Spring Boot 监听器详解

public class MyApplicationRunListener implements SpringApplicationRunListener {

    @Override
    public void starting(ConfigurableBootstrapContext bootstrapContext) {
        System.out.println("Application 启动");
    }

    @Override
    public void environmentPrepared(ConfigurableBootstrapContext bootstrapContext, ConfigurableEnvironment environment) {
        System.out.println("环境已准备完毕");
    }

    @Override
    public void contextPrepared(ConfigurableApplicationContext context) {
        System.out.println("在创建和准备ApplicationContext之后，但在加载源之前调用");
    }

    @Override
    public void contextLoaded(ConfigurableApplicationContext context) {
        System.out.println("上下文准备完毕，未刷新");
    }

    @Override
    public void started(ConfigurableApplicationContext context, Duration timeTaken) {
        System.out.println("上下文已刷新，应用程序已启动，但尚未调用CommandLineRunners和ApplicationRunners");
    }

    @Override
    public void ready(ConfigurableApplicationContext context, Duration timeTaken) {
        System.out.println("在刷新应用程序上下文并且调用了所有CommandLineRunner和ApplicationRunner之后，在运行方法完成之前立即调用");
    }

    @Override
    public void failed(ConfigurableApplicationContext context, Throwable exception) {
        System.out.println("当运行应用程序时发生故障时调用");
    }
}

org.springframework.boot.SpringApplicationRunListener=com.springboot.demo.listeners.MyApplicationRunListener

@Slf4j
public class MyApplicationListener implements ApplicationListener<ApplicationStartedEvent> {

    @Override
    public void onApplicationEvent(ApplicationStartedEvent event) {
        log.info("应用启动完成");
    }
}

org.springframework.context.ApplicationListener=com.springboot.demo.listeners.MyApplicationListener

SpringApplication springApplication = new SpringApplication(SpringBootDemoApplication.class);
springApplication.addListeners(new MyApplicationListener());

context:
  listener:
    classes: com.springboot.demo.listeners.MyApplicationListener

@Component
@Slf4j
public class MyApplicationListener2 {

    //监听单个事件
    @EventListener
    public void listenerApplicationStarted(ApplicationStartedEvent event) {
        log.info("应用启动完成");
    }
    
    @EventListener({ApplicationEnvironmentPreparedEvent.class})
    public void listenerApplicationEnv() {
        //实际测试，没有监听到，后面说明原理
        log.info("监听到了环境准备完成事件");
    }
    
    //监听多个事件
    @EventListener({ApplicationReadyEvent.class, ApplicationStartedEvent.class})
    public void listenerApplication() {
        log.info("监听到了多个事件");
    }

    //自己发布了一个Person事件，Person并没有继承ApplicationEvent
    @EventListener
    public void myCustomListener(Person person) {
        log.info("监听到自己发布的事件,{}", person);
    }

    //只有Person事件中name属性值为csdn时才接收到
    @EventListener(condition = "#person.name == 'csdn'")
    public void myCustomListener2(Person person) {
        log.info("SpEL表达式监听到自己发布的事件,{}", person);
    }
}

private void multicastInitialEvent(ApplicationEvent event) {
	refreshApplicationListeners();
	this.initialMulticaster.multicastEvent(event);
}

private void refreshApplicationListeners() {
		this.application.getListeners().forEach(this.initialMulticaster::addApplicationListener);
	}

public void multicastEvent(ApplicationEvent event) {
	multicastEvent(event, null);
}

@Override
public void multicastEvent(ApplicationEvent event, @Nullable ResolvableType eventType) {
	//获取事件类的类型信息
	ResolvableType type = (eventType != null ? eventType : ResolvableType.forInstance(event));
	// 获取执行事件的线程池，如果设置了，可以异步执行
	Executor executor = getTaskExecutor();
	//获取指定事件类型的监听器集合
	for (ApplicationListener<?> listener : getApplicationListeners(event, type)) {
	//如果定义了执行线程池，则用线程池调用
		if (executor != null) {
			executor.execute(() -> invokeListener(listener, event));
		}
		else {
		//同步调用监听器
			invokeListener(listener, event);
		}
	}
}

protected void invokeListener(ApplicationListener<?> listener, ApplicationEvent event) {
//获取失败处理器
	ErrorHandler errorHandler = getErrorHandler();
	if (errorHandler != null) {
		try {
			doInvokeListener(listener, event);
		}
		catch (Throwable err) {
			errorHandler.handleError(err);
		}
	}
	else {
		doInvokeListener(listener, event);
	}
}

