EventBus VS Spring Event

本地異步處理，采用事件機制 可以使 代碼解耦，更易讀。事件機制實現模式是 觀察者模式（或發布訂閱模式），主要分為三部分：發布者、監聽者、事件。

Guava EventBus

Guava EventBus實現是觀察者模式，用法很簡單，先上代碼。

不止是代碼

/*** Desc: 事件對象*/ @Data @NoArgsConstructor @AllArgsConstructor public class HelloEvent {private String eventName; }@Data @NoArgsConstructor public class WorldEvent extends HelloEvent {private int eventNo;public WorldEvent(String name, int no) {setEventName(name);setEventNo(no);} }/*** Desc: 事件監聽器，可以監聽多個事件。處理方法添加 @Subscribe 注解即可。*/ public class GeventListener {/*** 監聽 HelloEvent 類型及其父類型（Object）的事件*/@Subscribepublic void processEvent(HelloEvent event){System.out.println("process hello event, name:" + event.getEventName());}/*** 監聽 WorldEvent 類型及其父類型（HelloEvent 和 Object）的事件*/@Subscribepublic void processWorldEvent(WorldEvent event) {System.out.println("process world eventV1, no:" + event.getEventNo() + ", name:" + event.getEventName());}/*** 注冊多個監聽器 監聽同一事件* @param event*/@Subscribepublic void processWorldEventV2(WorldEvent event) {System.out.println("process world eventV2, no:" + event.getEventNo() + ", name:" + event.getEventName());}@Subscribepublic void processObject(Object object) {System.out.println("process common event, class:" + object.getClass().getSimpleName());} }public class GuavaTest {public static void main(String[] args) {EventBus eventBus = new EventBus();GeventListener listener = new GeventListener();eventBus.register(listener);eventBus.post(new HelloEvent("hello"));eventBus.post(new WorldEvent("world", 23333));} }

結果如下：

//HelloEvent被兩個監聽器處理（HelloEvent類及Object類的監聽器） process hello event, name:hello process common event, class:HelloEvent //WorldEvent被四個監聽器處理（兩個自己的，兩個父類的） process world eventV1, no:23333, name:world process world eventV2, no:23333, name:world process hello event, name:world process common event, class:WorldEvent

由上可知：Guava EventBus把類當做事件，是以class為key注冊和管理事件的，value是事件監聽器的method；事件監聽器只處理某一類（及其父類）事件。

事件注冊與發布

//com.google.common.eventbus.EventBus#registerpublic void register(Object object) {//key為Class， value為EventSubscriber(Object target, Method method)【集合】。注意這里Multimap 為HashMultimap， 即HashMap<K, Collection<V>>Multimap<Class<?>, EventSubscriber> methodsInListener =finder.findAllSubscribers(object);subscribersByTypeLock.writeLock().lock();try {subscribersByType.putAll(methodsInListener);} finally {subscribersByTypeLock.writeLock().unlock();}}//com.google.common.eventbus.EventBus#postpublic void post(Object event) {//找到event類及其所有父類Set<Class<?>> dispatchTypes = flattenHierarchy(event.getClass());boolean dispatched = false;for (Class<?> eventType : dispatchTypes) {subscribersByTypeLock.readLock().lock();try {//找到所有事件訂閱者（事件監聽器）Set<EventSubscriber> wrappers = subscribersByType.get(eventType);if (!wrappers.isEmpty()) {dispatched = true;for (EventSubscriber wrapper : wrappers) {//事件入隊列enqueueEvent(event, wrapper);}}} finally {subscribersByTypeLock.readLock().unlock();}}//如果沒有訂閱者訂閱此類消息，則為 DeadEventif (!dispatched && !(event instanceof DeadEvent)) {post(new DeadEvent(this, event));}dispatchQueuedEvents();}

