springboot 源码解析之监听器

 

事件发布的入口

applicationContext.publishEvent(new BlackListEvent());

 

实际上是AbstractApplicationContext 类的方法执行

 

@Override public void publishEvent(ApplicationEvent event) { publishEvent(event, null); }

 

protected void publishEvent(Object event, @Nullable ResolvableType eventType) {
   Assert.notNull(event, "Event must not be null");

   // Decorate event as an ApplicationEvent if necessary
   ApplicationEvent applicationEvent;
   if (event instanceof 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 {
       // 这里是发布事件的地方,
      getApplicationEventMulticaster().multicastEvent(applicationEvent, eventType);
   }

   // Publish event via parent context as well...
   if (this.parent != null) {
      if (this.parent instanceof AbstractApplicationContext) {
         ((AbstractApplicationContext) this.parent).publishEvent(event, eventType);
      }
      else {
         this.parent.publishEvent(event);
      }
   }
}

 

这个ApplicationEventMulticaster 维护着所有的 listener 使用了监听器模式, 他有一个默认实现是 SimpleApplicationEventMulticaster

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;
}

 

 

看一下 SimpleApplicationEventMulticaster 是如何发布的

获取所有的监听器,

@Override
public void multicastEvent(final ApplicationEvent event, @Nullable ResolvableType eventType) {
   ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));
   //  这里获取一个线程池, 默认是没有配置的, 意味着事件的发布与监听 是一个线程, 所以事件监听过程中如果有阻塞 ,主流程将会被阻塞,那将是致命的, 
   // 可以自己配置一个线程池, 然后异步执行监听动作, 但意味着 其不能使用 事件发布动作的上下文环境, (变量的值)
   Executor executor = getTaskExecutor();
   // 获取所有监听器并执行的动作
   for (ApplicationListener listener : getApplicationListeners(event, type)) {
      if (executor != null) {
         executor.execute(() -> invokeListener(listener, event));
      }
      else {
         invokeListener(listener, event);
      }
   }
}

 

 

protected Collection> getApplicationListeners(
      ApplicationEvent event, ResolvableType eventType) {

   Object source = event.getSource();
   Class sourceType = (source != null ? source.getClass() : null);
   ListenerCacheKey cacheKey = new ListenerCacheKey(eventType, sourceType);

   // 先从缓存中获取监听器, 如果获取到 直接返回
   // Quick check for existing entry on ConcurrentHashMap...
   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);
         Collection> listeners =
               retrieveApplicationListeners(eventType, sourceType, retriever);
         this.retrieverCache.put(cacheKey, retriever);
         return listeners;
      }
   }
   else {
      // No ListenerRetriever caching -> no synchronization necessary
      return retrieveApplicationListeners(eventType, sourceType, null);
   }
}

 

下边这个方法是执行获取监听器动作的

private Collection> retrieveApplicationListeners(
      ResolvableType eventType, @Nullable Class sourceType, @Nullable ListenerRetriever retriever) {

   List> allListeners = new ArrayList<>();
   Set> listeners;
   Set listenerBeans;
   synchronized (this.retrievalMutex) {
      listeners = new LinkedHashSet<>(this.defaultRetriever.applicationListeners);
      listenerBeans = new LinkedHashSet<>(this.defaultRetriever.applicationListenerBeans);
   }
   //  遍历所有监听器
   for (ApplicationListener listener : listeners) {
       // 这个supportsEvent 方法是判断监听器是否适用于此事件的
      if (supportsEvent(listener, eventType, sourceType)) {
         if (retriever != null) {
            retriever.applicationListeners.add(listener);
         }
         allListeners.add(listener);
      }
   }
   if (!listenerBeans.isEmpty()) {
      BeanFactory beanFactory = getBeanFactory();
      for (String listenerBeanName : listenerBeans) {
         try {
            Class listenerType = beanFactory.getType(listenerBeanName);
            if (listenerType == null || supportsEvent(listenerType, eventType)) {
               ApplicationListener listener =
                     beanFactory.getBean(listenerBeanName, ApplicationListener.class);
               if (!allListeners.contains(listener) && supportsEvent(listener, eventType, sourceType)) {
                  if (retriever != null) {
                     if (beanFactory.isSingleton(listenerBeanName)) {
                        retriever.applicationListeners.add(listener);
                     }
                     else {
                        retriever.applicationListenerBeans.add(listenerBeanName);
                     }
                  }
                  allListeners.add(listener);
               }
            }
         }
         catch (NoSuchBeanDefinitionException ex) {
            // Singleton listener instance (without backing bean definition) disappeared -
            // probably in the middle of the destruction phase
         }
      }
   }
   AnnotationAwareOrderComparator.sort(allListeners);
   if (retriever != null && retriever.applicationListenerBeans.isEmpty()) {
      retriever.applicationListeners.clear();
      retriever.applicationListeners.addAll(allListeners);
   }
   return allListeners;
}

具体是怎么做的呢

这里运用了适配器模式 将ApplicationListener 适配成了 GenericApplicationListener 同时将监听器的 泛型提取了出来 ,

 

protected boolean supportsEvent(
      ApplicationListener listener, ResolvableType eventType, @Nullable Class sourceType) {

   GenericApplicationListener smartListener = (listener instanceof GenericApplicationListener ?
         (GenericApplicationListener) listener : new GenericApplicationListenerAdapter(listener));
   return (smartListener.supportsEventType(eventType) && smartListener.supportsSourceType(sourceType));
}

 

public GenericApplicationListenerAdapter(ApplicationListener delegate) {
   Assert.notNull(delegate, "Delegate listener must not be null");
   this.delegate = (ApplicationListener) delegate;
   // 这里提取了泛型
   this.declaredEventType = resolveDeclaredEventType(this.delegate);
}

 

@Override
@SuppressWarnings("unchecked")
public boolean supportsEventType(ResolvableType eventType) {
   if (this.delegate instanceof SmartApplicationListener) {
      Class eventClass = (Class) eventType.resolve();
      return (eventClass != null && ((SmartApplicationListener) this.delegate).supportsEventType(eventClass));
   }
   else {
      return (this.declaredEventType == null || this.declaredEventType.isAssignableFrom(eventType));
   }
}

 

获取到监听器后 开始执行监听动作

回到 SimpleApplicationEventMulticaster 类

@Override
public void multicastEvent(final ApplicationEvent event, @Nullable ResolvableType eventType) {
   ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));
   Executor executor = getTaskExecutor();
   for (ApplicationListener listener : getApplicationListeners(event, type)) {
      if (executor != null) {
         executor.execute(() -> invokeListener(listener, event));
      }
      else {
          // 调用监听器
         invokeListener(listener, event);
      }
   }
}



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);
   }
}


private void doInvokeListener(ApplicationListener listener, ApplicationEvent event) {
   try {
       // 注意这一行代码 , 就调用到了自定义的监听器
      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.isTraceEnabled()) {
            logger.trace("Non-matching event type for listener: " + listener, ex);
         }
      }
      else {
         throw ex;
      }
   }
}