EventListener原理

目录

• 目录
• 流程
• initApplicationEventMulticaster()
• registerListeners()
• finishRefresh()
• publishEvent()
• @EventListener处理

流程

容器的刷新流程如下：

public void refresh() throws BeansException, IllegalStateException { synchronized (this.startupShutdownMonitor) { // Prepare this context for refreshing. prepareRefresh(); // Tell the subclass to refresh the internal bean factory. ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory(); // Prepare the bean factory for use in this context. prepareBeanFactory(beanFactory); try { // Allows post-processing of the bean factory in context subclasses. postProcessBeanFactory(beanFactory); // Invoke factory processors registered as beans in the context. invokeBeanFactoryPostProcessors(beanFactory); // Register bean processors that intercept bean creation. registerBeanPostProcessors(beanFactory); // Initialize message source for this context. initMessageSource(); // Initialize event multicaster for this context. initApplicationEventMulticaster(); // Initialize other special beans in specific context subclasses. onRefresh(); // Check for listener beans and register them. registerListeners(); // Instantiate all remaining (non-lazy-init) singletons. finishBeanFactoryInitialization(beanFactory); // Last step: publish corresponding event. finishRefresh(); } finally { // Reset common introspection caches in Spring's core, since we // might not ever need metadata for singleton beans anymore... resetCommonCaches(); } } } 

其中与EventListener有关联的步骤

• initApplicationEventMulticaster(); 初始化事件多播器
• registerListeners(); 注册Listener到多播器
• finishBeanFactoryInitialization(beanFactory); 涉及将@EventListener转为普通Listener
• finishRefresh(); 发布容器刷新完成事件ContextRefreshedEvent

initApplicationEventMulticaster()

protected void initApplicationEventMulticaster() { ConfigurableListableBeanFactory beanFactory = getBeanFactory(); //找applicationEventMulticaster的组件 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 { //没有就创建一个 this.applicationEventMulticaster = new SimpleApplicationEventMulticaster(beanFactory); //注册到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 + "]"); } } } 

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) { getApplicationEventMulticaster().addApplicationListenerBean(listenerBeanName); } // Publish early application events now that we finally have a multicaster... Set 
   
     earlyEventsToProcess = this.earlyApplicationEvents; this.earlyApplicationEvents = null; if (earlyEventsToProcess != null) { for (ApplicationEvent earlyEvent : earlyEventsToProcess) { getApplicationEventMulticaster().multicastEvent(earlyEvent); } } } 
   

finishRefresh()

protected void finishRefresh() { // Clear context-level resource caches (such as ASM metadata from scanning). clearResourceCaches(); // Initialize lifecycle processor for this context. initLifecycleProcessor(); // Propagate refresh to lifecycle processor first. getLifecycleProcessor().onRefresh(); // Publish the final event. //发布事件 publishEvent(new ContextRefreshedEvent(this)); // Participate in LiveBeansView MBean, if active. LiveBeansView.registerApplicationContext(this); } 

publishEvent() 发布事件，我们也可以手动调用容器的publishEvent() 方法来发布事件

ApplicationContext.publishEvent(new MyEvent("test")); 

publishEvent()

protected void publishEvent(Object event, @Nullable ResolvableType eventType) { // Decorate event as an ApplicationEvent if necessary ApplicationEvent applicationEvent; if (event instanceof ApplicationEvent) { applicationEvent = (ApplicationEvent) event; } else { applicationEvent = new PayloadApplicationEvent<>(this, event); if (eventType == null) { eventType = ((PayloadApplicationEvent) applicationEvent).getResolvableType(); } } // Multicast right now if possible - or lazily once the multicaster is initialized if (this.earlyApplicationEvents != null) { this.earlyApplicationEvents.add(applicationEvent); } else { //获取多播器 getApplicationEventMulticaster().multicastEvent(applicationEvent, eventType); } // Publish event via parent context as well... //父容器发布事件 if (this.parent != null) { if (this.parent instanceof AbstractApplicationContext) { ((AbstractApplicationContext) this.parent).publishEvent(event, eventType); } else { this.parent.publishEvent(event); } } } 

@EventListener处理

原理：通过EventListenerMethodProcessor来处理@Eventlstener

public class EventListenerMethodProcessor implements SmartInitializingSingleton, ApplicationContextAware 

