executor.rb

require 'java'

class Task
  include Java::JavaUtilConcurrent::Callable

  def initialize(num)
    @num = num
  end

  def call
    puts "This is task #{@num}"
    sleep 0.2
    @num
  end
end

Executors = Java::JavaUtilConcurrent::Executors
executor = Executors.newFixedThreadPool(8)

futures = (1..100).map do |i|
  executor.submit Task.new(i)
end

rets = futures.map { |f| f.get }
puts "return values: #{rets.join(',')}"

executor.shutdown