EventBus與Spring Event區(qū)別詳解(EventBus 事件機(jī)制,Spring Event事件機(jī)制)

更新時(shí)間：2020年02月07日 14:25:22 作者：漫夭

這篇文章主要介紹了EventBus與Spring Event區(qū)別,需要的朋友可以參考下

本地異步處理，采用事件機(jī)制可以使代碼解耦，更易讀。事件機(jī)制實(shí)現(xiàn)模式是觀察者模式（或發(fā)布訂閱模式），主要分為三部分：發(fā)布者、監(jiān)聽(tīng)者、事件。

Guava EventBus

Guava EventBus實(shí)現(xiàn)是觀察者模式，用法很簡(jiǎn)單，先上代碼。

/**
 * Desc: 事件對(duì)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(jiān)聽(tīng)器，可以監(jiān)聽(tīng)多個(gè)事件。處理方法添加 @Subscribe 注解即可。
 */
public class GeventListener {
 
  /**
   * 監(jiān)聽(tīng) HelloEvent 類型及其父類型（Object）的事件
   */
  @Subscribe
  public void processEvent(HelloEvent event){
    System.out.println("process hello event, name:" + event.getEventName());
  }
 
  /**
   * 監(jiān)聽(tīng) WorldEvent 類型及其父類型（HelloEvent 和 Object）的事件
   */
  @Subscribe
  public void processWorldEvent(WorldEvent event) {
    System.out.println("process world eventV1, no:" + event.getEventNo() + ", name:" + event.getEventName());
  }
  /**
   * 注冊(cè)多個(gè)監(jiān)聽(tīng)器 監(jiān)聽(tīng)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));
  }
}

結(jié)果如下：

//HelloEvent被兩個(gè)監(jiān)聽(tīng)器處理（HelloEvent類及Object類的監(jiān)聽(tīng)器）
process hello event, name:hello
process common event, class:HelloEvent
//WorldEvent被四個(gè)監(jiān)聽(tīng)器處理（兩個(gè)自己的，兩個(gè)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(dāng)做事件，是以class為key注冊(cè)和管理事件的，value是事件監(jiān)聽(tīng)器的method；事件監(jiān)聽(tīng)器只處理某一類（及其父類）事件。

事件注冊(cè)與發(fā)布

//com.google.common.eventbus.EventBus#register
 public 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#post
 public 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(jiān)聽(tīng)器）
    Set<EventSubscriber> wrappers = subscribersByType.get(eventType);
    if (!wrappers.isEmpty()) {
     dispatched = true;
     for (EventSubscriber wrapper : wrappers) {
     //事件入隊(duì)列
      enqueueEvent(event, wrapper);
     }
    }
   } finally {
    subscribersByTypeLock.readLock().unlock();
   }
  }
//如果沒(méi)有訂閱者訂閱此類消息，則為 DeadEvent
  if (!dispatched && !(event instanceof DeadEvent)) {
   post(new DeadEvent(this, event));
  }
  dispatchQueuedEvents();
 }

事件隔離

多個(gè)EventBus可以隔離事件。

public class AnotherListener {
  /**
   * 監(jiān)聽(tīng) WorldEvent 類型及其父類型（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));
  }
}

結(jié)果是

//eventBus結(jié)果與之前相同

process hello event, name:hello

//anotherEventBus 發(fā)布的事件，只被其注冊(cè)的監(jiān)聽(tīng)器處理

process common event, class:HelloEvent

process another world event, no:666, name:AnotherWorld

適用場(chǎng)景：

按照類區(qū)分事件
訂閱事件簇
支持自定義event，可以根據(jù)event自己寫(xiě)分發(fā)器
事件隔離

spring event

spring 新版事件機(jī)制也比較簡(jiǎn)單，看代碼。

/**
 * 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(jiān)聽(tīng)器類，spring也支持一個(gè)類中監(jiān)聽(tīng)多個(gè)事件
 */
@Component("springListener")
public class SpringListener {
  /**
   * 監(jiān)聽(tīng)所有ApplicationEvent類型 及其子類型 的事件
   */
  @EventListener
  public void processApplicationEvent(ApplicationEvent event) {
    System.out.println("process common event, class:" + event.getClass().getSimpleName());
  }
  /**
   * 監(jiān)聽(tīng) HelloEvent類型 事件
   */
  @EventListener
  public void processHelloEvent(HelloEvent event) {
    System.out.println("process helloEvent, name:" + event.getEventName());
  }
  /**
   * 監(jiān)聽(tīng) 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(jiān)聽(tīng) 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
  @EventListener
  public void processAsyncCustomerEvent(CustomerEvent event) {
    System.out.println("Async process CustomerEvent, name:" + event.getName());
  }
}
//執(zhí)行類，測(cè)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(jié)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?yàn)槭茿pplicationEvent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(huì)被processApplicationEvent處理
process common event, class:HelloEvent
//監(jiān)聽(tīng) 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?xiàng)l件
process customer CustomerEvent, name:customer
//監(jiān)聽(tīng)CustomerEvent類型，處理結(jié)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?？梢酝ㄟ^(guò)監(jiān)聽(tīng)這些事件，參與到spring生命周期中去。這種無(wú)侵入性交互方式，在做平臺(tái)服務(wù)時(shí)，是一種很好的方式。

注冊(cè)監(jiān)聽(tīng)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(diào)用 initApplicationEventMulticaster 初始化事件發(fā)布管理器applicationEventMulticaster，然后調(diào)用registerListeners() 注冊(cè)監(jiān)聽(tīng)器。

發(fā)布事件

spring 起初只支持 ApplicationEvent類型事件，后來(lái)優(yōu)化之后，支持自定義事件。自定義事件的處理，默認(rèn)為PayloadApplicationEvent，相當(dāng)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 initialized
    if (this.earlyApplicationEvents != null) {
      this.earlyApplicationEvents.add(applicationEvent);
    }
    else {
//核心操作，初始化 event    
  getApplicationEventMulticaster().multicastEvent(applicationEvent, eventType);
    }
    //調(diào)用父類，發(fā)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(zhí)行事件

  @Override
  public void multicastEvent(final ApplicationEvent event, ResolvableType eventType) {
    ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));
    //獲取事件的監(jiān)聽(tīng)器集合，并逐個(gè)觸發(fā)執(zhí)行監(jiān)聽(tīng)器
    for (final ApplicationListener<?> listener : getApplicationListeners(event, type)) {
    //異步的話，就放在線程池中執(zhí)行
      Executor executor = getTaskExecutor();
      if (executor != null) {
        executor.execute(new Runnable() {
          @Override
          public void run() {
            invokeListener(listener, event);
          }
        });
      }
      else {
      //本線程調(diào)用
        invokeListener(listener, event);
      }
    }
  }

可以看到，spring的事件機(jī)制更復(fù)雜，但是功能同樣強(qiáng)大。

適用場(chǎng)景：

按照類區(qū)分事件
訂閱事件簇
支持自定義event
按照condition過(guò)濾同類型事件

比較EventBus與Spring Event

使用方式比較

項(xiàng)目	事件	發(fā)布者	發(fā)布方法	是否異步	監(jiān)聽(tīng)者	注冊(cè)方式
EventBus	任意對(duì)象	EventBus	EventBus#post	是	注解Subscribe方法	手動(dòng)注冊(cè)EventBus#register
Spring Event	任意對(duì)象	ApplicationEventPublisher	ApplicationEventPublisher#publishEvent	支持同步異步	注解EventListener方法	系統(tǒng)注冊(cè)