的监听器
protected void initApplicationEventMulticaster() {ConfigurableListableBeanFactory beanFactory = getBeanFactory();if (beanFactory.containsLocalBean(APPLICATION_EVENT_MULTICASTER_BEAN_NAME)) {//创建bean实例this.applicationEventMulticaster =beanFactory.getBean(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, ApplicationEventMulticaster.class);}else {//创建bean实例(默认)this.applicationEventMulticaster = new SimpleApplicationEventMulticaster(beanFactory);beanFactory.registerSingleton(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, this.applicationEventMulticaster);}
}

• 用户可以在配置文件中为容器定义一个自定义的事件广播器，只要实现ApplicationEventMulticaster就可以了，Spring会通过反射的机制将其注册成容器的事件广播器，如果没有找到配置的外部事件广播器，Spring自动使用 SimpleApplicationEventMulticaster作为事件广播器。

2 注册事件监听器

//把我们的事件监听器注册到多播器上
protected void registerListeners() {//手动调用AnnotationConfigApplicationContext.setApplicationListener()注册进来的for (ApplicationListener<?> listener : getApplicationListeners()) {getApplicationEventMulticaster().addApplicationListener(listener);}//ApplicationListener所有实现了ApplicationListener接口的bean定义String[] listenerBeanNames = getBeanNamesForType(ApplicationListener.class, true, false);for (String listenerBeanName : listenerBeanNames) {//把监听器的名称注册到我们的多播器上,接口方式的监听就是在这里注册到多播器里面的//为了防止懒加载的监听器//ApplicationListenerDetector解析接口方式的监听器.在postProcessAfterInitialization方法中在初始化后调用,如果实现了加了@Lazy注解,那么在后置处理器里面就不会处理,要靠当前方法防止漏掉 	
getApplicationEventMulticaster().addApplicationListenerBean(listenerBeanName);}//获取早期事件Set<ApplicationEvent> earlyEventsToProcess = this.earlyApplicationEvents;this.earlyApplicationEvents = null;if (earlyEventsToProcess != null) {for (ApplicationEvent earlyEvent : earlyEventsToProcess) {//通过多播器进行播发早期事件getApplicationEventMulticaster().multicastEvent(earlyEvent);}}
}

• Spring根据反射机制，使用ListableBeanFactory的getBeansOfType方法，从BeanDefinitionRegistry中找出所有实现 org.springframework.context.ApplicationListener的Bean，将它们注册为容器的事件监听器，实际的操作就是将其添加到事件广播器所提供的监听器注册表中。

3 发布事件

跟着 finishRefresh();方法进入publishEvent(new ContextRefreshedEvent(this));方法如下:

protected void publishEvent(Object event, ResolvableType eventType) {Assert.notNull(event, "Event must not be null");if (logger.isTraceEnabled()) {logger.trace("Publishing event in " + getDisplayName() + ": " + event);}// Decorate event as an ApplicationEvent if necessaryApplicationEvent applicationEvent;if (event instanceof ApplicationEvent) {applicationEvent = (ApplicationEvent) event;}else {applicationEvent = new PayloadApplicationEvent<Object>(this, event);if (eventType == null) {eventType = ((PayloadApplicationEvent)applicationEvent).getResolvableType();}}// Multicast right now if possible - or lazily once the multicaster is initializedif (this.earlyApplicationEvents != null) {this.earlyApplicationEvents.add(applicationEvent);}else {getApplicationEventMulticaster().multicastEvent(applicationEvent, eventType);}// Publish event via parent context as well...if (this.parent != null) {if (this.parent instanceof AbstractApplicationContext) {((AbstractApplicationContext) this.parent).publishEvent(event, eventType);}else {this.parent.publishEvent(event);}}
}

• 在AbstractApplicationContext的publishEvent方法中， Spring委托ApplicationEventMulticaster将事件通知给所有的事件监听器.