EventListenerMethodProcessor 是一个 SmartInitializingSingleton。

SmartInitializingSingleton什么时候执行呢？

在Refresh()

​ -> finishBeanFactoryInitialization(beanFactory);

​ -> DefaultListableBeanFactory#preInstantiateSingletons

public void preInstantiateSingletons() throws BeansException { // Iterate over a copy to allow for init methods which in turn register new bean definitions. // While this may not be part of the regular factory bootstrap, it does otherwise work fine. List 
   
     beanNames = new ArrayList<>(this.beanDefinitionNames); // Trigger initialization of all non-lazy singleton beans... for (String beanName : beanNames) { RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName); if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) { if (isFactoryBean(beanName)) { Object bean = getBean(FACTORY_BEAN_PREFIX + beanName); if (bean instanceof FactoryBean) { final FactoryBean 
     factory = (FactoryBean 
    ) bean; boolean isEagerInit; if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) { isEagerInit = AccessController.doPrivileged((PrivilegedAction 
    
      ) ((SmartFactoryBean 
     ) factory)::isEagerInit, getAccessControlContext()); } else { isEagerInit = (factory instanceof SmartFactoryBean && ((SmartFactoryBean 
     ) factory).isEagerInit()); } if (isEagerInit) { getBean(beanName); } } } else { // 创建bean getBean(beanName); } } } // Trigger post-initialization callback for all applicable beans... for (String beanName : beanNames) { Object singletonInstance = getSingleton(beanName); if (singletonInstance instanceof SmartInitializingSingleton) { final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance; if (System.getSecurityManager() != null) { AccessController.doPrivileged((PrivilegedAction 
      
     
   

preInstantiateSingletons 前半部分主要是遍历beanNames 创建Bean。创建完bean后判断各bean是不是SmartInitializingSingleton，如果是则执行 smartSingleton.afterSingletonsInstantiated()方法。

//org.springframework.context.event.EventListenerMethodProcessor#afterSingletonsInstantiated public void afterSingletonsInstantiated() { List 
   
     factories = getEventListenerFactories(); ConfigurableApplicationContext context = getApplicationContext(); String[] beanNames = context.getBeanNamesForType(Object.class); for (String beanName : beanNames) { if (!ScopedProxyUtils.isScopedTarget(beanName)) { Class 
     type = null; try { type = AutoProxyUtils.determineTargetClass(context.getBeanFactory(), beanName); } if (type != null) { if (ScopedObject.class.isAssignableFrom(type)) { try { Class 
     targetClass = AutoProxyUtils.determineTargetClass( context.getBeanFactory(), ScopedProxyUtils.getTargetBeanName(beanName)); if (targetClass != null) { type = targetClass; } } } try { //处理bean processBean(factories, beanName, type); } } } } } 
   

protected void processBean( final List 
   
     factories, final String beanName, final Class 
     targetType) { //没有注解的class if (!this.nonAnnotatedClasses.contains(targetType)) { Map 
    
      annotatedMethods = null; try { //查找被注解EventListener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)) { //添加到没有注解的集合 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 = getApplicationContext(); for (Method method : annotatedMethods.keySet()) { for (EventListenerFactory factory : factories) { if (factory.supportsMethod(method)) { Method methodToUse = AopUtils.selectInvocableMethod(method, context.getType(beanName)); //创建applicationListener，通过Adapter将注解形式的listener转换为普通的listener ApplicationListener 
       applicationListener = factory.createApplicationListener(beanName, targetType, methodToUse); if (applicationListener instanceof ApplicationListenerMethodAdapter) { ((ApplicationListenerMethodAdapter) applicationListener).init(context, this.evaluator); } //添加listner到applicationContext context.addApplicationListener(applicationListener); break; } } } if (logger.isDebugEnabled()) { logger.debug(annotatedMethods.size() + " @EventListener methods processed on bean '" + beanName + "': " + annotatedMethods); } } } } 
      
     
   

• 查找类中标注@EventListener的方法
• EventListenerFactory.createApplicationListener(beanName, targetType, methodToUse) 构造listener
• 添加listener到Context中
  • 如果有applicationEventMulticaster，添加到ApplicationContext.applicationEventMulticaster中
  • 如果没有applicationEventMulticaster，添加到ApplicationContext.applicationListeners中。