I have a recursive bulk task that I put into execution in a ForkJoinPool thread pool.

public class SomeTask extends RecursiveAction {
    @Override
    protected void compute() {
       //Some recursive logic...
    }
}

public class Main {

    public static void main(String[] args) {
        startForkJoinPool();
    }

    private void startForkJoinPool() {
       SomeTask someTask = new SomeTask();

       ForkJoinPool pool = new ForkJoinPool(4);
       pool.invoke(someTask);
       pool.shutdown();
    }
}

Now I need to execute this logic in two more parallel threads.

I decided to try to use the ExecutorService thread pool, and when I put entities into it for execution, I found out that they are not executed in parallel, but, as it were, one of the threads is parked for the duration of the first one.

public class SomeTask extends RecursiveAction {
    @Override
    protected void compute() {
       //Some recursive logic...
    }
}

public class Main {

    public static void main(String[] args) {
        List<Thread> threadList = new ArrayList<>();
        threadList.add(new Thread(() -> startForkJoinPool()));
        threadList.add(new Thread(() -> startForkJoinPool()));

        ExecutorService executorService = Executors.newFixedThreadPool(2);
        threadList.forEach(executorService::execute);
        executorService.shutdown();
    }

    private void startForkJoinPool() {
       SomeTask someTask = new SomeTask();

       ForkJoinPool pool = new ForkJoinPool(4);
       pool.invoke(someTask);
       pool.shutdown();
    }
}

Tell me, please, what can I do wrong? Many thanks in advance to anyone who can point me in the right direction.

tl;dr

• Change execute to submit.
• Await termination when shutting down an executor service.

Executor#execute is not necessarily async

You need to carefully read the documentation. This is a tricky area.

The ExecutorService#execute method is inherited from the super interface Executor.

The Javadoc for Executor says:

However, the Executor interface does not strictly require that execution be asynchronous

So any Runnable you pass to execute may or may not be run on a background thread.

ExecutorService#submit is always async

If you definitely want your Runnable to run on a background thread, pass to ExecutorService#submit.

Change this line:

threadList.forEach(executorService::execute);

… to this:

threadList.forEach(executorService::submit);

Shutdown

Your call to ExecutorService#shutdown is insufficient. You need to wait for submitted tasks to complete.

See the boilerplate method shutdownAndAwaitTermination given to you in the Javadoc for ExecutorService.

Or, in Java 19+, call ExecutorService#close.