The org.zkoss.zk.ui.util.WebAppCleanup interface
From Documentation
Listener:
org.zkoss.zk.ui.util.WebAppCleanup
A listener could implement WebAppCleanup to cleanup a ZK application, when it is being destroyed.
When a ZK application is going to be destroyed, it invokes the cleanup method of this interface such that developers could plug the application-specific codes to cleanup the application.