ReplaceCommonPool.java

import sun.misc.Unsafe;

import java.lang.reflect.Field;
import java.util.concurrent.ForkJoinPool;

/**
 * @author Jakub Kubrynski
 */
public class ReplaceCommonPool {

	public static void main(String[] args) throws Exception {
		System.out.println(ForkJoinPool.commonPool().getParallelism());
		replacePool(new ForkJoinPool(1));
		System.out.println(ForkJoinPool.commonPool().getParallelism());
	}

	private static void replacePool(ForkJoinPool newPool) throws ReflectiveOperationException {
		Field field = Unsafe.class.getDeclaredField("theUnsafe");
		field.setAccessible(true);
		Unsafe unsafe = (Unsafe) field.get(null);
		Field common = ForkJoinPool.class.getDeclaredField("common");
		long commonOffset = unsafe.staticFieldOffset(common);
		unsafe.putObject(ForkJoinPool.class, commonOffset, newPool);
	}
}

Hello Kuba,
that's not really the best way of doing this in runtime.

Because of how ForkJoinPool works in Java 8, if you submit a tasks to a FJP from within an existing ForkJoinPool, it's ForkJoinTasks will be submitted to the existing "enclosing" pool, instead of the global one. See ForkJoinTask#fork to read up on the details (inForkJoinPool() is used).

Here's how to do this:

final ForkJoinPool pool = ???
final List<Integer> myList = ???
pool.submit(() -> myList.parallel().filter(i -> i % 2 == 0).collect(toList())).get();

Happy hAkking!