事件隔離

多個EventBus可以隔離事件。

public class AnotherListener {/*** 監聽 WorldEvent 類型及其父類型（HelloEvent 和 Object）的事件*/@Subscribepublic void processAnotherWorldEvent(WorldEvent event) {System.out.println("process another world event, no:" + event.getEventNo() + ", name:" + event.getEventName());} }public class GuavaTest {public static void main(String[] args) {EventBus eventBus = new EventBus();GeventListener listener = new GeventListener();eventBus.register(listener);eventBus.post(new HelloEvent("hello"));EventBus anotherEventBus = new EventBus();AnotherListener anotherListener = new AnotherListener();anotherEventBus.register(anotherListener);anotherEventBus.post(new WorldEvent("AnotherWorld", 666));} }

結果是

//eventBus結果與之前相同 process hello event, name:hello //anotherEventBus 發布的事件，只被其注冊的監聽器處理 process common event, class:HelloEvent process another world event, no:666, name:AnotherWorld

適用場景：

• 按照類區分事件
• 訂閱 事件簇
• 支持自定義event，可以根據event自己寫分發器
• 事件隔離

spring event

spring 新版事件機制也比較簡單，看代碼。

不止是代碼

/*** 繼承 ApplicationEvent 的事件*/ @Data public class HelloEvent extends ApplicationEvent {private String eventName;public HelloEvent(String eventName) {super(eventName);setEventName(eventName);} }/*** 自定義事件*/ @Data @NoArgsConstructor @AllArgsConstructor public class CustomerEvent {private String name;private Boolean isCustomer; }/*** 監聽器類，spring也支持一個類中監聽多個事件*/ @Component("springListener") public class SpringListener {/*** 監聽所有ApplicationEvent類型 及其子類型 的事件*/@EventListenerpublic void processApplicationEvent(ApplicationEvent event) {System.out.println("process common event, class:" + event.getClass().getSimpleName());}/*** 監聽 HelloEvent類型 事件*/@EventListenerpublic void processHelloEvent(HelloEvent event) {System.out.println("process helloEvent, name:" + event.getEventName());}/*** 監聽 CustomerEvent 類型事件，但是需要滿足condition條件,即isCustomer=true*/@EventListener(condition = "#event.isCustomer")public void processCustomerEvent(CustomerEvent event) {System.out.println("process customer CustomerEvent, name:" + event.getName());}/*** 監聽 CustomerEvent 類型事件，但是需要滿足condition條件,即name="miaomiao"*/@EventListener(condition = "#event.getName().equals('miaomiao')")public void processMiaoMiaoEvent(CustomerEvent event) {System.out.println("process miaomiao's CustomerEvent, name:" + event.getName());}/*** 支持異步處理事件*/@Async@EventListenerpublic void processAsyncCustomerEvent(CustomerEvent event) {System.out.println("Async process CustomerEvent, name:" + event.getName());} }//執行類，測試入口 @SpringBootApplication @ComponentScan(basePackages = {"com.example.manyao.async"}) public class DemoApplication {public static void main(String[] args) throws TException {SpringApplication.run(DemoApplication.class, args);ApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");String[] names = context.getBeanDefinitionNames();for(int i=0; i<names.length; i++) {System.out.println(names[i]);}System.out.println("++++++++++");context.publishEvent(new HelloEvent("helloEvent"));context.publishEvent(new CustomerEvent("customer", true));context.publishEvent(new CustomerEvent("miaomiao", false));} }

