Spring Event 实现原理
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转载：
继承 ApplicationEvent 类
import lombok.Getter;
import org.springframework.context.ApplicationEvent;
@Getter
public class EventDemo extends ApplicationEvent {
private String message;
// Object source 具体事件源类
public EventDemo(Object source, String message) {
super(source);
this.message = message;
}
}实现 ApplicationListener 类，并重写 onApplicationEvent() 方法
import org.springframework.context.ApplicationListener;
import org.springframework.stereotype.Component;
@Component
public class EventDemoListern implements ApplicationListener<EventDemo> {
@Override
public void onApplicationEvent(EventDemo event) {
System.out.println("收到信息: " + event.getMessage());
}
}ApplicationEventPublisher Bean 发布事件
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.ApplicationEventPublisher;
import org.springframework.stereotype.Component;
@Component
public class EventDemoPublish {
@Autowired
private ApplicationEventPublisher applicationEventPublisher;
public void publish(String message) {
EventDemo eventDemo = new EventDemo(this, message);
applicationEventPublisher.publishEvent(eventDemo);
}
}从 demo 的代码来看 事件是通过
ApplicationEventPublisher类的publishEvent()方法发送的；那么通过断点追踪此方法的行为轨迹，我们会发现其实是调用了AbstractApplicationContext类的publishEvent()方法，那么请看下面的源码。
protected void publishEvent(Object event, @Nullable ResolvableType eventType) {
Assert.notNull(event, "Event must not be null");
ApplicationEvent applicationEvent;
if (event instanceof ApplicationEvent) {
//判断事件是否是 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 {
//发送事件的入口
getApplicationEventMulticaster().multicastEvent(applicationEvent, eventType);
}
// 有父类也发送事件
if (this.parent != null) {
if (this.parent instanceof AbstractApplicationContext) {
((AbstractApplicationContext) this.parent).publishEvent(event, eventType);
}
else {
this.parent.publishEvent(event);
}
}
}既然找到了发送事件的入口，那么就先追踪
applicationEventMulticaster()方法
/** Helper class used in event publishing. */
@Nullable
private ApplicationEventMulticaster applicationEventMulticaster;
ApplicationEventMulticaster getApplicationEventMulticaster() throws IllegalStateException {
if (this.applicationEventMulticaster == null) {
throw new IllegalStateException("ApplicationEventMulticaster not initialized - " +
"call 'refresh' before multicasting events via the context: " + this);
}
return this.applicationEventMulticaster;
}到这里我们只掌握了 ApplicationEventMulticaster 是个接口类，并不知道真正的实现类，而且也没发现关于此接口的构造函数(稍后再揭秘实现类究竟是怎么来的)；但是通过断点追踪 multicastEvent() 方法，不难发现其实是调用了 SimpleApplicationEventMulticaster 类的 multicastEvent()，源码如下：
public void multicastEvent(final ApplicationEvent event, @Nullable ResolvableType eventType) {
ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));
Executor executor = getTaskExecutor();
// getApplicationListeners() 获取 event 对应的监听者
for (ApplicationListener<?> listener : getApplicationListeners(event, type)) {
if (executor != null) {
executor.execute(() -> invokeListener(listener, event));
}
else {
// 调用实例
invokeListener(listener, event);
}
}
}走到这里我们会发现事件的监听者都来自 getApplicationListeners(event, type)；
通知监听者的其实是 invokeListener(listener, event)；
protected Collection<ApplicationListener<?>> getApplicationListeners(
ApplicationEvent event, ResolvableType eventType) {
Object source = event.getSource();
Class<?> sourceType = (source != null ? source.getClass() : null);
ListenerCacheKey cacheKey = new ListenerCacheKey(eventType, sourceType);
// Quick check for existing entry on ConcurrentHashMap...
