EventBus VS Spring Event

  • Z时代
  • 2024-01-10
  • 分类：综合

本文内容纲要：

- EventBus VS Spring Event
- Guava EventBus
- spring event
- 比较EventBus与Spring Event

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
     */
    @Subscribe
    public 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
     */
    @Subscribe
    public 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
     */
    @Subscribe
    public void processWorldEventV2(WorldEvent event) {
        System.out.println("process world eventV2, no:" + event.getEventNo() + ", name:" + event.getEventName());
    }
    @Subscribe
    public 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#register
  public 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#post
  public 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 DeadEvent
    if (!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
     */
    @Subscribe
    public 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
     */
    @EventListener
    public void processApplicationEvent(ApplicationEvent event) {
        System.out.println("process common event, class:" + event.getClass().getSimpleName());
    }
    /**
     * 监听 HelloEvent类型 事件
     */
    @EventListener
    public 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
    @EventListener
    public 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 necessary
		ApplicationEvent 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 initialized
		if (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

@Override
	public 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() {
					@Override
					public 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EventBusEventBus#post是注解Subscribe方法手动注册EventBus#register
Spring Event任意对象ApplicationEventPublisherApplicationEventPublisher#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
EventBusClass是是否是简单
Spring EventClass是是是否复杂

本文内容总结：EventBus VS Spring Event，Guava EventBus，spring event，比较EventBus与Spring Event，

原文链接：https://www.cnblogs.com/shoren/p/eventBus_springEvent.html

以上是 EventBus VS Spring Event 的全部内容， 来源链接： utcz.com/z/296594.html

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