org.apache.geode.cache.query

## Interface CqListener

- All Superinterfaces:
    CacheCallback, Declarable
  All Known Subinterfaces:
    CqStatusListener
  All Known Implementing Classes:
    CqListenerAdapter

      public interface CqListener
      extends CacheCallback

  Application plug-in interface for handling continuous query events after they occur. The listener has two methods, one that is called when there is an event satisfied by the CQ and the other called when there is an error during CQ processing.
  Since:
    GemFire 5.5

- - ### Method Summary

    All Methods Instance Methods Abstract Methods
    Modifier and Type | Method and Description
    --- | ---
    `void` | `onError(CqEvent aCqEvent)` This method is invoked when there is an error during CQ processing.
    `void` | `onEvent(CqEvent aCqEvent)` This method is invoked when an event is occurred on the region that satisfied the query condition of this CQ.

    - ### Methods inherited from interface org.apache.geode.cache.CacheCallback

      `close`

    - ### Methods inherited from interface org.apache.geode.cache.Declarable

      `init, initialize`

- - ### Method Detail

    - #### onEvent

          void onEvent(CqEvent aCqEvent)

      This method is invoked when an event is occurred on the region that satisfied the query condition of this CQ. This event does not contain an error. If CQ is executed using ExecuteWithInitialResults the returned result may already include the changes with respect to this event. This could arise when updates are happening on the region while CQ registration is in progress. The CQ does not block any region operation as it could affect the performance of region operation. Its up to the application to synchronize between the region operation and CQ registration to avoid duplicate event being delivered.
      See Also:
        `CqQuery.executeWithInitialResults()`

    - #### onError

          void onError(CqEvent aCqEvent)

      This method is invoked when there is an error during CQ processing. The error can appear while applying query condition on the event. e.g if the event doesn't has attributes as specified in the CQ query. This event does contain an error. The newValue may or may not be available, and will be null if not available.