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1. 初始化监听器

SpringBoot启动需要环境配置，在这个步骤之前Spring容器并没有刷新，无法获取自定义的监听器。所以在SpringApplication的构造器中，在spring.factories文件中定义了监听器。当然用户也能通过@Component定义监听器监听ApplicationStartedEvent事件。

public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {this.resourceLoader = resourceLoader;Assert.notNull(primarySources, "PrimarySources must not be null");this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));this.webApplicationType = WebApplicationType.deduceFromClasspath();setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));this.mainApplicationClass = deduceMainApplicationClass();
}

在创建SpringApplication构造器的时候，通过ApplicationListener类名对应的key获取到监听器的类名，并且实例化，保存到SprngApplication.listeners

2. 创建事件发布器 SpringApplicationRunListeners

在SpringApplication#run方法中，获取到了SpringApplicationRunListeners，并且对不同的事件进行了分发。

//SpringApplication#getRunListeners
private SpringApplicationRunListeners getRunListeners(String[] args) {Class<?>[] types = new Class<?>[] { SpringApplication.class, String[].class };return new SpringApplicationRunListeners(logger,getSpringFactoriesInstances(SpringApplicationRunListener.class, types, this, args));
}//EventPublishingRunListener#EventPublishingRunListener
public EventPublishingRunListener(SpringApplication application, String[] args) {this.application = application;this.args = args;this.initialMulticaster = new SimpleApplicationEventMulticaster();for (ApplicationListener<?> listener : application.getListeners()) {this.initialMulticaster.addApplicationListener(listener);}
}//AbstractApplicationEventMulticaster#addApplicationListener
public void addApplicationListener(ApplicationListener<?> listener) {synchronized (this.retrievalMutex) {// Explicitly remove target for a proxy, if registered already,// in order to avoid double invocations of the same listener.Object singletonTarget = AopProxyUtils.getSingletonTarget(listener);if (singletonTarget instanceof ApplicationListener) {this.defaultRetriever.applicationListeners.remove(singletonTarget);}this.defaultRetriever.applicationListeners.add(listener);this.retrieverCache.clear();}
}

实例化SpringApplicationRunListener对应的类名，也就是EventPublishingRunListener。其实真正负责事件发布的是initialMulticaster（SimpleApplicationEventMulticaster），从springApplication中拿到已经初始化完毕的监听器，放入到initialMulticaster，以便回调监听器。

	void starting() {for (SpringApplicationRunListener listener : this.listeners) {listener.starting();}}void environmentPrepared(ConfigurableEnvironment environment) {for (SpringApplicationRunListener listener : this.listeners) {listener.environmentPrepared(environment);}}void contextPrepared(ConfigurableApplicationContext context) {for (SpringApplicationRunListener listener : this.listeners) {listener.contextPrepared(context);}}void contextLoaded(ConfigurableApplicationContext context) {for (SpringApplicationRunListener listener : this.listeners) {listener.contextLoaded(context);}}void started(ConfigurableApplicationContext context) {for (SpringApplicationRunListener listener : this.listeners) {listener.started(context);}}void running(ConfigurableApplicationContext context) {for (SpringApplicationRunListener listener : this.listeners) {listener.running(context);}}void failed(ConfigurableApplicationContext context, Throwable exception) {for (SpringApplicationRunListener listener : this.listeners) {callFailedListener(listener, context, exception);}}

可以看到，SpringApplicationRunListeners中分发了很多事件，我们就拿environmentPrepared举例。

3. 事件分发流程

3.1 SimpleApplicationEventMulticaster#multicastEvent

@Override
public void multicastEvent(ApplicationEvent event) {multicastEvent(event, resolveDefaultEventType(event));
}@Override
public void multicastEvent(final ApplicationEvent event, @Nullable ResolvableType eventType) {ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));Executor executor = getTaskExecutor();for (ApplicationListener<?> listener : getApplicationListeners(event, type)) {if (executor != null) {executor.execute(() -> invokeListener(listener, event));}else {invokeListener(listener, event);}}
}//回调监听器
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;}}
}

1. 解析事件的类型。
2. 获取到监听器，这里需要通过不同的事件类型去匹配，不同的监听器处理不同的事件。
3. 这里可配置线程池执行器Executor，如果有Executor，那么就通过线程的方式去回调监听器onApplicationEvent方法。

