Adding manifest to a JAR

[Stefano Mancarella]

I have a jar file without any manifest (it's created with a JLINK task).
Is there a simple way of adding a manifest to it?
At the moment I'm using:

  <java fork="yes" classname="sun.tools.jar.Main">
   <arg value="uvfm"/>
   <arg value="my.jar"/>
   <arg value="manifest.txt"/>
  </java>

which seems too complex to me.