Listener:

org.zkoss.zk.ui.util.Monitor

A listener could implement org.zkoss.zk.ui.util.Monitor to monitor the statuses of ZK. Unlike other listeners, there is at most one monitor listener for each Web application. If you like, you can chain them together manually.

ZK provides an implementation named org.zkoss.zk.ui.util.Statistic, which accumulates the statistic data in the memory. It is a good starting point to understand the loading of your ZK application.

Instantiation: Unlike most other listeners, at most one monitor instance is created in one application. In addition, it is shared, so the implementation must be thread-safe.