This week's book giveaway is in the Mac OS forum. We're giving away four copies of a choice of "Take Control of Upgrading to Yosemite" or "Take Control of Automating Your Mac" and have Joe Kissell on-line! See this thread for details.
As long as that jar file is in your classpath, and you know the name of the class implementing the main method (which you can find in the manifest file of the jar), you can call the main method directly. Alternatively, you can call "java -jar xyz.jar" using Runtime.exec.