@SuppressWarnings({"rawtypes", "unchecked"})
private void doInvokeListener(ApplicationListener listener, ApplicationEvent event) {
	try {
	//此处执行事件监听器的onApplicationEvent方法
		listener.onApplicationEvent(event);
	}
	catch (ClassCastException ex) {
		String msg = ex.getMessage();
		if (msg == null || matchesClassCastMessage(msg, event.getClass()) ||
				(event instanceof PayloadApplicationEvent payloadEvent &&
						matchesClassCastMessage(msg, payloadEvent.getPayload().getClass()))) {
			// Possibly a lambda-defined listener which we could not resolve the generic event type for
			// -> let's suppress the exception.
			Log loggerToUse = this.lazyLogger;
			if (loggerToUse == null) {
				loggerToUse = LogFactory.getLog(getClass());
				this.lazyLogger = loggerToUse;
			}
			if (loggerToUse.isTraceEnabled()) {
				loggerToUse.trace("Non-matching event type for listener: " + listener, ex);
			}
		}
		else {
			throw ex;
		}
	}
}

protected Collection<ApplicationListener<?>> getApplicationListeners(
		ApplicationEvent event, ResolvableType eventType) {

	//获取事件发生的对象
	Object source = event.getSource();
	Class<?> sourceType = (source != null ? source.getClass() : null);
	//根据事件的类型信息和源对象组成一个监听器的缓存key
	ListenerCacheKey cacheKey = new ListenerCacheKey(eventType, sourceType);

	// 创建一个新的监听器检索缓存
	CachedListenerRetriever newRetriever = null;

	// 根据key从检索缓存中获取缓存的监听器封装类
	CachedListenerRetriever existingRetriever = this.retrieverCache.get(cacheKey);
	//如果不存在
	if (existingRetriever == null) {
		//判断事件类型和源对象能否用指定的classLoader加载
		// 创建并缓存一个新的ListenerRetriever
		if (this.beanClassLoader == null ||
				(ClassUtils.isCacheSafe(event.getClass(), this.beanClassLoader) &&
						(sourceType == null || ClassUtils.isCacheSafe(sourceType, this.beanClassLoader)))) {
			newRetriever = new CachedListenerRetriever();
			//如果指定键没有关联值，则存入新值，返回null，有关联值返回关联值
			existingRetriever = this.retrieverCache.putIfAbsent(cacheKey, newRetriever);
			//有关联值，就不填充新值，将创建的对象取消关联
			if (existingRetriever != null) {
				newRetriever = null;  
			}
		}
	}

	//缓存检索器中有值，就返回缓存的事件监听器列表
	if (existingRetriever != null) {
		Collection<ApplicationListener<?>> result = existingRetriever.getApplicationListeners();
		if (result != null) {
			return result;
		}
	}
	//缓存检索器中没有值的话，继续检索
	return retrieveApplicationListeners(eventType, sourceType, newRetriever);
}

private Collection<ApplicationListener<?>> retrieveApplicationListeners(
		ResolvableType eventType, @Nullable Class<?> sourceType, @Nullable CachedListenerRetriever retriever) {

	List<ApplicationListener<?>> allListeners = new ArrayList<>();
	Set<ApplicationListener<?>> filteredListeners = (retriever != null ? new LinkedHashSet<>() : null);
	Set<String> filteredListenerBeans = (retriever != null ? new LinkedHashSet<>() : null);

	Set<ApplicationListener<?>> listeners;
	Set<String> listenerBeans;
	//从默认检索器中读取监听器列表和监听器bean名称
	synchronized (this.defaultRetriever) {
		listeners = new LinkedHashSet<>(this.defaultRetriever.applicationListeners);
		listenerBeans = new LinkedHashSet<>(this.defaultRetriever.applicationListenerBeans);
	}

	// 循环添加已经注册的监听器，包括ApplicationListenerDetector加载的监听器
	for (ApplicationListener<?> listener : listeners) {
	//检查指定的监听器是否是需要关注的事件
		if (supportsEvent(listener, eventType, sourceType)) {
			if (retriever != null) {
				filteredListeners.add(listener);
			}
			allListeners.add(listener);
		}
	}

