In migration guide when describing how to migrate valves there is mentioned that it can be migrated to undertow handlers. There is only info how to configure handler and no info how to create one. Info how to create custom handler is important mainly when migrating custom valves.

Info how to create custom handler can be found in upstream documentation [1]

[1] http://undertow.io/undertow-docs/undertow-docs-1.3.0/index.html#undertow-handler-authors-guide