[Q] how can I start a shell process and return immediately?

I want to start a Java program from a Ruby program and have the Java
program run in another process and have control returned to my Ruby
program as soon as the Java process starts up successfully.

I can do this from a shell by suffixing an '&':

   java -cp <classpath> <main_class>&

starts the Java program in a background process thread and returns
control the shell.

When I try this from a Ruby program the program blocks until the Java
program is terminated.