Spring注解--监听事件:ApplicationListen

1. 基本用法

• 定义自己的ApplicationListener

@Component
public class MyApplicationListener implements ApplicationListener<ApplicationEvent> {
    @Override
    public void onApplicationEvent(ApplicationEvent event) {
        System.out.println(" my application listener" + event);
    }
}

• 定义自己的事件

public class MyApplicationEvent extends ApplicationEvent {
    public MyApplicationEvent(Object source) {
        super(source);
    }
}

• 发布事件

public static void main(String[] args) {
        AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(MainConfig.class);
        context.publishEvent(new MyApplicationEvent(new String("event")) {
        });
    }

2. 源码分析

• 在容器创建初始化的时候，initApplicationEventMulticaster()方法会初始化ApplicationEventMulticaster（事件分发器), 创建一个SimpleApplicationEventMulticaster类：

this.applicationEventMulticaster = new SimpleApplicationEventMulticaster(beanFactory);
beanFactory.registerSingleton(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, this.applicationEventMulticaster);

• 在容器创建的最后的finishRefresh()方法中有一个publishEvent(new ContextRefreshedEvent(this))

protected void publishEvent(Object event, ResolvableType eventType) {
        ...
    //将指定的event的publish给所有的listener
    getApplicationEventMulticaster().multicastEvent(applicationEvent, eventType);       
      ...
}

• multicastEvent，回调所有listener的onApplicationEvent方法

@Override
    public void multicastEvent(final ApplicationEvent event, ResolvableType eventType) {
          //找到容器中所有的listener，执行listener
        for (final ApplicationListener<?> listener : getApplicationListeners(event, type)) {
                ...//执行listener
                invokeListener(listener, event);
                ...
            }
        }
    }

• 容器中的监听器
  在容器初始化中， registerListeners()会将容器中的listener添加到ApplicationEventMulticaster中

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!
        String[] listenerBeanNames = getBeanNamesForType(ApplicationListener.class, true, false);
        for (String listenerBeanName : listenerBeanNames) {
            getApplicationEventMulticaster().addApplicationListenerBean(listenerBeanName);
        }
    }

在multicastEvent方法的getApplicationListeners方法中，会从ApplicationEventMulticaster中取出添加进去的listener

listeners = new LinkedHashSet<ApplicationListener<?>>(this.defaultRetriever.applicationListeners);
listenerBeans = new LinkedHashSet<String>(this.defaultRetriever.applicationListenerBeans);