A random program that I installed reset the default application (Java) for .jar files on my Macbook, and to reset the behaviour in Mac OSX took a bit more work than I thought. Turns out the “Java Launcher” isn’t in the list of applications that you can select from, in the “Open With…” list. You’ll need to select “Open With”, choose “Other”, and browse to the following:
Ensure that it’s set to “Always Open With”, and you should be set. All .jar files will now be opened with Java in the future.