十一、Spring之事件监听

Spring之事件监听

ApplicationListener

ApplicationListener是Spring事件机制的一部分，与抽象类ApplicationEvent类配合来完成ApplicationContext的事件机制。

如果容器中存在ApplicationListener的Bean，当ApplicationContext调用publishEvent方法时，对应的Bean会被触发。这一过程是典型的观察者模式的实现。

源码：

@FunctionalInterface
public interface ApplicationListener<E extends ApplicationEvent> extends EventListener {

    /**
     * Handle an application event.
     * @param event the event to respond to
     */
    void onApplicationEvent(E event);

}

ContextRefreshedEvent事件的监听

以Spring的内置事件ContextRefreshedEvent为例，当ApplicationContext被初始化或刷新时，会触发ContextRefreshedEvent事件，下面我们就实现一个ApplicationListener来监听此事件的发生。

@Component // 需对该类进行Bean的实例化
public class LearnListener implements ApplicationListener<ContextRefreshedEvent> {
   @Override
   public void onApplicationEvent(ContextRefreshedEvent event) {
      // 打印容器中出事Bean的数量
      System.out.println("监听器获得容器中初始化Bean数量：" + event.getApplicationContext().getBeanDefinitionCount());
   }
}

事件发布

在容器创建完成后，在finishRefresh()方法中发布了一个事件——ContextRefreshedEvent

我们来具体看一下这个事件是如何发布的

protected void publishEvent(Object event, @Nullable ResolvableType eventType) {
    Assert.notNull(event, "Event must not be null");

    // Decorate event as an ApplicationEvent if necessary
    ApplicationEvent applicationEvent;
    if (event instanceof ApplicationEvent) {
        applicationEvent = (ApplicationEvent) event;
    }
    else {
        applicationEvent = new PayloadApplicationEvent<>(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 {
        //获取事件的派发器
        getApplicationEventMulticaster().multicastEvent(applicationEvent, eventType);
    }

    // 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);
        }
    }
}

派发事件：getApplicationEventMulticaster().multicastEvent(applicationEvent, eventType);

这里的执行invokeListener主要是来回调listener的接口方法

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以上就是spring中事件发布的流程。

事件派发器

在事件发布的过程中，有一步是获取事件的派发器，那么事件派发器是在哪里创建的呢？

实际上在容器初始化时，执行了initApplicationEventMulticaster()这个方法，来为容器初始化事件派发器。

protected void initApplicationEventMulticaster() {
    ConfigurableListableBeanFactory beanFactory = getBeanFactory();
    //先来判断容器中有没有applicationEventMulticaster
    if (beanFactory.containsLocalBean(APPLICATION_EVENT_MULTICASTER_BEAN_NAME)) {
        this.applicationEventMulticaster =
            beanFactory.getBean(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, ApplicationEventMulticaster.class);
        if (logger.isTraceEnabled()) {
            logger.trace("Using ApplicationEventMulticaster [" + this.applicationEventMulticaster + "]");
        }
    }
    else {
        //如果没有则创建一个派发器
        this.applicationEventMulticaster = new SimpleApplicationEventMulticaster(beanFactory);
        beanFactory.registerSingleton(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, this.applicationEventMulticaster);
        if (logger.isTraceEnabled()) {
            logger.trace("No '" + APPLICATION_EVENT_MULTICASTER_BEAN_NAME + "' bean, using " +
                         "[" + this.applicationEventMulticaster.getClass().getSimpleName() + "]");
        }
    }
}

APPLICATION_EVENT_MULTICASTER_BEAN_NAME：

public static final String APPLICATION_EVENT_MULTICASTER_BEAN_NAME = "applicationEventMulticaster";

监听器从哪里来

refresh()方法中执行了registerListeners()来给容器中注册监听器

protected void registerListeners() {
    // Register statically specified listeners first.
    for (ApplicationListener<?> listener : getApplicationListeners()) {
        getApplicationEventMulticaster().addApplicationListener(listener);
    }

    // Do not initialize FactoryBeans here: We need to leave all regular beans
    // uninitialized to let post-processors apply to them!
    //根据类型获取所有的监听器的Bean名称
    String[] listenerBeanNames = getBeanNamesForType(ApplicationListener.class, true, false);
    for (String listenerBeanName : listenerBeanNames) {
       //将监听器加入到派发器当中
       getApplicationEventMulticaster().addApplicationListenerBean(listenerBeanName);
    }

    // Publish early application events now that we finally have a multicaster...
    Set<ApplicationEvent> earlyEventsToProcess = this.earlyApplicationEvents;
    this.earlyApplicationEvents = null;
    if (earlyEventsToProcess != null) {
        for (ApplicationEvent earlyEvent : earlyEventsToProcess) {
            getApplicationEventMulticaster().multicastEvent(earlyEvent);
        }
    }
}

@EventListener

除了实现ApplicationListener接口来完成事件监听以外，@EventListener这个注解也同样可以监听事件的发生

只需要将@EventListener标注在方法上面：

@EventListener(classes = {ApplicationEvent.class})
public void listen(ApplicationEvent applicationEvent){
    System.out.println("监听到："+applicationEvent);
}