4 Spring默认的事件广播器SimpleApplicationEventMulticaster

@Override
public void multicastEvent(final ApplicationEvent event, ResolvableType eventType) {ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));//从多播器里面取出所有的监听器for (final ApplicationListener<?> listener : getApplicationListeners(event, type)) {//判断多播器是否支持异步多播Executor executor = getTaskExecutor();if (executor != null) {//异步多播executor.execute(new Runnable() {@Overridepublic void run() {invokeListener(listener, event);}});}else {//同步invokeListener(listener, event);}}
}@SuppressWarnings({"unchecked", "rawtypes"})
protected void invokeListener(ApplicationListener listener, ApplicationEvent event) {ErrorHandler errorHandler = getErrorHandler();if (errorHandler != null) {try {//直接调用//注解版会调用ApplicationListenerMethodAdapte,通过反射的机制调用listener.onApplicationEvent(event);}catch (Throwable err) {errorHandler.handleError(err);}}else {try {listener.onApplicationEvent(event);}catch (ClassCastException ex) {// Possibly a lambda-defined listener which we could not resolve the generic event type forLogFactory.getLog(getClass()).debug("Non-matching event type for listener: " + listener, ex);}}
}protected Collection<ApplicationListener<?>> getApplicationListeners(ApplicationEvent event, ResolvableType eventType) {Object source = event.getSource();Class<?> sourceType = (source != null ? source.getClass() : null);ListenerCacheKey cacheKey = new ListenerCacheKey(eventType, sourceType);// Quick check for existing entry on ConcurrentHashMap...ListenerRetriever retriever = this.retrieverCache.get(cacheKey);if (retriever != null) {return retriever.getApplicationListeners();}if (this.beanClassLoader == null ||(ClassUtils.isCacheSafe(event.getClass(), this.beanClassLoader) &&(sourceType == null || ClassUtils.isCacheSafe(sourceType, this.beanClassLoader)))) {// Fully synchronized building and caching of a ListenerRetrieversynchronized (this.retrievalMutex) {retriever = this.retrieverCache.get(cacheKey);if (retriever != null) {return retriever.getApplicationListeners();}retriever = new ListenerRetriever(true);//根据事件类型解析出所有的对应的监听器Collection<ApplicationListener<?>> listeners =retrieveApplicationListeners(eventType, sourceType, retriever);this.retrieverCache.put(cacheKey, retriever);return listeners;}}else {// No ListenerRetriever caching -> no synchronization necessaryreturn retrieveApplicationListeners(eventType, sourceType, null);}}private Collection<ApplicationListener<?>> retrieveApplicationListeners(ResolvableType eventType, @Nullable Class<?> sourceType, @Nullable ListenerRetriever retriever) {List<ApplicationListener<?>> allListeners = new ArrayList<>();Set<ApplicationListener<?>> listeners;Set<String> listenerBeans;synchronized (this.retrievalMutex) {//这个集合保存的是通过Bean的后置处理器注册进来的事件监听器listeners = new LinkedHashSet<>(this.defaultRetriever.applicationListeners);//这个集合保存的是通过registerListeners注册的事件监听器listenerBeans = new LinkedHashSet<>(this.defaultRetriever.applicationListenerBeans);}//循环for (ApplicationListener<?> listener : listeners) {//如果支持这个类型,就放到集合中if (supportsEvent(listener, eventType, sourceType)) {if (retriever != null) {retriever.applicationListeners.add(listener);}allListeners.add(listener);}}//循环,因为这里面保存的保存的只是一个名字,所以需要创建Beanif (!listenerBeans.isEmpty()) {ConfigurableBeanFactory beanFactory = getBeanFactory();for (String listenerBeanName : listenerBeans) {try {//如果支持.创建Bean.放到集合中if (supportsEvent(beanFactory, listenerBeanName, eventType)) {ApplicationListener<?> listener =beanFactory.getBean(listenerBeanName, ApplicationListener.class);if (!allListeners.contains(listener) && supportsEvent(listener, eventType, sourceType)) {if (retriever != null) {if (beanFactory.isSingleton(listenerBeanName)) {retriever.applicationListeners.add(listener);}else {retriever.applicationListenerBeans.add(listenerBeanName);}}allListeners.add(listener);}}else {// Remove non-matching listeners that originally came from// ApplicationListenerDetector, possibly ruled out by additional// BeanDefinition metadata (e.g. factory method generics) above.Object listener = beanFactory.getSingleton(listenerBeanName);if (retriever != null) {retriever.applicationListeners.remove(listener);}allListeners.remove(listener);}}catch (NoSuchBeanDefinitionException ex) {// Singleton listener instance (without backing bean definition) disappeared -// probably in the middle of the destruction phase}}}AnnotationAwareOrderComparator.sort(allListeners);if (retriever != null && retriever.applicationListenerBeans.isEmpty()) {retriever.applicationListeners.clear();retriever.applicationListeners.addAll(allListeners);}return allListeners;}

• 遍历注册的每个监听器，并启动来调用每个监听器的onApplicationEvent方法。由于SimpleApplicationEventMulticaster的taskExecutor的实现类是SyncTaskExecutor，因此，事件监听器对事件的处理，是同步进行的。

