2406. Java Concurrency - Fork Join
RecursiveTask and Fork Join

Learn the fork-join framework

1. Fork-join Framework

The fork-join framework allows to break a certain task on several workers and then wait for the result to combine them. It leverages multi-processor machine’s capacity to great extent.

  • Fork - A process in which a task splits itself into smaller and independent sub-tasks which can be executed concurrently.
  • Join - A process in which a task join all the results of sub-tasks once the subtasks have finished executing, otherwise it keeps waiting.


  • ForkJoinPool - A special thread pool designed to work with fork-and-join task splitting.
  • RecursiveTask<V> - RecursiveTask represents a task which returns a value.
  • RecursiveAction - Just like RecursiveTask except it does not return a result
  • ForkJoinTask<V> - Superclass of RecursiveTask and RecursiveAction. fork() and join() are methods defined in this class.

2. Example

Create an array with random numbers, then count how many numbers are larger than 0.5.

2.1 Counter

import java.util.concurrent.RecursiveTask;
import java.util.function.Predicate;

public class Counter extends RecursiveTask<Integer> {
    static final int THRESHOLD = 100;
    private double[] numbers;
    private int from;
    private int to;
    private Predicate<Double> filter;

    public Counter(double[] numbers, int from, int to, Predicate<Double> filter) {
        this.numbers = numbers;
        this.from = from;
        this.to = to;
        this.filter = filter;

    protected Integer compute() {
        if (to - from < THRESHOLD) { // no need to split, calculate the result
            int count = 0;
            for (int i = from; i < to; i++) {
                if (filter.test(numbers[i])) {
            return count;
        } else { // split to smaller tasks
            int mid = (to - from ) / 2 + from;
            Counter first = new Counter(numbers, from, mid, filter);
            Counter second = new Counter(numbers, mid, to, filter);
            invokeAll(first, second); //
            return first.join() + second.join();
  • Override the compute() method to generate and invoke subtasks, and to combine their results.
  • THRESHOLD is used to determine whether to execute the calculation or continue splitting.
  • The invokeAll method receives a number of tasks and blocks until all of them have completed.
  • The join method yields the result.
public class ForkJoinExample {
    static final int size = 10000;
    public static void main(String[] args) {
        double[] numbers = new double[size];
        // generate random numbers
        for (int i = 0; i < size; i++) {
            numbers[i] = Math.random();
        Counter counter = new Counter(numbers, 0, numbers.length, x->x > 0.5);
        ForkJoinPool pool = new ForkJoinPool();
        pool.invoke(counter); // Performs the given task, returning its result upon completion.
        System.out.println(counter.join()); // output: 500305



3. Source Files

4. References