You will need a keystore to sign jars. In my case I was given a p12 file from my Certificate Authority. It took a while but I finally found how to make a keystore from my p12.
You don’t need a keystore to exist to import a p12:
> keytool -v -importkeystore -srckeystore certificate.p12 -srcstoretype PKCS12 -destkeystore keystore.jks -deststoretype JKS
Now the keystore will have the contents of the p12, which is the certificate and the key.