从代码可以看出，applicationContext.publishEvent()方法，需要同步等待各个监听器处理完之后，才返回。

也就是说，Spring提供的事件机制，默认是同步的。如果想用异步的，可以自己实现ApplicationEventMulticaster接口，并在Spring容器中注册id为applicationEventMulticaster的Bean。例如下面所示:

public class AsyncApplicationEventMulticaster extends AbstractApplicationEventMulticaster {  private TaskExecutor taskExecutor = new SimpleAsyncTaskExecutor();  public void setTaskExecutor(TaskExecutor taskExecutor) {  this.taskExecutor = (taskExecutor != null ? taskExecutor : new SimpleAsyncTaskExecutor());  }  protected TaskExecutor getTaskExecutor() {  return this.taskExecutor;  }  @SuppressWarnings("unchecked")  public void multicastEvent(final ApplicationEvent event) {  for (Iterator<ApplicationListener> it = getApplicationListeners().iterator(); it.hasNext();) {  final ApplicationListener listener =  it.next();  getTaskExecutor().execute(new Runnable() {  public void run() {  listener.onApplicationEvent(event);  }  });  }  }  
}  

spring配置：

@Bean(name = "applicationEventMulticaster")
public ApplicationEventMulticaster simpleApplicationEventMulticaster() {SimpleApplicationEventMulticaster eventMulticaster= new SimpleApplicationEventMulticaster();//ThreadPoolTaskExecutoreventMulticaster.setTaskExecutor(new SimpleAsyncTaskExecutor());return eventMulticaster;
}

Spring发布事件之后，所有注册的事件监听器，都会收到该事件，因此，事件监听器在处理事件时，需要先判断该事件是否是自己关心的。
Sping事件体系所使用的设计模式是：观察者模式。ApplicationListener是观察者接口，接口中定义了onApplicationEvent方法，该方法的作用是对ApplicationEvent事件进行处理。

注解的事件监听器如果创建

解析@EventListener需要创世纪的类EventListenerMethodProcessor该方法实现了SmartInitializingSineleton接口

该接口会在所有的bean创建完之后,看是否有bean实现了这个接口,实现了就会调用里面的方法(afterSingletonsInstantiated)

@Overridepublic void afterSingletonsInstantiated() {ConfigurableListableBeanFactory beanFactory = this.beanFactory;Assert.state(this.beanFactory != null, "No ConfigurableListableBeanFactory set");String[] beanNames = beanFactory.getBeanNamesForType(Object.class);//处理所有的bean,查找当前Bean标注了@EventListener的方法for (String beanName : beanNames) {processBean(beanName, type);}}private void processBean(final String beanName, final Class<?> targetType) {if (!this.nonAnnotatedClasses.contains(targetType) &&AnnotationUtils.isCandidateClass(targetType, EventListener.class) &&!isSpringContainerClass(targetType)) {Map<Method, EventListener> annotatedMethods = null;try {//查找当前bean标注了@EventListener的方式annotatedMethods = MethodIntrospector.selectMethods(targetType,(MethodIntrospector.MetadataLookup<EventListener>) method ->AnnotatedElementUtils.findMergedAnnotation(method, EventListener.class))else {// Non-empty set of methodsConfigurableApplicationContext context = this.applicationContext;Assert.state(context != null, "No ApplicationContext set");List<EventListenerFactory> 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));//创建事件监听器ApplicationListener<?> applicationListener =factory.createApplicationListener(beanName, targetType, methodToUse);if (applicationListener instanceof ApplicationListenerMethodAdapter) {((ApplicationListenerMethodAdapter) applicationListener).init(context, this.evaluator);}//注册事件到Context中context.addApplicationListener(applicationListener);break;}}}if (logger.isDebugEnabled()) {logger.debug(annotatedMethods.size() + " @EventListener methods processed on bean '" +beanName + "': " + annotatedMethods);}}}}