結果是

//以下是spring上下文event，繼承自 ApplicationContextEvent。 用于用戶參與上下文生命周期的入口。因為是ApplicationEvent子類型，所以，由processApplicationEvent處理。 process common event, class:ContextRefreshedEvent process common event, class:EmbeddedServletContainerInitializedEvent process common event, class:ApplicationReadyEvent process common event, class:ContextRefreshedEvent //以下是上下文中的bean springListener org.springframework.context.annotation.internalConfigurationAnnotationProcessor org.springframework.context.annotation.internalAutowiredAnnotationProcessor org.springframework.context.annotation.internalRequiredAnnotationProcessor org.springframework.context.annotation.internalCommonAnnotationProcessor org.springframework.context.event.internalEventListenerProcessor org.springframework.context.event.internalEventListenerFactory ++++++++++ //HelloEvent 繼承 ApplicationEvent，會被processApplicationEvent處理 process common event, class:HelloEvent //監聽 HelloEvent類型 的 processHelloEvent 處理 process helloEvent, name:helloEvent //非 ApplicationEvent 的事件，則為 PayloadApplicationEvent process common event, class:PayloadApplicationEvent //isCustomer=true,符合processCustomerEvent處理條件 process customer CustomerEvent, name:customer //監聽CustomerEvent類型，處理結果 Async process CustomerEvent, name:customer process common event, class:PayloadApplicationEvent //符合processMiaoMiaoEvent條件 process miaomiao's CustomerEvent, name:miaomiao Async process CustomerEvent, name:miaomiao //spring 上下文事件 process common event, class:ContextClosedEvent

spring 上下文事件

上述例子中的
ContextRefreshedEvent，EmbeddedServletContainerInitializedEvent，ApplicationReadyEvent，ContextRefreshedEvent，ContextClosedEvent 等事件，都是spring上下文事件。可以通過監聽這些事件，參與到spring生命周期中去。這種無侵入性交互方式，在做平臺服務時，是一種很好的方式。

注冊監聽器

org.springframework.context.event.EventListenerMethodProcessor#processBean 將所有注解EventListener的方法，存入上下文的applicationListeners中。Listener的封裝類為ApplicationListenerMethodAdapter(String beanName, Class<?> targetClass, Method method)。
org.springframework.context.support.AbstractApplicationContext#refresh 中調用 initApplicationEventMulticaster 初始化事件發布管理器applicationEventMulticaster，然后調用registerListeners() 注冊監聽器。

發布事件

spring 起初只支持 ApplicationEvent類型事件，后來優化之后，支持自定義事件。自定義事件的處理，默認為PayloadApplicationEvent，相當于EventBus的DeadEvent。

//org.springframework.context.support.AbstractApplicationContext#publishEvent(java.lang.Object, org.springframework.core.ResolvableType)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 {//若不是ApplicationEvent類型，則使用PayloadApplicationEvent封裝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 { //核心操作，初始化 event getApplicationEventMulticaster().multicastEvent(applicationEvent, eventType);}//調用父類，發布事件// 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);}}}

執行事件

@Overridepublic void multicastEvent(final ApplicationEvent event, ResolvableType eventType) {ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));//獲取事件的監聽器集合，并逐個觸發執行監聽器for (final ApplicationListener<?> listener : getApplicationListeners(event, type)) {//異步的話，就放在線程池中執行Executor executor = getTaskExecutor();if (executor != null) {executor.execute(new Runnable() {@Overridepublic void run() {invokeListener(listener, event);}});}else {//本線程調用invokeListener(listener, event);}}}

可以看到，spring的事件機制更復雜，但是功能同樣強大。
適用場景：

• 按照類區分事件
• 訂閱 事件簇
• 支持自定義event
• 按照condition過濾同類型事件

比較EventBus與Spring Event

• 使用方式比較

項目事件發布者發布方法是否異步監聽者注冊方式

EventBus	任意對象	EventBus	EventBus#post	是	注解Subscribe方法	手動注冊EventBus#register
Spring Event	任意對象	ApplicationEventPublisher	ApplicationEventPublisher#publishEvent	支持同步異步	注解EventListener方法	系統注冊

• 使用場景比較

項目事件區分是否支持事件簇是否支持自定義event是否支持過濾是否支持事件隔離復雜程度

EventBus	Class	是	是	否	是	簡單
Spring Event	Class	是	是	是	否	復雜

轉載于:https://www.cnblogs.com/shoren/p/eventBus_springEvent.html

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