Does the current worker participate in work stealing?

It is the responsibility of the ForkJoinPool to manage threads. Client code should feed it tasks, not micromanage the threading. Note that tasks and threads are two different things; tasks are units of work to be executed, and threads execute that work.

ForkJoinTask.compute() should fork() into smaller subtasks if the task is large enough to benefit from running parts of the task in parallel, and simply process the task if the task is small enough that it would better be run in a single thread. If the work turns out to be more than expected, it can fork() some of the work and do the rest of it.

If ForkJoinTask.compute() forks into smaller subtasks, it can call join() before returning. The ForkJoinPool will then either free the thread to work on other tasks, or spawn a temporary thread to work on other tasks to ensure the available parallelism is fully utilized.

I think it's reasonable to assume that the appropriate number of worker threads are kept busy for as long as there are uncompleted tasks, unless you explicitly block the thread in the compute() method.

The Sun tutorial provides more specifics on how to use these classes:

https://docs.oracle.com/javase/tutorial/essential/concurrency/forkjoin.html