@EventListener的使用方式和原理

使用
1.定义事件

package com.toryxu.eventlistener.event;

import lombok.AllArgsConstructor;
import lombok.Getter;
import lombok.Setter;
import lombok.ToString;

/**
 * @author toryxu
 * @version 1.0
 * @date 2022/3/21 7:33 下午
 */
@AllArgsConstructor
@ToString
public class TestEvent {

    @Getter
    @Setter
    public String name;
}

2.定义监听器

package com.toryxu.eventlistener.listener;

import com.toryxu.eventlistener.event.TestEvent;
import lombok.extern.slf4j.Slf4j;
import org.springframework.context.event.EventListener;
import org.springframework.core.annotation.Order;
import org.springframework.stereotype.Component;

/**
 * @author toryxu
 * @version 1.0
 * @date 2022/3/21 7:33 下午
 */
@Component
@Slf4j
public class TestListener {

    @EventListener
    @Order(1)
    public void doSomething(TestEvent event) {
        log.info(event.getName());
        event.setName("Q");
    }

    @EventListener
    @Order(2)
    public void doAnotherThing(TestEvent event) {
        log.info(event.getName());
    }
}

3.通过applicationContext发布事件

package com.toryxu.eventlistener;

import com.toryxu.eventlistener.event.TestEvent;
import org.springframework.context.ApplicationContext;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

import javax.annotation.Resource;

/**
 * @author toryxu
 * @version 1.0
 * @date 2022/3/21 7:31 下午
 */
@RestController
public class TestController {

    @Resource
    ApplicationContext context;


    @GetMapping("/test")
    public void Test() {

        context.publishEvent(new TestEvent("ToryXu"));
    }
}

4.结果

原理：
1.查看EventListener的注释

/**
 * Annotation that marks a method as a listener for application events.
 *
 * 
If an annotated method supports a single event type, the method may
 * declare a single parameter that reflects the event type to listen to.
 * If an annotated method supports multiple event types, this annotation
 * may refer to one or more supported event types using the {@code classes}
 * attribute. See the {@link #classes} javadoc for further details.
 *
 * 
Events can be {@link ApplicationEvent} instances as well as arbitrary
 * objects.
 *
 * 
Processing of {@code @EventListener} annotations is performed via
 * the internal {@link EventListenerMethodProcessor} bean which gets
 * registered automatically when using Java config or manually via the
 * {@code } or {@code }
 * element when using XML config.
 ...

可以看到它是通过EventListenerMethodProcessor来实现的。

2.通过断点排查到是在AnnotationConfigUtils的registerAnnotationConfigProcessors中注册了这个后置处理器

public static Set<BeanDefinitionHolder> registerAnnotationConfigProcessors(
			BeanDefinitionRegistry registry, @Nullable Object source) {

		DefaultListableBeanFactory beanFactory = unwrapDefaultListableBeanFactory(registry);
		if (beanFactory != null) {
			if (!(beanFactory.getDependencyComparator() instanceof AnnotationAwareOrderComparator)) {
				beanFactory.setDependencyComparator(AnnotationAwareOrderComparator.INSTANCE);
			}
			if (!(beanFactory.getAutowireCandidateResolver() instanceof ContextAnnotationAutowireCandidateResolver)) {
				beanFactory.setAutowireCandidateResolver(new ContextAnnotationAutowireCandidateResolver());
			}
		}

		Set<BeanDefinitionHolder> beanDefs = new LinkedHashSet<>(8);

		if (!registry.containsBeanDefinition(CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME)) {
			RootBeanDefinition def = new RootBeanDefinition(ConfigurationClassPostProcessor.class);
			def.setSource(source);
			beanDefs.add(registerPostProcessor(registry, def, CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME));
		}

		if (!registry.containsBeanDefinition(AUTOWIRED_ANNOTATION_PROCESSOR_BEAN_NAME)) {
			RootBeanDefinition def = new RootBeanDefinition(AutowiredAnnotationBeanPostProcessor.class);
			def.setSource(source);
			beanDefs.add(registerPostProcessor(registry, def, AUTOWIRED_ANNOTATION_PROCESSOR_BEAN_NAME));
		}

		// Check for JSR-250 support, and if present add the CommonAnnotationBeanPostProcessor.
		if (jsr250Present && !registry.containsBeanDefinition(COMMON_ANNOTATION_PROCESSOR_BEAN_NAME)) {
			RootBeanDefinition def = new RootBeanDefinition(CommonAnnotationBeanPostProcessor.class);
			def.setSource(source);
			beanDefs.add(registerPostProcessor(registry, def, COMMON_ANNOTATION_PROCESSOR_BEAN_NAME));
		}

