spring事件驱动过程分析

在看Spring源码之前确保理解了JDK中的 EventObject,EventListener的使用场景，其实体现的是观察者模式，适用于很多场景。

在Spring中，事件基类ApplicationEvent，在应用的周期中触发（发布）相应的事件。

public abstract class ApplicationEvent extends EventObject {

	/** use serialVersionUID from Spring 1.2 for interoperability */
	private static final long serialVersionUID = 7099057708183571937L;

	/** System time when the event happened */
	private final long timestamp;


	/**
	 * Create a new ApplicationEvent.
	 * @param source the object on which the event initially occurred (never {@code null})
	 */
	public ApplicationEvent(Object source) {
		super(source);
		this.timestamp = System.currentTimeMillis();
	}


	/**
	 * Return the system time in milliseconds when the event happened.
	 */
	public final long getTimestamp() {
		return this.timestamp;
	}

}

监听器基类如下，在接收到事件后如果是自己感兴趣的就进行相应处理。

public interface ApplicationListener<E extends ApplicationEvent> extends EventListener {

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

}

Spring boot在启动的时候会加载一部分 ApplicationListener。Spring Context加载初始化完成（refresh）后会再次检测应用中的 ApplicationListener，并且注册，此时会将我们实现的 ApplicationListener 就会加入到 SimpleApplicationEventMulticaster 维护的 Listener 集合中。

事件触发的流程是怎样的？以 ContextRefreshedEvent 为例。在Context初始化完成后，在 finishRefresh 方法中会发布 ContextRefreshedEvent 事件。

protected void finishRefresh() {
	// Initialize lifecycle processor for this context.
	initLifecycleProcessor();

	// Propagate refresh to lifecycle processor first.
	getLifecycleProcessor().onRefresh();

	// Publish the final event.
	publishEvent(new ContextRefreshedEvent(this));

	// Participate in LiveBeansView MBean, if active.
	LiveBeansView.registerApplicationContext(this);
}
protected void publishEvent(Object event, ResolvableType eventType) {
	// Decorate event as an ApplicationEvent if necessary
	ApplicationEvent applicationEvent;
	if (event instanceof ApplicationEvent) {
		applicationEvent = (ApplicationEvent) event;
	}
	else {
		applicationEvent = new PayloadApplicationEvent<Object>(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 { // 多播事件
		getApplicationEventMulticaster().multicastEvent(applicationEvent, eventType);
	}

	// Publish event via parent context as well... 上层的Context也会响应该事件
	if (this.parent != null) {
		if (this.parent instanceof AbstractApplicationContext) {
			((AbstractApplicationContext) this.parent).publishEvent(event, eventType);
		}
		else {
			this.parent.publishEvent(event);
		}
	}
}
public void multicastEvent(final ApplicationEvent event, ResolvableType eventType) {
	ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));
	for (final ApplicationListener listener : getApplicationListeners(event, type)) {
		Executor executor = getTaskExecutor();
		// 如果设置了 Executor 则异步执行
		if (executor != null) {
			executor.execute(new Runnable() {
				@Override
				public void run() {
					invokeListener(listener, event);
				}
			});
		}
		else {// 否则同步调用listener的事件处理方法
			invokeListener(listener, event);
		}
	}
}

内部 listener 是如何维护的？使用的哪种集合？是否线程安全？

org.springframework.context.event.AbstractApplicationEventMulticaster.ListenerRetriever

private class ListenerRetriever {
    // 这里
	public final Set<ApplicationListener> applicationListeners;

	public final Set<String> applicationListenerBeans;

	private final boolean preFiltered;

	public ListenerRetriever(boolean preFiltered) {
		this.applicationListeners = new LinkedHashSet<ApplicationListener>();
		this.applicationListenerBeans = new LinkedHashSet<String>();
		this.preFiltered = preFiltered;
	}

	public Collection<ApplicationListener> getApplicationListeners() {
		LinkedList<ApplicationListener> allListeners = new LinkedList<ApplicationListener>();
		for (ApplicationListener listener : this.applicationListeners) {
			allListeners.add(listener);
		}
		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
				}
			}
		}
		// 根据Order注解排序
		AnnotationAwareOrderComparator.sort(allListeners);
		return allListeners;
	}
}

在增加元素到Set的时候有同步操作。

// 使用 ListenerRetriever 作为加锁对象
private Object retrievalMutex = this.defaultRetriever;

public void addApplicationListener(ApplicationListener listener) {
	synchronized (this.retrievalMutex) {
		this.defaultRetriever.applicationListeners.add(listener);
		this.retrieverCache.clear();
	}
}

public void addApplicationListenerBean(String listenerBeanName) {
	synchronized (this.retrievalMutex) {
		this.defaultRetriever.applicationListenerBeans.add(listenerBeanName);
		this.retrieverCache.clear();
	}
}