Java - Fork/Join Framework

Introduction

Typically, the fork/join framework is suitable in a situation where

A task can be divided in multiple subtasks that can be executed in parallel.
The partial results can be combined to get the final result.

The following four classes are the main classes in fork/join framework:

ForkJoinPool
ForkJoinTask
RecursiveAction
RecursiveTask

ForkJoinPool class represents a thread pool.

ForkJoinTask class represents a task.

Abstract ForkJoinTask class has two concrete subclasses: RecursiveAction and RecursiveTask.

An abstract subclass of the ForkJoinTask class that is called CountedCompleter.

The framework supports two types of tasks: a task that does not yield a result and a task that yields a result.

RecursiveAction class represents a task that does not yield a result.

RecursiveTask class represents a task that yields a result.

A CountedCompleter task may or may not yield a result.

Your class which represents a fork/join task should inherit from RecursiveAction or RecursiveTask and implement for the compute() abstract method.

Typically, the logic inside the compute() method is written similar to the following:

if (Task is small) {
    Solve the task.
}
else {
        Divide the task into subtsaks.
        Launch the subtasks asynchronously (fork stage).
        Wait for the subtasks to finish (join stage).
        Combine the results of all subtasks.
}

The following two methods of the ForkJoinTask class provide two important features during a task execution:

fork() method launches a new subtask from a task for an asynchronous execution.
join() method lets a task wait for another task to complete.

Steps in Using the Fork/Join Framework

Step 1: Declaring a Class to Represent a Task

class MyTask extends RecursiveTask<Long> {
        // Code for your task goes here
}

Step 2: Implementing the compute() Method

class MyTask extends RecursiveTask<Long> {
        public Long compute() {
                // Logic for the task goes here
        }
}

Step 3: Creating a Fork/Join Thread Pool

ForkJoinPool pool = new ForkJoinPool();

Step 4: Creating the Fork/Join Task

MyTask task = MyTask();

Step 5: Submitting the Task to the Fork/Join Pool for Execution

long result = pool.invoke(task);

Fork/Join Example

The following code uses fork/join framework compute sum of random integers.

Demo

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.RecursiveTask;

class MyTask extends RecursiveTask<Long> {
  private int count;

  public MyTask(int count) {
    this.count = count;
  }//from ww  w .j ava  2 s .  c  om

  @Override
  protected Long compute() {
    long result = 0;

    if (this.count <= 0) {
      return 0L; // We do not have anything to do
    }

    if (this.count == 1) {
      return 10L;
    }
    // Multiple numbers. Divide them into many single tasks. Keep the references
    // of all tasks to call their join() method later
    List<RecursiveTask<Long>> forks = new ArrayList<>();

    for (int i = 0; i < this.count; i++) {
      MyTask subTask = new MyTask(1);
      subTask.fork(); // Launch the subtask
      // Keep the subTask references to combine the results later
      forks.add(subTask);
    }

    // Now wait for all subtasks to finish and combine the result
    for (RecursiveTask<Long> subTask : forks) {
      result = result + subTask.join();
    }

    return result;
  }
}

public class Main {
  public static void main(String[] args) {
    ForkJoinPool pool = new ForkJoinPool();
    MyTask task = new MyTask(3);
    long sum = pool.invoke(task);
    System.out.println("Sum is " + sum);
  }
}

Java - Fork/Join Framework

Introduction

Steps in Using the Fork/Join Framework

Fork/Join Example

Demo

Result