EventBus VS Spring Event

本地异步处理，采用事件机制 可以使 代码解耦，更易读。事件机制实现模式是 观察者模式（或发布订阅模式），主要分为三部分：发布者、监听者、事件。

Guava EventBus

Guava EventBus实现是观察者模式，用法很简单，先上代码。

不止是代码

/*** Desc: 事件对象*/
@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: 事件监听器，可以监听多个事件。处理方法添加 @Subscribe 注解即可。*/
public class GeventListener {/*** 监听 HelloEvent 类型及其父类型（Object）的事件*/@Subscribepublic void processEvent(HelloEvent event){System.out.println("process hello event, name:" + event.getEventName());}/*** 监听 WorldEvent 类型及其父类型（HelloEvent 和 Object）的事件*/@Subscribepublic void processWorldEvent(WorldEvent event) {System.out.println("process world eventV1, no:" + event.getEventNo() + ", name:" + event.getEventName());}/*** 注册多个监听器 监听同一事件* @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));}
}

结果如下：

//HelloEvent被两个监听器处理（HelloEvent类及Object类的监听器）
process hello event, name:hello
process common event, class:HelloEvent
//WorldEvent被四个监听器处理（两个自己的，两个父类的）
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把类当做事件，是以class为key注册和管理事件的，value是事件监听器的method；事件监听器只处理某一类（及其父类）事件。

事件注册与发布

//com.google.common.eventbus.EventBus#registerpublic void register(Object object) {//key为Class， value为EventSubscriber(Object target, Method method)【集合】。注意这里Multimap 为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类及其所有父类Set<Class<?>> dispatchTypes = flattenHierarchy(event.getClass());boolean dispatched = false;for (Class<?> eventType : dispatchTypes) {subscribersByTypeLock.readLock().lock();try {//找到所有事件订阅者（事件监听器）Set<EventSubscriber> wrappers = subscribersByType.get(eventType);if (!wrappers.isEmpty()) {dispatched = true;for (EventSubscriber wrapper : wrappers) {//事件入队列enqueueEvent(event, wrapper);}}} finally {subscribersByTypeLock.readLock().unlock();}}//如果没有订阅者订阅此类消息，则为 DeadEventif (!dispatched && !(event instanceof DeadEvent)) {post(new DeadEvent(this, event));}dispatchQueuedEvents();}

事件隔离

多个EventBus可以隔离事件。

public class AnotherListener {/*** 监听 WorldEvent 类型及其父类型（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));}
}

结果是

//eventBus结果与之前相同
process hello event, name:hello
//anotherEventBus 发布的事件，只被其注册的监听器处理
process common event, class:HelloEvent
process another world event, no:666, name:AnotherWorld

适用场景：

• 按照类区分事件
• 订阅 事件簇
• 支持自定义event，可以根据event自己写分发器
• 事件隔离

spring event

spring 新版事件机制也比较简单，看代码。

不止是代码

/*** 继承 ApplicationEvent 的事件*/
@Data
public class HelloEvent extends ApplicationEvent {private String eventName;public HelloEvent(String eventName) {super(eventName);setEventName(eventName);}
}/*** 自定义事件*/
@Data
@NoArgsConstructor
@AllArgsConstructor
public class CustomerEvent {private String name;private Boolean isCustomer;
}/*** 监听器类，spring也支持一个类中监听多个事件*/
@Component("springListener")
public class SpringListener {/*** 监听所有ApplicationEvent类型 及其子类型 的事件*/@EventListenerpublic void processApplicationEvent(ApplicationEvent event) {System.out.println("process common event, class:" + event.getClass().getSimpleName());}/*** 监听 HelloEvent类型 事件*/@EventListenerpublic void processHelloEvent(HelloEvent event) {System.out.println("process helloEvent, name:" + event.getEventName());}/*** 监听 CustomerEvent 类型事件，但是需要满足condition条件,即isCustomer=true*/@EventListener(condition = "#event.isCustomer")public void processCustomerEvent(CustomerEvent event) {System.out.println("process customer CustomerEvent, name:" + event.getName());}/*** 监听 CustomerEvent 类型事件，但是需要满足condition条件,即name="miaomiao"*/@EventListener(condition = "#event.getName().equals('miaomiao')")public void processMiaoMiaoEvent(CustomerEvent event) {System.out.println("process miaomiao's CustomerEvent, name:" + event.getName());}/*** 支持异步处理事件*/@Async@EventListenerpublic void processAsyncCustomerEvent(CustomerEvent event) {System.out.println("Async process CustomerEvent, name:" + event.getName());}
}//执行类，测试入口
@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));}
}

结果是

//以下是spring上下文event，继承自 ApplicationContextEvent。 用于用户参与上下文生命周期的入口。因为是ApplicationEvent子类型，所以，由processApplicationEvent处理。
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 继承 ApplicationEvent，会被processApplicationEvent处理
process common event, class:HelloEvent
//监听 HelloEvent类型 的 processHelloEvent 处理
process helloEvent, name:helloEvent
//非 ApplicationEvent 的事件，则为 PayloadApplicationEvent
process common event, class:PayloadApplicationEvent
//isCustomer=true,符合processCustomerEvent处理条件
process customer CustomerEvent, name:customer
//监听CustomerEvent类型，处理结果
Async process CustomerEvent, name:customer
process common event, class:PayloadApplicationEvent
//符合processMiaoMiaoEvent条件
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。可以通过监听这些事件，参与到spring生命周期中去。这种无侵入性交互方式，在做平台服务时，是一种很好的方式。

注册监听器

org.springframework.context.event.EventListenerMethodProcessor#processBean 将所有注解EventListener的方法，存入上下文的applicationListeners中。Listener的封装类为ApplicationListenerMethodAdapter(String beanName, Class<?> targetClass, Method method)。
org.springframework.context.support.AbstractApplicationContext#refresh 中调用 initApplicationEventMulticaster 初始化事件发布管理器applicationEventMulticaster，然后调用registerListeners() 注册监听器。

发布事件

spring 起初只支持 ApplicationEvent类型事件，后来优化之后，支持自定义事件。自定义事件的处理，默认为PayloadApplicationEvent，相当于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类型，则使用PayloadApplicationEvent封装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);}//调用父类，发布事件// 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);}}}

执行事件

    @Overridepublic void multicastEvent(final ApplicationEvent event, ResolvableType eventType) {ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));//获取事件的监听器集合，并逐个触发执行监听器for (final ApplicationListener<?> listener : getApplicationListeners(event, type)) {//异步的话，就放在线程池中执行Executor executor = getTaskExecutor();if (executor != null) {executor.execute(new Runnable() {@Overridepublic void run() {invokeListener(listener, event);}});}else {//本线程调用invokeListener(listener, event);}}}

可以看到，spring的事件机制更复杂，但是功能同样强大。
适用场景：

• 按照类区分事件
• 订阅 事件簇
• 支持自定义event
• 按照condition过滤同类型事件

比较EventBus与Spring Event

• 使用方式比较

项目	事件	发布者	发布方法	是否异步	监听者	注册方式
EventBus	任意对象	EventBus	EventBus#post	是	注解Subscribe方法	手动注册EventBus#register
Spring Event	任意对象	ApplicationEventPublisher	ApplicationEventPublisher#publishEvent	支持同步异步	注解EventListener方法	系统注册

• 使用场景比较

项目	事件区分	是否支持事件簇	是否支持自定义event	是否支持过滤	是否支持事件隔离	复杂程度
EventBus	Class	是	是	否	是	简单
Spring Event	Class	是	是	是	否	复杂