Threads in bash?

Bash doesn’t support threading per se, but you could launch multiple java processes in the background, like:

java myprog &
java myprog &
java myprog &

Anything more than that you might look into Python or Ruby, which have thread management utilities, you could wait for each one to finish and collect output/exit status, etc.

Edit: Borrowing the suggestion from @CédricJulien to use wait, here’s a more thorough example. Given this MyProg.java program:

public class MyProg {
    public static void main(String[] args) {
        System.exit(Integer.parseInt(args[0]));
    }
}

you could write the following bash-threads.sh script to launch multiple instances of it in parallel:

#!/bin/bash
set -o errexit

java MyProg 1 &
pid1=$!
java MyProg 0 &
pid2=$!
java MyProg 2 &
pid3=$!

wait $pid1 && echo "pid1 exited normally" || echo "pid1 exited abnormally with status $?"
wait $pid2 && echo "pid2 exited normally" || echo "pid2 exited abnormally with status $?"
wait $pid3 && echo "pid3 exited normally" || echo "pid3 exited abnormally with status $?"

Its output is:

pid1 exited abnormally with status 1
pid2 exited normally
pid3 exited abnormally with status 2