How do you open a jar file on a mac?

You double-click on it.

You can indeed launch a jar file from the command line, with the following command:

java -jar yourfile.jar

As well as this you can assign "Jar Launcher" as the default app. to use when you double-click a jar file, as follows (I don't believe you need the developer tools installed for this):

Click once on the .jar file in the Finder and then from the menubar in the Finder select File -> Get Info". Click on "Open with" and from the popup menu select "Other". A file browser window will open. In this window, go to the /System/Library/CoreServices folder and select 'Jar Launcher'. Then make sure the "Always Open With" checkbox is checked and then click Add. Then click the "Change all" button so that any jar file will be opened automatically. Finally, close the Info window and now when you double-click any of your jar files they should run automatically.