Alternative for spawning a process with 'fork' in jRuby?

In MRI Ruby I can do this:

def transfer
  internal_server = self.init_server
  pid = fork do
    internal_server.run
  end

  # Make the server process run independently.
  Process.detach(pid)

  internal_client = self.init_client
  # Do other stuff with connecting to internal_server...
  internal_client.post('some data')    
ensure
  # Kill server
  Process.kill('KILL', pid) if pid
end

However the above code will not run in jRuby, because it does not support 'fork' method:

NotImplementedError: fork is not available on this platform

Is there any alternative solution for this in jRuby?

Thanks.