How to kill a process spawn from $zf(-2)

I use zf(-2) to spawn a external a Java application in a *nix instance.

I would like to kill this process after some conditions met.

I would like to leverage $zf("kill ... ") but this requires its the pid of this child process.

So is there a way to acquire the pid for the child process when I create it ?

If not, how is the suggested way to kill this process?

Thanks.