Ruby on Railsの初期化プロセスを解説！railsコマンドからアプリケーションの起動まで

#!/usr/bin/env ruby
APP_PATH = File.expand_path("../config/application", __dir__)

#config/boot.rbを読み込む
#https://github.com/rails/rails/blob/v7.0.5/railties/lib/rails/generators/rails/app/templates/config/boot.rb.tt
require_relative "../config/boot"

#引数に応じてrailsコマンドを実行させる
#https://github.com/rails/rails/blob/v7.0.5/railties/lib/rails/commands.rb
require "rails/commands"

#https://github.com/rails/rails/blob/v7.0.5/railties/lib/rails/commands/server/server_command.rb#L129-L148
module Rails
  module Command
    class ServerCommand < Base # :nodoc:
      ...

      def perform
        extract_environment_option_from_argument
        set_application_directory!
        prepare_restart

        Rails::Server.new(server_options).tap do |server|
          # NOTE: APP_PATH には /path/to/config/application が設定されている
          # そのため、 config/application.rb が読み込まれる
          require APP_PATH
          Dir.chdir(Rails.application.root)

          if server.serveable?
            print_boot_information(server.server, server.served_url)
            after_stop_callback = -> { say "Exiting" unless options[:daemon] }
            server.start(after_stop_callback)
          else
            say rack_server_suggestion(options[:using])
          end
        end
      end

require_relative "boot"

require "rails/all"

# ...

#https://github.com/rails/rails/blob/v7.0.5/railties/lib/rails/all.rb

require "rails"

%w(
  active_record/railtie
  active_storage/engine
  action_controller/railtie
  action_view/railtie
  action_mailer/railtie
  active_job/railtie
  action_cable/engine
  action_mailbox/engine
  action_text/engine
  rails/test_unit/railtie
).each do |railtie|
  begin
    require railtie
  rescue LoadError
  end
end

# https://github.com/rails/rails/blob/v7.0.5/activerecord/lib/active_record/railtie.rb#L301-L316

initializer "active_record.clear_active_connections" do
  config.after_initialize do
    # ActiveRecord::Baseが読み込まれたときに実行される
    ActiveSupport.on_load(:active_record) do
      # アクティブなコネクションを切断する
      clear_active_connections!
      flush_idle_connections!
    end
  end
end

require_relative "boot"

require "rails/all"

# ここまで読んだ

# :test, :development, :production のグループに応じて
# Gemfileのgemがロードされる
Bundler.require(*Rails.groups)

# ...

require_relative "boot"

require "rails/all"

Bundler.require(*Rails.groups)

# ここまで読んだ

# Rails::Applicationを継承した自分のApplicationクラスがロードされる
module RailsInitializatoinTest
  class Application < Rails::Application
    # Initialize configuration defaults for originally generated Rails version.
    config.load_defaults 7.0

    # Configuration for the application, engines, and railties goes here.
    #
    # These settings can be overridden in specific environments using the files
    # in config/environments, which are processed later.
    #
    # config.time_zone = "Central Time (US & Canada)"
    # config.eager_load_paths << Rails.root.join("extras")
  end
end

# https://github.com/rails/rails/blob/v7.0.5/railties/lib/rails/commands/server/server_command.rb#L129-L148
module Rails
  module Command
    class ServerCommand < Base # :nodoc:
      ...

      def perform
        extract_environment_option_from_argument
        set_application_directory!
        prepare_restart

        Rails::Server.new(server_options).tap do |server|
          require APP_PATH
          Dir.chdir(Rails.application.root)

          if server.serveable?
            # 「Booting Puma...」がコンソールに表示される
            print_boot_information(server.server, server.served_url)
            after_stop_callback = -> { say "Exiting" unless options[:daemon] }
            # サーバーを起動させる
            server.start(after_stop_callback)
          else
            say rack_server_suggestion(options[:using])
          end
        end
      end

# アプリケーションを起動させるためにRackベースのサバーとして利用されるファイル

# config/environment.rbを読み込む
require_relative "config/environment"

# サーバーを起動させる
run Rails.application
Rails.application.load_server

# Railsアプリケーションを読み込む
# 前の方の処理で既に読み込んでいるためスルー
require_relative "application"

# Railsアプリケーションを初期化する
Rails.application.initialize!

# ...

# サーバーを起動させる
run Rails.application
Rails.application.load_server