# Package org.zkoss.zkex.zul.impl

- Interface Summary
  Interface | Description
  --- | ---
  Operation | This interface is for model sharer developer only, you rarely need to use this interface.
  OperationQueueListener | This class is for model sharer developer only, you rarely need to use this class.

- Class Summary
  Class | Description
  --- | ---
  JFreeChartEngine | A chart engine implemented with JFreeChart.
  JHLabsCaptchaEngine | A captcha engine implemented with JH Labs libraries.
  OperationQueue | A queue for storing `Operation` and is thread-safe.
  OperationThread | This class is for model sharer developer only, you rarely need to use this class.