// 内部维护的缓存 (ConcurrentHashMap) 方便下次获取对应的监听者
ListenerRetriever retriever = this.retrieverCache.get(cacheKey);
if (retriever != null) {
return retriever.getApplicationListeners();
}
if (this.beanClassLoader == null ||
(ClassUtils.isCacheSafe(event.getClass(), this.beanClassLoader) &&
(sourceType == null || ClassUtils.isCacheSafe(sourceType, this.beanClassLoader)))) {
// Fully synchronized building and caching of a ListenerRetriever
synchronized (this.retrievalMutex) {
retriever = this.retrieverCache.get(cacheKey);
if (retriever != null) {
return retriever.getApplicationListeners();
}
retriever = new ListenerRetriever(true);
//第一次获取事件对应的监听者并放置缓存
Collection<ApplicationListener<?>> listeners =
retrieveApplicationListeners(eventType, sourceType, retriever);
this.retrieverCache.put(cacheKey, retriever);
return listeners;
}
}
else {
// No ListenerRetriever caching -> no synchronization necessary
return retrieveApplicationListeners(eventType, sourceType, null);
}
}
// 只截取了核心代码
private Collection<ApplicationListener<?>> retrieveApplicationListeners(
ResolvableType eventType, @Nullable Class<?> sourceType, @Nullable ListenerRetriever retriever) {
List<ApplicationListener<?>> allListeners = new ArrayList<>();
Set<ApplicationListener<?>> listeners;
Set<String> listenerBeans;
synchronized (this.retrievalMutex) {
// 获取所有 ApplicationListener 的实现类
// 注： listeners 和 listenerBeans 均是取内部类 ListenerRetriever 维护的 LinkedHashSet
listeners = new LinkedHashSet<>(this.defaultRetriever.applicationListeners);
listenerBeans = new LinkedHashSet<>(this.defaultRetriever.applicationListenerBeans);
}
for (ApplicationListener<?> listener : listeners) {
// 判断 listener 的 event 是否一致
if (supportsEvent(listener, eventType, sourceType)) {
if (retriever != null) {
retriever.applicationListeners.add(listener);
}
allListeners.add(listener);
}
}
...
return allListeners;
}
// 内部类
private class ListenerRetriever {
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;
}
public Collection<ApplicationListener<?>> getApplicationListeners() {
...
}
}这里同样也会产生一个问题。我们知道 listeners 取自内部类 ListenerRetriever 维护的 applicationListeners 对象，那么 applicationListeners 何时 add 了呢？稍后再揭秘。
总结 获取所有 ApplicationListener 的实现类后，依次判断是否支持给定的事件，也就是事件的类型是否一致；最终通过特定的 cachKey 放置缓存。
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);
}
}
@SuppressWarnings({"rawtypes", "unchecked"})
private void doInvokeListener(ApplicationListener listener, ApplicationEvent event) {
try {
// 直接调用 onApplicationEvent
listener.onApplicationEvent(event);
}
catch (ClassCastException ex) {
...
}
}核心代码：listener.onApplicationEvent(event); 拿到监听者的实例，直接调用 onApplicationEvent() 方法
为了揭秘上面两个问题，我们继续观察 AbstractApplicationContext 类，这个类里有我们想要的答案；
@Override
public void addApplicationListener(ApplicationListener<?> listener) {
Assert.notNull(listener, "ApplicationListener must not be null");
if (this.applicationEventMulticaster != null) {
this.applicationEventMulticaster.addApplicationListener(listener);
}
this.applicationListeners.add(listener);
}
/**
* Return the list of statically specified ApplicationListeners.
*/
public Collection<ApplicationListener<?>> getApplicationListeners() {
return this.applicationListeners;
}
@Override
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
...
try {
...
// Initialize event multicaster for this context.
// 初始化 ApplicationEventMulticaster
// Initialize the ApplicationEventMulticaster.
// Uses SimpleApplicationEventMulticaster if none defined in the context.
initApplicationEventMulticaster();
// Initialize other special beans in specific context subclasses.
onRefresh();
// Check for listener beans and register them.
registerListeners();
// Instantiate all remaining (non-lazy-init) singletons.
// 自己实现的 ApplicationListener 从这里实例化，经过复杂的 bean 实例过程，
// 最终由 ApplicationListenerDetector 类调用 addApplicationListener();
finishBeanFactoryInitialization(beanFactory);
// Last step: publish corresponding event.
finishRefresh();
}
...
}也就是说在容器启动过程中会装载ApplicationEventMulticaster，以及存放applicationListeners到容器中。