3.其他方法介绍

//声明早期的监听器和事件,不需要手动调用publishEvent
protected void prepareRefresh() {this.closed.set(false);//设为激活状态,只有此状态,才能getBeanthis.active.set(true);//下面两个方法是配合使用的,如果一个类重新了该方法,在该方法中设置了必须的环境变量//getEnvironment().setRequiredProperties("necessary")initPropertySources();//用来校验我们容器启动时必须依赖的环境变量的值 getEnvironment().validateRequiredProperties();//创建一个早期事件监听器对象if (this.earlyApplicationListeners == null) {this.earlyApplicationListeners = new LinkedHashSet<>(this.applicationListeners);}else {// Reset local application listeners to pre-refresh state.this.applicationListeners.clear();this.applicationListeners.addAll(this.earlyApplicationListeners);}//创建一个容器i用于保存早期待发布的事件集合//什么是早期事件???//就是我们的事件监听器还没有注册到还没有注册到多播器上的时候都成为早期事件//早期事件不需要手动publishEvent发布,在registerListener中会自动发布this.earlyApplicationEvents = new LinkedHashSet<>();}//注册解析接口方式的监听器的bean的后置处理器
protected void prepareBeanFactory(ConfigurableListableBeanFactory beanFactory) {//设置当前bean工厂的类加载器为当前application应用的加载器beanFactory.setBeanClassLoader(getClassLoader());//为Bean工厂设置我们标准的SPEL表达式解析器对象StandardBeanExpressionResolverbeanFactory.setBeanExpressionResolver(new StandardBeanExpressionResolver(beanFactory.getBeanClassLoader()));//为我们的bean工厂设置了一个propertityEditor属性资源编辑器对象(用于给后面的bean对象`夫赋值)beanFactory.addPropertyEditorRegistrar(new ResourceEditorRegistrar(this, getEnvironment()));//注册了一个完整的ApplicationContextAwareProcessor后置处理器,里面会调用很多awarebeanFactory.addBeanPostProcessor(new ApplicationContextAwareProcessor(this));//忽略以下接口的bean的接口函数方法,在populateBean时,因为以下接口都是通过setxxx方法时,这些方法不特殊处理会自动注入到容器中的beanbeanFactory.ignoreDependencyInterface(EnvironmentAware.class);beanFactory.ignoreDependencyInterface(EmbeddedValueResolverAware.class);beanFactory.ignoreDependencyInterface(ResourceLoaderAware.class);beanFactory.ignoreDependencyInterface(ApplicationEventPublisherAware.class);beanFactory.ignoreDependencyInterface(MessageSourceAware.class);beanFactory.ignoreDependencyInterface(ApplicationContextAware.class);//为什么可以在一个类里面@Autowired自动装配下面四个,就因为在这里设置了//当注册了依赖解析后,如果当注册了对BeanFactory.class的解析依赖后,当bean的属性注入的时候,一旦检测到属性为BeanFactory类型会将beanFactory的实例赋值beanFactory.registerResolvableDependency(BeanFactory.class, beanFactory);beanFactory.registerResolvableDependency(ResourceLoader.class, this);beanFactory.registerResolvableDependency(ApplicationEventPublisher.class, this);beanFactory.registerResolvableDependency(ApplicationContext.class, this);//注册了一个事件监听器探测器后置处理器接口,ApplicationListenerDetector解析接口方式的监听器.在postProcessAfterInitialization方法中在初始化后调用//在这里也会注册监听器beanFactory.addBeanPostProcessor(new ApplicationListenerDetector(this));// 设置环境if (beanFactory.containsBean(LOAD_TIME_WEAVER_BEAN_NAME)) {beanFactory.addBeanPostProcessor(new LoadTimeWeaverAwareProcessor(beanFactory));// Set a temporary ClassLoader for type matching.beanFactory.setTempClassLoader(new ContextTypeMatchClassLoader(beanFactory.getBeanClassLoader()));}// 设置环境系统属性if (!beanFactory.containsLocalBean(ENVIRONMENT_BEAN_NAME)) {beanFactory.registerSingleton(ENVIRONMENT_BEAN_NAME, getEnvironment());}if (!beanFactory.containsLocalBean(SYSTEM_PROPERTIES_BEAN_NAME)) {beanFactory.registerSingleton(SYSTEM_PROPERTIES_BEAN_NAME, getEnvironment().getSystemProperties());}//设置系统环境if (!beanFactory.containsLocalBean(SYSTEM_ENVIRONMENT_BEAN_NAME)) {beanFactory.registerSingleton(SYSTEM_ENVIRONMENT_BEAN_NAME, getEnvironment().getSystemEnvironment());}}

if (!beanFactory.containsLocalBean(ENVIRONMENT_BEAN_NAME)) {beanFactory.registerSingleton(ENVIRONMENT_BEAN_NAME, getEnvironment());}if (!beanFactory.containsLocalBean(SYSTEM_PROPERTIES_BEAN_NAME)) {beanFactory.registerSingleton(SYSTEM_PROPERTIES_BEAN_NAME, getEnvironment().getSystemProperties());}//设置系统环境if (!beanFactory.containsLocalBean(SYSTEM_ENVIRONMENT_BEAN_NAME)) {beanFactory.registerSingleton(SYSTEM_ENVIRONMENT_BEAN_NAME, getEnvironment().getSystemEnvironment());}
}