	// 通过bean名称来添加监听器，可能与上面的方式重叠，但这里会有一些新的元数据
	if (!listenerBeans.isEmpty()) {
	//获取bean工厂
		ConfigurableBeanFactory beanFactory = getBeanFactory();
		for (String listenerBeanName : listenerBeans) {
			try {
			//判断指定的监听器bean是否是需要关注的事件
				if (supportsEvent(beanFactory, listenerBeanName, eventType)) {
				//获取监听器bean
					ApplicationListener<?> listener =
							beanFactory.getBean(listenerBeanName, ApplicationListener.class);
							//最终判断
					if (!allListeners.contains(listener) && supportsEvent(listener, eventType, sourceType)) {
						if (retriever != null) {
							if (beanFactory.isSingleton(listenerBeanName)) {
								filteredListeners.add(listener);
							}
							else {
								filteredListenerBeans.add(listenerBeanName);
							}
						}
						allListeners.add(listener);
					}
				}
				else {
					// 移除不支持的监听器
					Object listener = beanFactory.getSingleton(listenerBeanName);
					if (retriever != null) {
						filteredListeners.remove(listener);
					}
					allListeners.remove(listener);
				}
			}
			catch (NoSuchBeanDefinitionException ex) {
				
			}
		}
	}

	//排序
	AnnotationAwareOrderComparator.sort(allListeners);
	if (retriever != null) {
		if (filteredListenerBeans.isEmpty()) {
			retriever.applicationListeners = new LinkedHashSet<>(allListeners);
			retriever.applicationListenerBeans = filteredListenerBeans;
		}
		else {
			retriever.applicationListeners = filteredListeners;
			retriever.applicationListenerBeans = filteredListenerBeans;
		}
	}
	return allListeners;
}

protected boolean supportsEvent(
		ApplicationListener<?> listener, ResolvableType eventType, @Nullable Class<?> sourceType) {

	GenericApplicationListener smartListener = (listener instanceof GenericApplicationListener gal ? gal :
			new GenericApplicationListenerAdapter(listener));
			//通过判断给定的事件类型是否与要关注的事件类型一致，并且支持给定的源类型
	return (smartListener.supportsEventType(eventType) && smartListener.supportsSourceType(sourceType));
}

public GenericApplicationListenerAdapter(ApplicationListener<?> delegate) {
	Assert.notNull(delegate, "Delegate listener must not be null");
	this.delegate = (ApplicationListener<ApplicationEvent>) delegate;
	//解析出事件类型信息
	this.declaredEventType = resolveDeclaredEventType(this.delegate);
}

public boolean supportsEventType(ResolvableType eventType) {
//如果是GenericApplicationListener 的实现，它扩展了SmartApplicationListener
	if (this.delegate instanceof GenericApplicationListener gal) {
		return gal.supportsEventType(eventType);
	}
	//如果是SmartApplicationListener的实现
	else if (this.delegate instanceof SmartApplicationListener sal) {
		Class<? extends ApplicationEvent> eventClass = (Class<? extends ApplicationEvent>) eventType.resolve();
		return (eventClass != null && sal.supportsEventType(eventClass));
	}
	else {
	//其他类型判断
		return (this.declaredEventType == null || this.declaredEventType.isAssignableFrom(eventType));
	}
}

@Override
public boolean supportsSourceType(@Nullable Class<?> sourceType) {
	return (!(this.delegate instanceof SmartApplicationListener sal) || sal.supportsSourceType(sourceType));
}

public void onApplicationEvent(ApplicationEvent event) {
	//环境准备完毕
	if (event instanceof ApplicationEnvironmentPreparedEvent preparedEvent) {
	//从context.listener.classes加载配置的事件监听器
		List<ApplicationListener<ApplicationEvent>> delegates = getListeners(preparedEvent.getEnvironment());
		if (delegates.isEmpty()) {
			return;
		}
		//新创建一个SimpleApplicationEventMulticaster，跟以前用的不是同一个
		this.multicaster = new SimpleApplicationEventMulticaster();
		for (ApplicationListener<ApplicationEvent> listener : delegates) {
			this.multicaster.addApplicationListener(listener);
		}
	}
	if (this.multicaster != null) {
	//监听到其他事件的时候向所有注册在该广播器上的监听器广播事件
		this.multicaster.multicastEvent(event);
	}
}

protected void invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory beanFactory) {

