custom fork join pool is being used with common pool in java program

Question

I have created the below program into which I am trying to pass the custom fork join pool and I do not want to use the common join pool but still I see that common pool is being used even after passing the fork join pool please explain why it is happening

package com.example.javanewfeatures;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.ForkJoinPool;
import java.util.stream.Collectors;
import java.util.stream.IntStream;

public class ForkJoinPoolExample {

    public static void main(String args[]) throws InterruptedException {

        List<Integer> numbers = buildIntRange();

        ForkJoinPool forkJoinPool = new ForkJoinPool(4);
        Thread t1 = new Thread(() -> forkJoinPool.submit(() -> {
            numbers.parallelStream().forEach(n -> {
                try {
                    Thread.sleep(5);
                    System.out.println("Loop 1 : " + Thread.currentThread());
                } catch (InterruptedException e) {

                }
            });
        }).invoke());

        ForkJoinPool forkJoinPool2 = new ForkJoinPool(4);
        Thread t2 = new Thread(() -> forkJoinPool2.submit(() -> {
            numbers.parallelStream().forEach(n -> {
                try {
                    Thread.sleep(5);
                    System.out.println("Loop 2 : " + Thread.currentThread());
                } catch (InterruptedException e) {

                }
            });
        }).invoke());

        t1.start();
        t2.start();
        t1.join();
        t2.join();

    }

    private static List<Integer> buildIntRange() {
        return IntStream.range(0, 10).boxed().collect(Collectors.toUnmodifiableList());
    }

}

Prasanna · Answer 1 · 2020-11-06T15:42:39.603

...but still I see that common pool is being used even after passing the fork join pool

Definitely, when you create a instance of ForkJoinPool the common pool will not be used. You can print the below statements to ensure that is not the case.

System.out.printf("Common Pool:%s\n", ForkJoinPool.commonPool());
System.out.printf("Custom Pool:%s\n", new ForkJoinPool(4));

But in your case, the common pool is not used by your tasks, rather it is used by Streams for parallel computation.

Now, if you want streams to use a custom pool ,then you can refer this post - Parallel streams in custom pool.

In your current implemention this is the behaviour.

You are submitting the task to the pool using ForkJoinPool.submit(). This will ensure the tasks is executed in the custom pool.
But post that on the returned task you are calling ForkJoinTask.invoke(). This time, the task will be triggered on Thread t1 which is not a FJ Thread so task will be submitted to the common pool. Refer ForkJoinTask.doInvoke() source code below:

  private int doInvoke() {
      int s; Thread t; ForkJoinWorkerThread wt;
      return (s = doExec()) < 0 ? s :
          ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
          (wt = (ForkJoinWorkerThread)t).pool.awaitJoin(wt.workQueue, this) :
          externalAwaitDone();
  }

  private int externalAwaitDone() {
      int s;
      ForkJoinPool cp = ForkJoinPool.common;
  //...
  }

If you observe the output you will have duplicate results. Executions through submit would have used the custom pool and through invoke would have used the common pool.
```
...
Loop 1 : Thread[ForkJoinPool-2-worker-0,5,main]
Loop 1 : Thread[ForkJoinPool.commonPool-worker-3,5,main]
...
```

To correct your implementation, as mentioned in the above referred post, you can make the following changes

Thread t2 = new Thread(() -> {
    try {
        forkJoinPool2.submit(() -> {
            numbers.stream().forEach(n -> {
                try {
                    Thread.sleep(5);
                    System.out.println("Loop 2 : " + Thread.currentThread());
                } catch (InterruptedException e) {

                }
            });
        }).get(); /*change invoke to get and catch the exception*/
    } catch (InterruptedException e) {
        e.printStackTrace();
    } catch (ExecutionException e) {
        e.printStackTrace();
    }
});

Output:

Loop 2 : Thread[ForkJoinPool-2-worker-1,5,main]
Loop 1 : Thread[ForkJoinPool-1-worker-1,5,main]
Loop 1 : Thread[ForkJoinPool-1-worker-3,5,main]
Loop 1 : Thread[ForkJoinPool-1-worker-2,5,main]
Loop 1 : Thread[ForkJoinPool-1-worker-0,5,main]
Loop 2 : Thread[ForkJoinPool-2-worker-1,5,main]
Loop 1 : Thread[ForkJoinPool-1-worker-1,5,main]
Loop 1 : Thread[ForkJoinPool-1-worker-3,5,main]
Loop 1 : Thread[ForkJoinPool-1-worker-2,5,main]
Loop 1 : Thread[ForkJoinPool-1-worker-0,5,main]
Loop 2 : Thread[ForkJoinPool-2-worker-1,5,main]
Loop 1 : Thread[ForkJoinPool-1-worker-1,5,main]
Loop 1 : Thread[ForkJoinPool-1-worker-3,5,main]
Loop 2 : Thread[ForkJoinPool-2-worker-1,5,main]
Loop 2 : Thread[ForkJoinPool-2-worker-1,5,main]
Loop 2 : Thread[ForkJoinPool-2-worker-1,5,main]
Loop 2 : Thread[ForkJoinPool-2-worker-1,5,main]
Loop 2 : Thread[ForkJoinPool-2-worker-1,5,main]
Loop 2 : Thread[ForkJoinPool-2-worker-1,5,main]
Loop 2 : Thread[ForkJoinPool-2-worker-1,5,main]

custom fork join pool is being used with common pool in java program

1 Answers1