Apparently, Oracle is abandoning JNLP (otherwise known as Java Web Start), and as of Java 11 they are not shipping it anymore, and as such it seems we need something else to replace it. I've been checking out the options that are out there already, and they seemed less than fitting for our purposes, and so in the end I've had to write my own launcher.
I'd like to start using it fairly soon, but I'd like to ask you to try it out first, as I'm sure there may very well be edge-cases out there that it doesn't handle well yet. If you feel up for it, please use this link:
http://game.havenandhearth.com/java/hafen-launcher.jar
You'll notice it's a .jar instead of a .jnlp file. Jar files should be just as directly executable as JNLP files, so all you should have to do is "just run it". Please report any problems with it!
As a reflective note, JNLP was by far and away the most important reason why I originally chose to write the client in Java, and while there are many things I like about JNLP, it has been taking a stranger and stranger route in the years since then (the most important one being when Oracle decided to require an "officially" signed certificate for JNLP programs), and so if I had known about that back then, I would have probably taken a diffeent approach. There are other things that I appreciate about the client being in Java (it's still pretty easy to make it run on a variety of platform, and the dynamic code loading system of Java is particularly nice), but if it hadn't been for JNLP, I would have probably not chosen to use Java, so it's interesting how things turn out.
I also think it is very strange that Oracle decided to deprecate JNLP, and I can't really fathom why they did so. Their official reason seems to be that they think that "app stores" are the future, which I both hope and think is untrue, and also strikes me as a weird reason to deprecate a technology that is not only mature and functional, but also seems to be widely used in practice. Weird stuff. I see other games (such as Wurm Online) still use it, seemingly without second thoughts or official plans to transition from it. Nevertheless, Oracle seems steadfast in its decision, so I guess all I can do is to deal with it.