Spring的事件驱动模型

Spring事件驱动模型的三个概念：事件，事件监听者、事件发布者。

事件：如下图所示,Spring中定义了事件的抽象类ApplicationEvent,该类继承自JDK的 EventObjectl类,ApplicationEvent中的timestamp字段用来存储事件发生的时间戳,父类中的source表示事件发生的对象。当我们要定义时间的时候只需要继承ApplicationEvent,如下面自定义的PublishEvent事件

        //自定义事件  
    public class PublishEvent extends ApplicationEvent {  
    private String name;
    public PublishEvent(Object source) {
        super(source);
        this.name = (String) source;
    }
    
    @Override
    public String toString() {
        return "PublishEvent{" +
                "name='" + name + '\'' +
                '}';
    }
}  

事件监听者: 事件监听者负责监听和处理事件,如下图所示,Spring定义了事件的监听接口ApplicationListener,该接口继承自JDK的EventListener接口,并且ApplicationListener规定了onApplicatioEvent(E)函数的参数必须为ApplicationEvent的子类,所以每个时间监听者都只是针对某个ApplicationEvent的子类的监听。

 //自定义时间监听者
 public class EventListener implements   ApplicationListener {

     public void onApplicationEvent(PublishEvent publishEvent) {

         System.out.println("EventListener>>>>>>>>>>>>"+publishEvent.toString());

    }
}

事件发布者：事件发布者的作用显而易见,就是发布事件,将事件交给对应的发布者处理,那么Spring是如何做到的呢? 我们来看下图。Spring定义了ApplicationEventPublisher接口和ApplicationEventMulticaster接口。ApplicationEventPublisher用来发布事件,ApplicationEventMulticaster是用来维护事件监听者和将事件多播给事件的监听者,其实事件的真正发布就是在ApplicationEventPublisher的multicaster方法中实现的。

实现原理：
ApplicationEventPublisher的publishEvent的实现是在AbstractApplicationContext中事件的，代码如下：

 protected void publishEvent(Object event, @Nullable ResolvableType eventType) {
    Assert.notNull(event, "Event must not be null");
    if (this.logger.isTraceEnabled()) {
        this.logger.trace("Publishing event in " + this.getDisplayName() + ": " + event);
    }

    Object applicationEvent;
    if (event instanceof ApplicationEvent) {
        applicationEvent = (ApplicationEvent)event;
    } else {
        applicationEvent = new PayloadApplicationEvent(this, event);
        if (eventType == null) {
            eventType = ((PayloadApplicationEvent)applicationEvent).getResolvableType();
        }
    }

    if (this.earlyApplicationEvents != null) {
        this.earlyApplicationEvents.add(applicationEvent);
    } else {
        //调用ApplicationEventMulticaster的实现类SimpleApplicationEventMulticaster的multicastEvent方法多播事件
        this.getApplicationEventMulticaster().multicastEvent((ApplicationEvent)applicationEvent, eventType);
    }

    if (this.parent != null) {
        if (this.parent instanceof AbstractApplicationContext) {
            ((AbstractApplicationContext)this.parent).publishEvent(event, eventType);
        } else {
            this.parent.publishEvent(event);
        }
    }

}

SimpleApplicationEventMulticaster中的相关代码

public void multicastEvent(ApplicationEvent event, @Nullable ResolvableType eventType) {
    ResolvableType type = eventType != null ? eventType : this.resolveDefaultEventType(event);
    //得到event的集合
    Iterator var4 = this.getApplicationListeners(event, type).iterator();
   //遍历集合执行调用
    while(var4.hasNext()) {
        ApplicationListener listener = (ApplicationListener)var4.next();
        Executor executor = this.getTaskExecutor();
        if (executor != null) {
            executor.execute(() -> {
                this.invokeListener(listener, event);
            });
        } else {
            this.invokeListener(listener, event);
        }
    }

}

protected void invokeListener(ApplicationListener listener, ApplicationEvent event) {
    ErrorHandler errorHandler = this.getErrorHandler();
    if (errorHandler != null) {
        try {
            this.doInvokeListener(listener, event);
        } catch (Throwable var5) {
            errorHandler.handleError(var5);
        }
    } else {
        this.doInvokeListener(listener, event);
    }

}


private void doInvokeListener(ApplicationListener listener, ApplicationEvent event) {
    try {
        //最终调用listener的onApplicationEvent方法
        listener.onApplicationEvent(event);
    } catch (ClassCastException var6) {
        String msg = var6.getMessage();
        if (msg != null && !this.matchesClassCastMessage(msg, event.getClass().getName())) {
            throw var6;
        }

        Log logger = LogFactory.getLog(this.getClass());
        if (logger.isDebugEnabled()) {
            logger.debug("Non-matching event type for listener: " + listener, var6);
        }
    }

}

编写测试用例,在用例中我们注入ApplicationEventPublisher

@SpringBootTest(classes = Application.class)
@RunWith(SpringRunner.class)
public class EventTest {

    @Autowired
    private ApplicationEventPublisher applicationEventPublisher;

    @Test
    public void testEvent(){
        PublishEvent publishEvent = new PublishEvent("abc");
        applicationEventPublisher.publishEvent(publishEvent);
    }

}

源码地址:https://github.com/zuijiazhenrong/spring-event-demo