Ok. I have read through probably two-dozen posts and articles, and still can't seem to find an answer. I have installed OpenJDK-7 and am trying to set it as the default to open .jar files (specifically, I'm trying to set it as the default for minecraft.jar, freshly downloaded from the minecraft.org site).
I am running the 64-bit version of Ubuntu 13.04 (fresh install), and installed OpenJDK through the Software Center. Everything looks to be fine, OpenJDK shows up in the /usr/share/applications directory. If I right-click on minecraft.jar I see "OpenJDK Java 7 Runtime" as an option under the "Open With" context menu. That's fine. It launches the minecraft launcher, checks for updates, and I can fire up the game. The problem is, if I go to "Properties" and then to the "Open With" tab, OpenJDK is nowhere to be found in the list of options. I want to set it up as the default for .jar files so I can set up a launcher to drop into the unity launcher.
I have also used gedit in a terminal window to change the "defaults.list" file in the /applications directory to use openjdk-7-java.desktop for both jar and java-archive.
No matter what I try, the outcome is always the same: I can launch it if I right-click > open with > OpenJDK... but I cannot set OpenJDK as the default for .jar files.