A thread-safe state storage (intended to use with JRuby)

session.rb

# coding: utf-8

require "json"
require "java"
import "java.util.concurrent.ConcurrentHashMap"

class Session

  def self.current
    @current ||= ConcurrentHashMap.new
  end

  def self.get(id)
    unless session = current.get(id)                          # get by id
      new_session = create(id)                                # but unless it exists, create one
      unless session = current.put_if_absent(id, new_session) # put it back
        session = new_session                                 # but unless some other thread already put it first, use this one
      end
    end
    session
  end

  def self.create(id)
    data = JSON.parse(File.read("#{id}.json"), :symbolize_names => true)
    new(id, data)
  end

  # Blocking but thread-safe access to a session
  def self.with(id)
    get(id).lock.synchronize { yield session }
  end

  attr_reader :id, :lock

  def initialize(id, data)
    @id = id
    @data = data
    @lock = Mutex.new
  end

end