3.2 获取监听器 AbstractApplicationEventMulticaster#getApplicationListeners

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);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);}
}

先从缓存中获取，缓存有，就直接返回；缓存没有，就通过retrieveApplicationListeners返回监听器。缓存key为事件类型ApplicationEnvironmentPreparedEvent和springApplication。

3.3 AbstractApplicationEventMulticaster#retrieveApplicationListeners

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) {listeners = new LinkedHashSet<>(this.defaultRetriever.applicationListeners);listenerBeans = new LinkedHashSet<>(this.defaultRetriever.applicationListenerBeans);}// Add programmatically registered listeners, including ones coming// from ApplicationListenerDetector (singleton beans and inner beans).for (ApplicationListener<?> listener : listeners) {if (supportsEvent(listener, eventType, sourceType)) {if (retriever != null) {retriever.applicationListeners.add(listener);}allListeners.add(listener);}}// Add listeners by bean name, potentially overlapping with programmatically// registered listeners above - but here potentially with additional metadata.if (!listenerBeans.isEmpty()) {ConfigurableBeanFactory beanFactory = getBeanFactory();for (String listenerBeanName : listenerBeans) {try {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;
}

1. 循环所有的listeners，调用supportsEvent方法判断listener是否支持当前的事件，支持就放入retriever里面。
2. listenerBeans原理一样，只是从Spring容器中拿到所属的bean而已。

3.4 AbstractApplicationEventMulticaster#supportsEvent

这是真正判断事件与监听器匹配关系的方法

protected boolean supportsEvent(ApplicationListener<?> listener, ResolvableType eventType, @Nullable Class<?> sourceType) {GenericApplicationListener smartListener = (listener instanceof GenericApplicationListener ?(GenericApplicationListener) listener : new GenericApplicationListenerAdapter(listener));return (smartListener.supportsEventType(eventType) && smartListener.supportsSourceType(sourceType));
}

1. 当前的监听器是否属于GenericApplicationListener 类型，不是就包装成GenericApplicationListenerAdapter。

public GenericApplicationListenerAdapter(ApplicationListener<?> delegate) {Assert.notNull(delegate, "Delegate listener must not be null");this.delegate = (ApplicationListener<ApplicationEvent>) delegate;this.declaredEventType = resolveDeclaredEventType(this.delegate);
}
//resolveDeclaredEventType 解析监听器中声明的泛型类型
@Nullable
private static ResolvableType resolveDeclaredEventType(ApplicationListener<ApplicationEvent> listener) {ResolvableType declaredEventType = resolveDeclaredEventType(listener.getClass());if (declaredEventType == null || declaredEventType.isAssignableFrom(ApplicationEvent.class)) {Class<?> targetClass = AopUtils.getTargetClass(listener);if (targetClass != listener.getClass()) {declaredEventType = resolveDeclaredEventType(targetClass);}}return declaredEventType;
}@Nullable
static ResolvableType resolveDeclaredEventType(Class<?> listenerType) {ResolvableType eventType = eventTypeCache.get(listenerType);if (eventType == null) {eventType = ResolvableType.forClass(listenerType).as(ApplicationListener.class).getGeneric();eventTypeCache.put(listenerType, eventType);}return (eventType != ResolvableType.NONE ? eventType : null);
}

值得注意的是，在GenericApplicationListenerAdapter构造器中，resolveDeclaredEventType方法解析了监听器中声明的泛型。方便后面将监听器中声明的泛型与事件的类型进行对比，如果支持，就说明这个监听器需要被回调。
2. 判断监听器会否支持类型，并且监听器是否支持sourceType，也就是springApplication，默认支持。

public boolean supportsEventType(ResolvableType eventType) {if (this.delegate instanceof SmartApplicationListener) {Class<? extends ApplicationEvent> eventClass = (Class<? extends ApplicationEvent>) eventType.resolve();return (eventClass != null && ((SmartApplicationListener) this.delegate).supportsEventType(eventClass));}else {return (this.declaredEventType == null || this.declaredEventType.isAssignableFrom(eventType));}
}

3. 获取到事件所对应的监听器已结束，回调监听器。