I had earlier written about a generic way of Running executable jar files on GNOME. I found a much cooler way of executing jar files. It involves adding a menu entry(.desktop file) in GNOME 3.X desktop and then forcing this menu entry to appear in open with list using a "%U" at the end. This will make the jar execution command appear on the context menu or right click menu of the jar. Ensure that you have a JRE or JDK on your system. For installing JRE on your openSUSE 12.2 you can take a look at this post.
Launch Alacarte through quick launch window(use command "alacarte" after pressing Alt+F2) and add menu entry "java -jar %U" as shown below. A new entry can be created by clicking the "New Item" button. In the pop up window you can give any name like "Jar" against Name entry and provide comments like "Cool way to run jar". The %U ensures that the launcher will appear on "open with" list.
After making the menu entry right click on any jar and choose the "Open With Other Application" menu.
Then choose the menu entry named "Jar" in the dialogue box as shown below.
Now the "Jar" entry will get permanently added to your context menu and will always be handy when you need to run a Jar
To set the application's default action on double click you need to right click and choose "Properties". You need to choose "Open With" tab and then choose the relevant application launcher and the "Set as default" button.