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Java Concurrency 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. Following are the core concepts and objects used in fork-join framework.


Fork is a process in which a task splits itself into smaller and independent sub-tasks which can be executed concurrently.


Sum left  = new Sum(array, low, mid);

Here Sum is a subclass of RecursiveTask and left.fork() spilts the task into sub-tasks.


Join is a process in which a task join all the results of sub-tasks once the subtasks have finished executing, otherwise it keeps waiting.



Here left is an object of Sum class.


it is a special thread pool designed to work with fork-and-join task splitting.


ForkJoinPool forkJoinPool = new ForkJoinPool(4);

Here a new ForkJoinPool with a parallelism level of 4 CPUs.


RecursiveAction represents a task which does not return any value.


class Writer extends RecursiveAction {
   protected void compute() { }


RecursiveTask represents a task which returns a value.


class Sum extends RecursiveTask<Long> {
   protected Long compute() { return null; }


The following TestThread program shows usage of Fork-Join framework in thread based environment.

Live Demo
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.RecursiveTask;

public class TestThread {

   public static void main(final String[] arguments) throws
      ExecutionException {
      int nThreads = Runtime.getRuntime().availableProcessors();
      int[] numbers = new int[1000]; 

      for(int i = 0; i < numbers.length; i++) {
         numbers[i] = i;

      ForkJoinPool forkJoinPool = new ForkJoinPool(nThreads);
      Long result = forkJoinPool.invoke(new

   static class Sum extends RecursiveTask<Long> {
      int low;
      int high;
      int[] array;

      Sum(int[] array, int low, int high) {
         this.array = array;
         this.low   = low;
         this.high  = high;

      protected Long compute() {
         if(high - low <= 10) {
            long sum = 0;
            for(int i = low; i < high; ++i) 
               sum += array[i];
               return sum;
         } else {	    	
            int mid = low + (high - low) / 2;
            Sum left  = new Sum(array, low, mid);
            Sum right = new Sum(array, mid, high);
            long rightResult = right.compute();
            long leftResult  = left.join();
            return leftResult + rightResult;

This will produce the following result.


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