What determines the number of threads a Java ForkJoinPool creates?

Question

As far as I had understood ForkJoinPool, that pool creates a fixed number of threads (default: number of cores) and will never create more threads (unless the a

Answer 1

From the source comments:

Compensating: Unless there are already enough live threads, method tryPreBlock() may create or re-activate a spare thread to compensate for blocked joiners until they unblock.

I think what's happening is that you're not finishing any of the tasks very quickly, and since there aren't available worker threads when you submit a new task, a new thread gets created.