init.rb

module MaintenanceMode
  def self.included(base)
    base.class_eval do
      unloadable
      prepend_before_filter(:show_maintenance_mode_page)

      def show_maintenance_mode_page
        unless User.current.admin?
          render :text => "This site is currently under maintenance. Please check back later"
          return false
        end
      end
    end
  end
end


# Patches to the Redmine core.
require 'dispatcher'

Dispatcher.to_prepare do
  require_dependency 'application_controller'
  ApplicationController.send(:include, MaintenanceMode)
end