gistfile1.rb

#!/usr/bin/env jruby

require 'monitor'

class WorkerToken
end

class WorkerGroup
  LIMIT = 1

  def initialize
    @worker_mutex ||= Monitor.new
    @worker_queue ||= @worker_mutex.new_cond
    @worker_slots ||= []
  end

  def get_slot
    @worker_mutex.synchronize do
      loop do
        if @worker_slots.size >= LIMIT
          @worker_queue.wait
        end

        token = WorkerToken.new
        @worker_slots << token

        return token
      end
    end
  end

  def free_slot(token)
    @worker_mutex.synchronize do
      if @worker_slots.delete(token)
        @worker_queue.signal
      end
    end
  end
end

group = WorkerGroup.new

t1 = Thread.new do
  token_1 = group.get_slot
  Thread.new { sleep 1; group.free_slot(token_1) }
  token_2 = group.get_slot
  Thread.new { sleep 1; group.free_slot(token_2) }
end

t2 = Thread.new do
  token_3 = group.get_slot
  Thread.new { sleep 1; group.free_slot(token_3) }
  token_4 = group.get_slot
  Thread.new { sleep 1; group.free_slot(token_4) }
end

t1.join
t2.join