scorer.rb

require "java"
import java.util.concurrent.RecursiveAction

class Scorer < RecursiveAction

  attr_reader :documents

  def compute
    if documents.size <= 10000
      score_each
    else
      invokeAll(
        Scorer.new(documents.slice!(0, documents.length / 2)),
        Scorer.new(documents)
      )
    end
    nil
  end

  def initialize(documents)
    super()
    @documents = documents
  end

  def score_each
    documents.each do |doc|
      # Just set to one for testing purposes to see if this fork/join works.
      doc[:score] = 1
    end
  end
end

scorer_spec.rb

require "java"
import java.util.concurrent.ForkJoinPool

require "spec_helper"

describe Engine do

  let(:documents) do
    (1...1000000).reduce([]) do |docs, n|
      docs.tap { |d| d.push({ score: n }) }
    end
  end

  let(:pool) do
    ForkJoinPool.new
  end

  let!(:results) do
    pool.invoke(Scorer.new(documents))
  end

  it "scores each of the documents" do
    documents.first["score"].should eq(1)
  end
end