Undertow’s logging sort of sucks. Add default debug logging into the predicate handler that logs each handler as its executed and what the result of that predicate is and what handlers are run as a result.
The first place I would recommend we add some logging to is the /undertow/core/src/main/java/io/undertow/server/handlers/PredicateHandler.java class in the handleRequest() method
We can also see about logging the data inside the predicate's PREDICATE_CONTEXT as a trace level log message
We may need a custom build of Undertow to use this at first. We need to send any changes we make to Undertow’s upstream repo as a pull.
Build Undertow like this: