Java 7: Fork/Join Framework Example

The Fork/Join Framework in Java 7 is designed for work that can be broken down into smaller tasks and the results of those tasks combined to produce the final result.

In general, classes that use the Fork/Join Framework follow the following simple algorithm:

// pseudocode
Result solve(Problem problem) {
  if (problem.size < SEQUENTIAL_THRESHOLD)
    return solveSequentially(problem);
  else {
    Result left, right;
    INVOKE-IN-PARALLEL {
      left = solve(extractLeftHalf(problem));
      right = solve(extractRightHalf(problem));
    }
    return combine(left, right);
  }
}

In order to demonstrate this, I have created an example to find the maximum number from a large array using fork/join:

import java.util.Random;
import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.RecursiveTask;

public class MaximumFinder extends RecursiveTask<Integer> {

  private static final int SEQUENTIAL_THRESHOLD = 5;

  private final int[] data;
  private final int start;
  private final int end;

  public MaximumFinder(int[] data, int start, int end) {
    this.data = data;
    this.start = start;
    this.end = end;
  }

  public MaximumFinder(int[] data) {
    this(data, 0, data.length);
  }

  @Override
  protected Integer compute() {
    final int length = end - start;
    if (length < SEQUENTIAL_THRESHOLD) {
      return computeDirectly();
    }
    final int split = length / 2;
    final MaximumFinder left = new MaximumFinder(data, start, start + split);
    left.fork();
    final MaximumFinder right = new MaximumFinder(data, start + split, end);
    return Math.max(right.compute(), left.join());
  }

  private Integer computeDirectly() {
    System.out.println(Thread.currentThread() + " computing: " + start
                       + " to " + end);
    int max = Integer.MIN_VALUE;
    for (int i = start; i < end; i++) {
      if (data[i] > max) {
        max = data[i];
      }
    }
    return max;
  }

  public static void main(String[] args) {
    // create a random data set
    final int[] data = new int[1000];
    final Random random = new Random();
    for (int i = 0; i < data.length; i++) {
      data[i] = random.nextInt(100);
    }

    // submit the task to the pool
    final ForkJoinPool pool = new ForkJoinPool(4);
    final MaximumFinder finder = new MaximumFinder(data);
    System.out.println(pool.invoke(finder));
  }
}

The MaximumFinder class is a RecursiveTask which is responsible for finding the maximum number from an array. If the size of the array is less than a threshold (5) then find the maximum directly, by iterating over the array. Otherwise, split the array into two halves, recurse on each half and wait for them to complete (join). Once we have the result of each half, we can find the maximum of the two and return it.