The org.zkoss.zk.ui.util.PerformanceMeter interface

Listener:

org.zkoss.zk.ui.util.PerformanceMeter

A listener could implement PerformanceMeter to measure the performance. Unlike other listeners, there is at most one performance meter listener for each Web application. If you like, you can chain them together manually.

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