EventBus与Spring Event区别详解(EventBus 事件机制,Spring Event事件机制)

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。

/**
 * 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：

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

spring event

spring 新版事件机制也比较简单，看代码。

/**
 * 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：

1. 按照类区分事件
2. 订阅 事件簇
3. 支持自定义event
4. 按照condition过滤同类型事件

比较EventBus与Spring Event

使用方式比较

使用场景比较

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