Fork Join Framework

Fork Join technology was introduced with Java version 7, to be able to deal with parallel concurrency.

This technology is based in splitting a task recursively until it is small enough to be performed. The sub-tasks will be executed in parallel in different threads. When the sub-tasks have finished, then a join is invoked to merge the results into one.

Example. The following example is very simple. It consists on a function that calculates consecutive natural numbers in an interval. The class will receive two parameters (number min. and max.). If this interval is greater than 6 (in the example) the task will be split into two sub-tasks.

The class extends RecursiveTask<Long> and implements method compute(). As our class is a task it will be invoked by ForkJoinPool

	import java.util.concurrent.ForkJoinPool;
	import java.util.concurrent.RecursiveTask;
	class NewTask extends RecursiveTask<Long> {
	private static final long serialVersionUID = 1895156518961346797L;
	static final int MAX = 6;
	int min;
	int max;
	NewTask(int min, int max) {
	this.min = min;
	this.max = max;
	}
	protected Long compute() {
	if(max - min <= MAX) {
	System.out.println("Computing");
	long result = 0;
	for(int i=min; i <= max; ++i)
	result += i;
	System.out.println("Result1: "+ result);
	return result;
	} else {
	System.out.println("Splitting task");
	int mid = min + (max - min) / 2;
	NewTask left = new NewTask(min, mid);
	NewTask right = new NewTask(mid+1, max);
	left.fork();
	right.fork();
	long sumLeft = left.join();
	long sumRight = right.join();
	long result = sumRight + sumLeft;
	System.out.println("Result2: "+ result);
	return result;
	}
	}
	public static void main (String args[]){
	ForkJoinPool.commonPool().invoke(new NewTask(0,12));
	}
	}

view raw NewTask.java hosted with ❤ by GitHub

Output console:
Splitting task
Computing
Result1: 21
Computing
Result1: 57
Result2: 78