SpringBoot基础-监听器实现分析

自定义监听器

定义监听器

实现ApplicationListener接口

@Order(1)
public class FirstListener implements ApplicationListener<ApplicationStartedEvent> {
    @Override
    public void onApplicationEvent(ApplicationStartedEvent event) {
        System.out.println("FirstListener:" + event.toString());
    }
}

实现SmartApplicationListener接口

@Order(1)
public class FourListener implements SmartApplicationListener {

    @Override
    public boolean supportsEventType(Class<? extends ApplicationEvent> eventType) {
        //对哪些事件感兴趣
        return ApplicationStartedEvent.class.isAssignableFrom(eventType) || ApplicationPreparedEvent.class.isAssignableFrom(eventType);
    }

    @Override
    public void onApplicationEvent(ApplicationEvent event) {
        System.out.println("FourListener:" + event.toString());
    }
}

添加监听器到Spring中

META-INF/spring.factories 中添加

org.springframework.context.ApplicationListener=com.demo.silly.spring.boot.listener.spring.FirstListener

硬编码添加

@SpringBootApplication
public class SpringBootDemoApplication {
    public static void main(String[] args) throws InterruptedException {
        SpringApplication springApplication = new SpringApplication((SpringBootDemoApplication.class));
        springApplication.addListeners(new FirstListener());
        springApplication.run(args);
    }
}

application.yml添加

context.listener.classes: com.demo.silly.spring.boot.listener.spring.ThirdListener

源码

监听器定义

//EventListener 
package java.util;

/**
 * A tagging interface that all event listener interfaces must extend.
 * @since JDK1.1
 */
public interface EventListener {
}

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

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


	/**
	 * Create a new {@code ApplicationListener} for the given payload consumer.
	 * @param consumer the event payload consumer
	 * @param <T> the type of the event payload
	 * @return a corresponding {@code ApplicationListener} instance
	 * @since 5.3
	 * @see PayloadApplicationEvent
	 */
	static <T> ApplicationListener<PayloadApplicationEvent<T>> forPayload(Consumer<T> consumer) {
		return event -> consumer.accept(event.getPayload());
	}

}

系统广播器定义

public interface ApplicationEventMulticaster {

	/**
	 * Add a listener to be notified of all events.
	 * @param listener the listener to add
	 */
	void addApplicationListener(ApplicationListener<?> listener);

	/**
	 * Add a listener bean to be notified of all events.
	 * @param listenerBeanName the name of the listener bean to add
	 */
	void addApplicationListenerBean(String listenerBeanName);

	/**
	 * Remove a listener from the notification list.
	 * @param listener the listener to remove
	 */
	void removeApplicationListener(ApplicationListener<?> listener);

	/**
	 * Remove a listener bean from the notification list.
	 * @param listenerBeanName the name of the listener bean to remove
	 */
	void removeApplicationListenerBean(String listenerBeanName);

	/**
	 * Remove all listeners registered with this multicaster.
	 * <p>After a remove call, the multicaster will perform no action
	 * on event notification until new listeners are registered.
	 */
	void removeAllListeners();

	/**
	 * Multicast the given application event to appropriate listeners.
	 * <p>Consider using {@link #multicastEvent(ApplicationEvent, ResolvableType)}
	 * if possible as it provides better support for generics-based events.
	 * @param event the event to multicast
	 */
	void multicastEvent(ApplicationEvent event);

	/**
	 * Multicast the given application event to appropriate listeners.
	 * <p>If the {@code eventType} is {@code null}, a default type is built
	 * based on the {@code event} instance.
	 * @param event the event to multicast
	 * @param eventType the type of event (can be {@code null})
	 * @since 4.2
	 */
	void multicastEvent(ApplicationEvent event, @Nullable ResolvableType eventType);

}

系统事件

监听器注册

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);
	}

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