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I was recently looking for a boardgame engine, and briefly considered vassal. It looks really unlikely. The documentation is all a decade old. It's written in Java, but there's no Javadoc. There are a ton of modules written for it, so somebody clearly cares about it, but it looked very outdated in terms of its technology.
I ended up going with boardgame.io, which provides far less, but for my simple project I didn't want all that much (mostly, solving the hassle of getting game state synchronized among players). Not that I really wanted to write Javascript rather than Java, but the web made a better target platform than Java Swing.