The jar doc states "=" can be used in either style of option, ex:

-f=FILE or --file=FILE

but only the GNU style option (--file) accepts "=", ex:

$ jar -c --main-class=CheckIfJarIsSigned -f=CheckIfJarIsSigned.jar CheckIfJarIsSigned.class
unrecognized option : -f=CheckIfJarIsSigned.jar

$ jar -c --main-class=CheckIfJarIsSigned --file=CheckIfJarIsSigned.jar CheckIfJarIsSigned.class