Java - C-Like Fork?

This answer is probably a little late but:

http://akuma.kohsuke.org/

seems to be exactly what your looking for

Funnily, I am just working on this: a Java process running other Java processes. I used the article From Runtime.exec() to ProcessBuilder as a solid base, and When Runtime.exec() won't as a good advice how to gobble the output streams.

PS.: For those wondering, I had to do that (instead of spawning new threads) because yet another Java process is checking the presence of these processes which are, normally, ran separately with shell commands.

The Application Isolation API (JSR 121) introduces Isolate which addresses this use case.