tez-common/src/main/java/org/apache/tez/common/AsyncDispatcherConcurrent.java [229:250]:
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  public void register(Class<? extends Enum> eventType,
      EventHandler handler) {
    Preconditions.checkState(getServiceState() == STATE.NOTINITED);
    /* check to see if we have a listener registered */
    EventHandler<Event> registeredHandler = (EventHandler<Event>) eventHandlers.get(eventType);
    checkForExistingDispatchers(false, eventType);
    LOG.info("Registering " + eventType + " for " + handler.getClass());
    if (registeredHandler == null) {
      eventHandlers.put(eventType, handler);
    } else if (!(registeredHandler instanceof MultiListenerHandler)){
      /* for multiple listeners of an event add the multiple listener handler */
      MultiListenerHandler multiHandler = new MultiListenerHandler();
      multiHandler.addHandler(registeredHandler);
      multiHandler.addHandler(handler);
      eventHandlers.put(eventType, multiHandler);
    } else {
      /* already a multilistener, just add to it */
      MultiListenerHandler multiHandler
      = (MultiListenerHandler) registeredHandler;
      multiHandler.addHandler(handler);
    }
  }
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tez-common/src/main/java/org/apache/tez/common/AsyncDispatcher.java [257:278]:
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  public void register(Class<? extends Enum> eventType,
      EventHandler handler) {
    Preconditions.checkState(getServiceState() == STATE.NOTINITED);
    /* check to see if we have a listener registered */
    EventHandler<Event> registeredHandler = (EventHandler<Event>) eventHandlers.get(eventType);
    checkForExistingDispatchers(false, eventType);
    LOG.info("Registering " + eventType + " for " + handler.getClass());
    if (registeredHandler == null) {
      eventHandlers.put(eventType, handler);
    } else if (!(registeredHandler instanceof MultiListenerHandler)){
      /* for multiple listeners of an event add the multiple listener handler */
      MultiListenerHandler multiHandler = new MultiListenerHandler();
      multiHandler.addHandler(registeredHandler);
      multiHandler.addHandler(handler);
      eventHandlers.put(eventType, multiHandler);
    } else {
      /* already a multilistener, just add to it */
      MultiListenerHandler multiHandler
      = (MultiListenerHandler) registeredHandler;
      multiHandler.addHandler(handler);
    }
  }
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