Bean相关的处理器 二、ApplicationListenerDetector

 ApplicationListenerDetector：主要作用：

    1、在Bean初始化完成之后：如果Bean是单例的则并且bean instanceof ApplicationListener。加入到this.applicationListeners中。

    2、在Bean销毁之前搞事情： 如果Bean是一个ApplicationListener，则会从ApplicationEventMulticaster（事件广播器）中提前删除了。

class ApplicationListenerDetector implements DestructionAwareBeanPostProcessor, MergedBeanDefinitionPostProcessor {

   private static final Log logger = LogFactory.getLog(ApplicationListenerDetector.class);
   private final transient AbstractApplicationContext applicationContext;
   private final transient Map<String, Boolean> singletonNames = new ConcurrentHashMap<>(256);
   public ApplicationListenerDetector(AbstractApplicationContext applicationContext) {
      this.applicationContext = applicationContext;
   }

   @Override
   public void postProcessMergedBeanDefinition(RootBeanDefinition beanDefinition, Class<?> beanType, String beanName) {
      this.singletonNames.put(beanName, beanDefinition.isSingleton());
   }
    
   @Override
   public Object postProcessBeforeInitialization(Object bean, String beanName) {
      return bean;
   }
    //初始化之前
   @Override
   public Object postProcessAfterInitialization(Object bean, String beanName) {
      if (bean instanceof ApplicationListener) {
         // potentially not detected as a listener by getBeanNamesForType retrieval
         Boolean flag = this.singletonNames.get(beanName);
         if (Boolean.TRUE.equals(flag)) {
            // singleton bean (top-level or inner): register on the fly
            this.applicationContext.addApplicationListener((ApplicationListener<?>) bean);
         }
         else if (Boolean.FALSE.equals(flag)) {
            if (logger.isWarnEnabled() && !this.applicationContext.containsBean(beanName)) {
               // inner bean with other scope - can't reliably process events
               logger.warn("Inner bean '" + beanName + "' implements ApplicationListener interface " +
                     "but is not reachable for event multicasting by its containing ApplicationContext " +
                     "because it does not have singleton scope. Only top-level listener beans are allowed " +
                     "to be of non-singleton scope.");
            }
            this.singletonNames.remove(beanName);
         }
      }
      return bean;
   }

    //销毁之前
   @Override
   public void postProcessBeforeDestruction(Object bean, String beanName) {
      if (bean instanceof ApplicationListener) {
         try {
            ApplicationEventMulticaster multicaster = this.applicationContext.getApplicationEventMulticaster();
            multicaster.removeApplicationListener((ApplicationListener<?>) bean);
            multicaster.removeApplicationListenerBean(beanName);
         }
         catch (IllegalStateException ex) {
            // ApplicationEventMulticaster not initialized yet - no need to remove a listener
         }
      }
   }

   @Override
   public boolean requiresDestruction(Object bean) {
      return (bean instanceof ApplicationListener);
   }

   @Override
   public boolean equals(Object other) {
      return (this == other || (other instanceof ApplicationListenerDetector &&
            this.applicationContext == ((ApplicationListenerDetector) other).applicationContext));
   }
   @Override
   public int hashCode() {
      return ObjectUtils.nullSafeHashCode(this.applicationContext);
   }


}