	//使用注册委托类处理BeanFactoryPostProcessor的实现
	PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors());

	// Detect a LoadTimeWeaver and prepare for weaving, if found in the meantime
	// (e.g. through an @Bean method registered by ConfigurationClassPostProcessor)
	if (!NativeDetector.inNativeImage() && beanFactory.getTempClassLoader() == null && beanFactory.containsBean(LOAD_TIME_WEAVER_BEAN_NAME)) {
		beanFactory.addBeanPostProcessor(new LoadTimeWeaverAwareProcessor(beanFactory));
		beanFactory.setTempClassLoader(new ContextTypeMatchClassLoader(beanFactory.getBeanClassLoader()));
	}
}

public static void invokeBeanFactoryPostProcessors(
		ConfigurableListableBeanFactory beanFactory, List<BeanFactoryPostProcessor> beanFactoryPostProcessors) {

	Set<String> processedBeans = new HashSet<>();

//首先处理是BeanDefinitionRegistry的实例
	if (beanFactory instanceof BeanDefinitionRegistry registry) {
		List<BeanFactoryPostProcessor> regularPostProcessors = new ArrayList<>();
		List<BeanDefinitionRegistryPostProcessor> registryProcessors = new ArrayList<>();

		for (BeanFactoryPostProcessor postProcessor : beanFactoryPostProcessors) {
			if (postProcessor instanceof BeanDefinitionRegistryPostProcessor registryProcessor) {
				registryProcessor.postProcessBeanDefinitionRegistry(registry);
				registryProcessors.add(registryProcessor);
			}
			else {
				regularPostProcessors.add(postProcessor);
			}
		}

		// 然后其中分别处理实现了 PriorityOrdered、Ordered 和其余的处理器
		List<BeanDefinitionRegistryPostProcessor> currentRegistryProcessors = new ArrayList<>();

		//处理实现了PriorityOrdered的处理器
		String[] postProcessorNames =
				beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
		for (String ppName : postProcessorNames) {
			if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
				currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
				processedBeans.add(ppName);
			}
		}
		sortPostProcessors(currentRegistryProcessors, beanFactory);
		registryProcessors.addAll(currentRegistryProcessors);
		invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry, beanFactory.getApplicationStartup());
		currentRegistryProcessors.clear();

		// 处理实现了Ordered的处理器
		postProcessorNames = beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
		for (String ppName : postProcessorNames) {
			if (!processedBeans.contains(ppName) && beanFactory.isTypeMatch(ppName, Ordered.class)) {
				currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
				processedBeans.add(ppName);
			}
		}
		sortPostProcessors(currentRegistryProcessors, beanFactory);
		registryProcessors.addAll(currentRegistryProcessors);
		invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry, beanFactory.getApplicationStartup());
		currentRegistryProcessors.clear();

		// 最后是剩下的处理器
		boolean reiterate = true;
		while (reiterate) {
			reiterate = false;
			postProcessorNames = beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
			for (String ppName : postProcessorNames) {
				if (!processedBeans.contains(ppName)) {
					currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
					processedBeans.add(ppName);
					reiterate = true;
				}
			}
			sortPostProcessors(currentRegistryProcessors, beanFactory);
			registryProcessors.addAll(currentRegistryProcessors);
			invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry, beanFactory.getApplicationStartup());
			currentRegistryProcessors.clear();
		}

		// 调用迄今为止处理的所有处理器的postProcessBeanFactory回调
		invokeBeanFactoryPostProcessors(registryProcessors, beanFactory);
		invokeBeanFactoryPostProcessors(regularPostProcessors, beanFactory);
	}

	else {
		// 其他情况调用在上下文实例中注册的工厂处理程序
		invokeBeanFactoryPostProcessors(beanFactoryPostProcessors, beanFactory);
	}

	// 处理 BeanFactoryPostProcessor 实现的实例
	String[] postProcessorNames =
			beanFactory.getBeanNamesForType(BeanFactoryPostProcessor.class, true, false);

