Is it possible to set the priority of the threads in Stream.parallel()?

If I want to run a Stream in parallel in a background task is it possible to run it in lower priority? And if so how?

Yes it is possible.

The procedure is as follows:

1. Create a ForkJoinWorkerThreadFactory that creates threads with an appropriate priority.

2. Create a ForkJoinPool using the above thread factory.

3. Instantiate the parallel stream.

4. Run the stream by submitting it to the ForkJoinPool

Something like this:

public class MyThread extends ForkJoinWorkerThread {
    public MyThread(ForkJoinPool pool, int priority) {
        super(pool);
        setPriority(priority);
    }
}

final int poolSize = ...
final int priority = ...

List<Long> aList = LongStream.rangeClosed(firstNum, lastNum).boxed()
  .collect(Collectors.toList());

ForkJoinWorkerThreadFactory factory = new ForkJoinWorkerThreadFactory() {
    public ForkJoinWorkerThread newThread(ForkJoinPool pool) {
         return new MyThread(pool, priority);
    }
};
/*
ForkJoinWorkerThreadFactory factory = pool -> new MyThread(
  pool,
  priority
);
*/

ForkJoinPool customThreadPool = new ForkJoinPool(
    poolSize, factory, null, false);
long actualTotal = customThreadPool.submit(
    () -> aList.parallelStream().reduce(0L, Long::sum)).get();

(Example code adapted from http://www.baeldung.com/java-8-parallel-streams-custom-threadpool)