Spring ApplicationListener源碼解析

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

@FunctionalInterface
public interface ApplicationListener<E extends ApplicationEvent> extends EventListener {
	//監聽者監聽事件的邏輯處理
	void onApplicationEvent(E event);
	static <T> ApplicationListener<PayloadApplicationEvent<T>> forPayload(Consumer<T> consumer) {
		return event -> consumer.accept(event.getPayload());
	}
}

//事件繼承Spring中的ApplicationEvent
public class MyEvent extends ApplicationEvent {
    private String name;
    public MyEvent(ApplicationContext source,String name) {
        super(source);
        this.name = name;
    }
    public String getName() {
        return name;
    }
}

//定義監聽者實現ApplicationListener，並通過泛型聲明監聽的事件
@Component
public class MyEventListener implements ApplicationListener<MyEvent> {
    @Override
    public void onApplicationEvent(MyEvent event) {
        System.out.println("監聽MyEvent：收到消息時間:"+event.getTimestamp()+"【消息name："+event.getName() + "】");
    }
}

@Component
public class MyEventProcessor implements ApplicationContextAware {
    private ApplicationContext applicationContext;
    @Override
    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
        this.applicationContext = applicationContext;
    }
    public ApplicationContext getApplicationContext() {
        return applicationContext;
    }
}

@SpringBootApplication
public class BootApplication {
    @Resource
    private DefaultListableBeanFactory defaultListableBeanFactory;
    @Resource
    private MySpringAware mySpringAware;
    @Resource
    private MyEventProcessor myEventProcessor;
    public static void main(String[] args) {
        SpringApplication.run(BootApplication.class,args);
    }
    @PostConstruct
    public void init() {
        ApplicationContext applicationContext = myEventProcessor.getApplicationContext();
        //發佈事件，事件發佈之後，前面訂閱的監聽者就會監聽到該事件發佈的消息
        applicationContext.publishEvent(new MyEvent(applicationContext,"陳湯姆"));
        applicationContext.publishEvent(new MyEvent(applicationContext,"陳湯姆2"));
    }
}

public abstract class AbstractApplicationContext extends DefaultResourceLoader
		implements ConfigurableApplicationContext {
    @Override
	public void refresh() throws BeansException, IllegalStateException {
		synchronized (this.startupShutdownMonitor) {
			// Prepare this context for refreshing.
			prepareRefresh();
			// Tell the subclass to refresh the internal bean factory.
			ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
			// Prepare the bean factory for use in this context.
			prepareBeanFactory(beanFactory);
			try {
				// Allows post-processing of the bean factory in context subclasses.
				postProcessBeanFactory(beanFactory);
				// Invoke factory processors registered as beans in the context.
				invokeBeanFactoryPostProcessors(beanFactory);
				// Register bean processors that intercept bean creation.
				registerBeanPostProcessors(beanFactory);
				// Initialize message source for this context.
				initMessageSource();
				// Initialize event multicaster for this context.
				initApplicationEventMulticaster();
				// Initialize other special beans in specific context subclasses.
				onRefresh();
				//註冊觀察者
				registerListeners();
				// Instantiate all remaining (non-lazy-init) singletons.
				finishBeanFactoryInitialization(beanFactory);
				// Last step: publish corresponding event.
				finishRefresh();
			}
			catch (BeansException ex) {
				if (logger.isWarnEnabled()) {
					logger.warn("Exception encountered during context initialization - " +
							"cancelling refresh attempt: " + ex);
				}
				// Destroy already created singletons to avoid dangling resources.
				destroyBeans();
				// Reset 'active' flag.
				cancelRefresh(ex);
				// Propagate exception to caller.
				throw ex;
			}
			finally {
				// Reset common introspection caches in Spring's core, since we
				// might not ever need metadata for singleton beans anymore...
				resetCommonCaches();
			}
		}
	}
    //將觀察者註入到集合中
	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) {
            //調用集合的add操作
			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) {
                //獲取觀察者的集合
				getApplicationEventMulticaster().multicastEvent(earlyEvent);
			}
		}
	}
}

public abstract class AbstractApplicationEventMulticaster
		implements ApplicationEventMulticaster, BeanClassLoaderAware, BeanFactoryAware {
	private class ListenerRetriever {
        //存儲觀察者的集合
		public final Set<ApplicationListener<?>> applicationListeners = new LinkedHashSet<>();
		public final Set<String> applicationListenerBeans = new LinkedHashSet<>();
		private final boolean preFiltered;
		public ListenerRetriever(boolean preFiltered) {
			this.preFiltered = preFiltered;
		}
        //獲取觀察者的集合
		public Collection<ApplicationListener<?>> getApplicationListeners() {
			List<ApplicationListener<?>> allListeners = new ArrayList<>(
					this.applicationListeners.size() + this.applicationListenerBeans.size());
			allListeners.addAll(this.applicationListeners);
			if (!this.applicationListenerBeans.isEmpty()) {
				BeanFactory beanFactory = getBeanFactory();
				for (String listenerBeanName : this.applicationListenerBeans) {
					try {
						ApplicationListener<?> listener = beanFactory.getBean(listenerBeanName, ApplicationListener.class);
						if (this.preFiltered || !allListeners.contains(listener)) {
							allListeners.add(listener);
						}
					}
					catch (NoSuchBeanDefinitionException ex) {
						// Singleton listener instance (without backing bean definition) disappeared -
						// probably in the middle of the destruction phase
					}
				}
			}
			if (!this.preFiltered || !this.applicationListenerBeans.isEmpty()) {
				AnnotationAwareOrderComparator.sort(allListeners);
			}
			return allListeners;
		}
	}
}

public class SimpleApplicationEventMulticaster extends AbstractApplicationEventMulticaster {
	@Override
	public void multicastEvent(final ApplicationEvent event, @Nullable ResolvableType eventType) {
		ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));
        //getApplicationListeners就是獲取ApplicationListener的集合
		for (final ApplicationListener<?> listener : getApplicationListeners(event, type)) {
			Executor executor = getTaskExecutor();
			if (executor != null) {
                //執行監聽者的
				executor.execute(() -> invokeListener(listener, event));
			}
			else {
				invokeListener(listener, event);
			}
		}
	}
    //執行監聽者邏輯
    protected void invokeListener(ApplicationListener<?> listener, ApplicationEvent event) {
		ErrorHandler errorHandler = getErrorHandler();
		if (errorHandler != null) {
			try {
				doInvokeListener(listener, event);
			}
			catch (Throwable err) {
				errorHandler.handleError(err);
			}
		}
		else {
			doInvokeListener(listener, event);
		}
	}
	//最終執行邏輯
    private void doInvokeListener(ApplicationListener listener, ApplicationEvent event) {
		try {
            //調用監聽者重寫的onApplicationEvent方法
			listener.onApplicationEvent(event);
		}
		catch (ClassCastException ex) {
			String msg = ex.getMessage();
			if (msg == null || matchesClassCastMessage(msg, event.getClass())) {
				// Possibly a lambda-defined listener which we could not resolve the generic event type for
				// -> let's suppress the exception and just log a debug message.
				Log logger = LogFactory.getLog(getClass());
				if (logger.isDebugEnabled()) {
					logger.debug("Non-matching event type for listener: " + listener, ex);
				}
			}
			else {
				throw ex;
			}
		}
	}
}