When trying to sign a jar file with Jarsigner, you get:
Jarsigner: Only one alias can be specified.
To fix this, put all the files you are using with the jarsigner switch in the working directory (the directory you are running the command from in the command prompt – i.e. The file path to the left of where you type in the command prompt). If you must reference other directories in the command line, try putting them in quotes or changing the directory names so there are no spaces in the name.