Strange — I swear this wasn't the case. I and several other devs tried for a good three weeks to get Javalin to accept such route bindings. This was a year or two ago, though. Maybe the interface type for a handler function was changed in some more recent version of Javalin?
Not sure about the older versions of Javalin, but from my experience with v5+ it just uses regular Single Abstract Method conversion for the Handler interface, which has a single method with just one parameter (the Context).