		// Check for JPA support, and if present add the PersistenceAnnotationBeanPostProcessor.
		if (jpaPresent && !registry.containsBeanDefinition(PERSISTENCE_ANNOTATION_PROCESSOR_BEAN_NAME)) {
			RootBeanDefinition def = new RootBeanDefinition();
			try {
				def.setBeanClass(ClassUtils.forName(PERSISTENCE_ANNOTATION_PROCESSOR_CLASS_NAME,
						AnnotationConfigUtils.class.getClassLoader()));
			}
			catch (ClassNotFoundException ex) {
				throw new IllegalStateException(
						"Cannot load optional framework class: " + PERSISTENCE_ANNOTATION_PROCESSOR_CLASS_NAME, ex);
			}
			def.setSource(source);
			beanDefs.add(registerPostProcessor(registry, def, PERSISTENCE_ANNOTATION_PROCESSOR_BEAN_NAME));
		}

		if (!registry.containsBeanDefinition(EVENT_LISTENER_PROCESSOR_BEAN_NAME)) {
			RootBeanDefinition def = new RootBeanDefinition(EventListenerMethodProcessor.class);
			def.setSource(source);
			beanDefs.add(registerPostProcessor(registry, def, EVENT_LISTENER_PROCESSOR_BEAN_NAME));
		}

		if (!registry.containsBeanDefinition(EVENT_LISTENER_FACTORY_BEAN_NAME)) {
			RootBeanDefinition def = new RootBeanDefinition(DefaultEventListenerFactory.class);
			def.setSource(source);
			beanDefs.add(registerPostProcessor(registry, def, EVENT_LISTENER_FACTORY_BEAN_NAME));
		}

		return beanDefs;
	}

3.继续看该processor的processbean方法

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 {
				annotatedMethods = MethodIntrospector.selectMethods(targetType,
						(MethodIntrospector.MetadataLookup<EventListener>) 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<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));
							ApplicationListener<?> applicationListener =
									factory.createApplicationListener(beanName, targetType, methodToUse);
							if (applicationListener instanceof ApplicationListenerMethodAdapter) {
								((ApplicationListenerMethodAdapter) applicationListener).init(context, this.evaluator);
							}
							context.addApplicationListener(applicationListener);
							break;
						}
					}
				}
				if (logger.isDebugEnabled()) {
					logger.debug(annotatedMethods.size() + " @EventListener methods processed on bean '" +
							beanName + "': " + annotatedMethods);
				}
			}
		}
	}

4.然后看context的publishEvent方法

protected void publishEvent(Object event, @Nullable ResolvableType eventType) {
		Assert.notNull(event, "Event must not be null");

		// 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 {
			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);
			}
		}
	}

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);

		// Potential new retriever to populate
		CachedListenerRetriever newRetriever = null;

		// Quick check for existing entry on ConcurrentHashMap
		CachedListenerRetriever existingRetriever = this.retrieverCache.get(cacheKey);
		if (existingRetriever == null) {
			// Caching a new ListenerRetriever if possible
			if (this.beanClassLoader == null ||
					(ClassUtils.isCacheSafe(event.getClass(), this.beanClassLoader) &&
							(sourceType == null || ClassUtils.isCacheSafe(sourceType, this.beanClassLoader)))) {
				newRetriever = new CachedListenerRetriever();
				existingRetriever = this.retrieverCache.putIfAbsent(cacheKey, newRetriever);
				if (existingRetriever != null) {
					newRetriever = null;  // no need to populate it in retrieveApplicationListeners
				}
			}
		}

		if (existingRetriever != null) {
			Collection<ApplicationListener<?>> result = existingRetriever.getApplicationListeners();
			if (result != null) {
				return result;
			}
			// If result is null, the existing retriever is not fully populated yet by another thread.
			// Proceed like caching wasn't possible for this current local attempt.
		}

		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;
		synchronized (this.defaultRetriever) {
			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) {
					filteredListeners.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)) {
									filteredListeners.add(listener);
								}
								else {
									filteredListenerBeans.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) {
							filteredListeners.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) {
			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 ?
				(GenericApplicationListener) listener : new GenericApplicationListenerAdapter(listener));
		return (smartListener.supportsEventType(eventType) && smartListener.supportsSourceType(sourceType));
	}

如果支持，则返回该listener
再回到multicastEvent

@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);
			}
		}
	}

invoke.