	//在实现PriorityOrdered、Ordered和其他的BeanFactoryPostProcessors之间分离
	List<BeanFactoryPostProcessor> priorityOrderedPostProcessors = new ArrayList<>();
	List<String> orderedPostProcessorNames = new ArrayList<>();
	List<String> nonOrderedPostProcessorNames = new ArrayList<>();
	for (String ppName : postProcessorNames) {
		if (processedBeans.contains(ppName)) {
			// 跳过-已在上面的第一阶段中处理
		}
		else if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
			priorityOrderedPostProcessors.add(beanFactory.getBean(ppName, BeanFactoryPostProcessor.class));
		}
		else if (beanFactory.isTypeMatch(ppName, Ordered.class)) {
			orderedPostProcessorNames.add(ppName);
		}
		else {
			nonOrderedPostProcessorNames.add(ppName);
		}
	}

	// 首先，调用实现PriorityOrdered的BeanFactoryPostProcessors
	sortPostProcessors(priorityOrderedPostProcessors, beanFactory);
	invokeBeanFactoryPostProcessors(priorityOrderedPostProcessors, beanFactory);

	
	//调用实现Ordered的BeanFactoryPostProcessors
	List<BeanFactoryPostProcessor> orderedPostProcessors = new ArrayList<>(orderedPostProcessorNames.size());
	for (String postProcessorName : orderedPostProcessorNames) {
		orderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class));
	}
	sortPostProcessors(orderedPostProcessors, beanFactory);
	invokeBeanFactoryPostProcessors(orderedPostProcessors, beanFactory);

	// 最后调用其他的BeanFactoryPostProcessors
	List<BeanFactoryPostProcessor> nonOrderedPostProcessors = new ArrayList<>(nonOrderedPostProcessorNames.size());
	for (String postProcessorName : nonOrderedPostProcessorNames) {
		nonOrderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class));
	}
	//EventListenerMethodProcessor会在此处被调用
	invokeBeanFactoryPostProcessors(nonOrderedPostProcessors, beanFactory);

	//清除缓存的合并bean定义，因为后处理程序可能已经修改了原始元数据，例如替换值中的占位符。。。
	beanFactory.clearMetadataCache();
}

private static void invokeBeanFactoryPostProcessors(
		Collection<? extends BeanFactoryPostProcessor> postProcessors, ConfigurableListableBeanFactory beanFactory) {

	for (BeanFactoryPostProcessor postProcessor : postProcessors) {
	//步骤记录器
		StartupStep postProcessBeanFactory = beanFactory.getApplicationStartup().start("spring.context.bean-factory.post-process")
				.tag("postProcessor", postProcessor::toString);
				//循环调用postProcessBeanFactory
		postProcessor.postProcessBeanFactory(beanFactory);
		postProcessBeanFactory.end();
	}
}

public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {
	this.beanFactory = beanFactory;

//获取EventListenerFactory实现类，其用于处理EventListener注解，
//将其封装成ApplicationListener
	Map<String, EventListenerFactory> beans = beanFactory.getBeansOfType(EventListenerFactory.class, false, false);
	List<EventListenerFactory> factories = new ArrayList<>(beans.values());
	AnnotationAwareOrderComparator.sort(factories);
	this.eventListenerFactories = factories;
}

//预实例化所有非懒加载的单例 bean，并触发所有适用 bean 的初始化后回调。
public void preInstantiateSingletons() throws BeansException {
	if (logger.isTraceEnabled()) {
		logger.trace("Pre-instantiating singletons in " + this);
	}

	// 访问 beanDefinitionNames，以允许初始化方法注册新的 bean 定义的列表的副本
	List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);

	// 触发所有非延迟加载的单例 bean 的实例化
	for (String beanName : beanNames) {
		RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
		if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
		//如果是工厂 bean，检查是否需要实例化
			if (isFactoryBean(beanName)) {
				Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
				if (bean instanceof SmartFactoryBean<?> smartFactoryBean && smartFactoryBean.isEagerInit()) {
					getBean(beanName);
				}
			}
			//如果不是工厂bean，则实例化 bean
			else {
				getBean(beanName);
			}
		}
	}

	// 触发所有适用bean的初始化后回调
	for (String beanName : beanNames) {
		Object singletonInstance = getSingleton(beanName);
		if (singletonInstance instanceof SmartInitializingSingleton smartSingleton) {
		// 启动一个 smart-initialize 的 StartupStep 作为性能分析；
       // 在执行完 smartSingleton.afterSingletonsInstantiated() 之后结束这个 StartupStep。
			StartupStep smartInitialize = this.getApplicationStartup().start("spring.beans.smart-initialize")
					.tag("beanName", beanName);
			smartSingleton.afterSingletonsInstantiated();
			smartInitialize.end();
		}
	}
}