Get fields from separately running jar

I know this is pretty generic but is there a way for me to run a jar, then using a completely separate java program access fields from that running jar? I would like to ultimately set the text of a known JTextField (I know the name of the JTextField and class). I know how I could do this with reflection if it all ran as one unit but I can't find any way to get an Object containing an instance of a running jar which is what you would need.