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

It is worth noting that the output of the code posted by elusive-code depends on the version of java. Running the code in the java 8 I see the output:

...
counter=0.73 activeThreads=45 runningThreads=5 poolSize=49 queuedTasks=105 queuedSubmissions=0 parallelism=2 stealCount=3056
counter=0.75 activeThreads=46 runningThreads=1 poolSize=51 queuedTasks=0 queuedSubmissions=0 parallelism=2 stealCount=3158
counter=0.77 activeThreads=47 runningThreads=3 poolSize=51 queuedTasks=0 queuedSubmissions=0 parallelism=2 stealCount=3157
counter=0.74 activeThreads=45 runningThreads=3 poolSize=51 queuedTasks=5 queuedSubmissions=0 parallelism=2 stealCount=3153

But running the same code in the java 11 the output is different:

...
counter=0.75 activeThreads=1 runningThreads=1 poolSize=2 queuedTasks=4 queuedSubmissions=0 parallelism=2 stealCount=0
counter=0.76 activeThreads=1 runningThreads=1 poolSize=2 queuedTasks=3 queuedSubmissions=0 parallelism=2 stealCount=0
counter=0.77 activeThreads=1 runningThreads=1 poolSize=2 queuedTasks=2 queuedSubmissions=0 parallelism=2 stealCount=0
counter=0.78 activeThreads=1 runningThreads=1 poolSize=2 queuedTasks=1 queuedSubmissions=0 parallelism=2 stealCount=0
counter=0.79 activeThreads=1 runningThreads=1 poolSize=2 queuedTasks=0 queuedSubmissions=0 parallelism=2 stealCount=0