Make many parts of Rails lazy. In order to facilitate this,

add lazy_load_hooks.rb, which allows us to declare code that
should be run at some later time. For instance, this allows
us to defer requiring ActiveRecord::Base at boot time purely
to apply configuration. Instead, we register a hook that should
apply configuration once ActiveRecord::Base is loaded.

With these changes, brings down total boot time of a
new app to 300ms in production and 400ms in dev.

TODO: rename base_hook

actionmailer/lib/action_mailer.rb

@@ -34,6 +34,7 @@
 require 'active_support/core_ext/module/attr_internal'
 require 'active_support/core_ext/module/delegation'
 require 'active_support/core_ext/string/inflections'
+require 'active_support/lazy_load_hooks'
 
 module ActionMailer
   extend ::ActiveSupport::Autoload

actionmailer/lib/action_mailer/base.rb

@@ -291,6 +291,8 @@ class Base < AbstractController::Base
       :parts_order  => [ "text/plain", "text/enriched", "text/html" ]
     }.freeze
 
+    ActionMailer.run_base_hooks(self)
+
     class << self
 
       def mailer_name

actionmailer/lib/action_mailer/railtie.rb

@@ -6,19 +6,21 @@ class Railtie < Rails::Railtie
     railtie_name :action_mailer
 
     initializer "action_mailer.url_for", :before => :load_environment_config do |app|
-      ActionMailer::Base.send(:include, app.routes.url_helpers)
+      ActionMailer.base_hook { include app.routes.url_helpers }
     end
 
     require "action_mailer/railties/log_subscriber"
     log_subscriber ActionMailer::Railties::LogSubscriber.new
 
     initializer "action_mailer.logger" do
-      ActionMailer::Base.logger ||= Rails.logger
+      ActionMailer.base_hook { self.logger ||= Rails.logger }
     end
 
     initializer "action_mailer.set_configs" do |app|
-      app.config.action_mailer.each do |k,v|
-        ActionMailer::Base.send "#{k}=", v
+      ActionMailer.base_hook do
+        app.config.action_mailer.each do |k,v|
+          send "#{k}=", v
+        end
       end
     end
   end

actionpack/lib/action_controller/base.rb

@@ -58,6 +58,8 @@ def self.filter_parameter_logging(*args, &block)
       filter
     end
 
+    ActionController.run_base_hooks(self)
+
   end
 end
 

actionpack/lib/action_controller/railtie.rb

@@ -40,7 +40,7 @@ class Railtie < Rails::Railtie
     log_subscriber ActionController::Railties::LogSubscriber.new
 
     initializer "action_controller.logger" do
-      ActionController::Base.logger ||= Rails.logger
+      ActionController.base_hook { self.logger ||= Rails.logger }
     end
 
     initializer "action_controller.set_configs" do |app|
@@ -51,19 +51,23 @@ class Railtie < Rails::Railtie
       ac.stylesheets_dir = paths.public.stylesheets.to_a.first
       ac.secret = app.config.cookie_secret
 
-      ActionController::Base.config.replace(ac)
+      ActionController.base_hook { self.config.replace(ac) }
     end
 
     initializer "action_controller.initialize_framework_caches" do
-      ActionController::Base.cache_store ||= RAILS_CACHE
+      ActionController.base_hook { self.cache_store ||= RAILS_CACHE }
     end
 
     initializer "action_controller.set_helpers_path" do |app|
-      ActionController::Base.helpers_path = app.config.paths.app.helpers.to_a
+      ActionController.base_hook do
+        self.helpers_path = app.config.paths.app.helpers.to_a
+      end
     end
 
     initializer "action_controller.url_helpers" do |app|
-      ActionController::Base.extend ::ActionController::Railtie::UrlHelpers.with(app.routes)
+      ActionController.base_hook do
+        extend ::ActionController::Railtie::UrlHelpers.with(app.routes)
+      end
 
       message = "ActionController::Routing::Routes is deprecated. " \
                 "Instead, use Rails.application.routes"

actionpack/lib/action_dispatch/middleware/params_parser.rb

@@ -1,4 +1,3 @@
-require 'active_support/json'
 require 'action_dispatch/http/request'
 
 module ActionDispatch

actionpack/lib/action_dispatch/middleware/session/cookie_store.rb

@@ -1,5 +1,4 @@
 require 'active_support/core_ext/hash/keys'
-require 'rack/request'
 
 module ActionDispatch
   module Session

actionpack/lib/action_dispatch/middleware/stack.rb

@@ -58,7 +58,7 @@ def ==(middleware)
           if lazy_compare?(@klass) && lazy_compare?(middleware)
             normalize(@klass) == normalize(middleware)
           else
-            klass == ActiveSupport::Inflector.constantize(middleware.to_s)
+            klass.name == middleware.to_s
           end
         end
       end

actionpack/lib/action_dispatch/routing/mapper.rb

@@ -1,3 +1,5 @@
+require "active_support/core_ext/hash/except"
+
 module ActionDispatch
   module Routing
     class Mapper
@@ -85,7 +87,7 @@ def conditions
           end
 
           def requirements
-            @requirements ||= returning(@options[:constraints] || {}) do |requirements|
+            @requirements ||= (@options[:constraints] || {}).tap do |requirements|
               requirements.reverse_merge!(@scope[:constraints]) if @scope[:constraints]
               @options.each { |k, v| requirements[k] = v if v.is_a?(Regexp) }
             end

actionpack/lib/action_view.rb

@@ -41,6 +41,7 @@ module ActionView
       autoload :Rendering
     end
 
+    autoload :Base
     autoload :MissingTemplate,   'action_view/base'
     autoload :Resolver,          'action_view/template/resolver'
     autoload :PathResolver,      'action_view/template/resolver'
@@ -56,6 +57,5 @@ module ActionView
 end
 
 require 'active_support/core_ext/string/output_safety'
-require 'action_view/base'
 
 I18n.load_path << "#{File.dirname(__FILE__)}/action_view/locale/en.yml"

actionpack/lib/action_view/base.rb

@@ -177,6 +177,8 @@ module Subclasses
 
     extend ActiveSupport::Memoizable
 
+    ActionView.run_base_hooks(self)
+
     attr_accessor :base_path, :assigns, :template_extension
     attr_internal :captures
 

actionpack/lib/action_view/helpers/active_model_helper.rb

@@ -5,9 +5,11 @@
 require 'active_support/core_ext/kernel/reporting'
 
 module ActionView
-  class Base
-    @@field_error_proc = Proc.new{ |html_tag, instance| "<div class=\"fieldWithErrors\">#{html_tag}</div>".html_safe }
-    cattr_accessor :field_error_proc
+  ActionView.base_hook do
+    class ActionView::Base
+      @@field_error_proc = Proc.new{ |html_tag, instance| "<div class=\"fieldWithErrors\">#{html_tag}</div>".html_safe }
+      cattr_accessor :field_error_proc
+    end
   end
 
   module Helpers

actionpack/lib/action_view/helpers/form_helper.rb

@@ -1211,8 +1211,10 @@ def nested_child_index(name)
     end
   end
 
-  class Base
-    cattr_accessor :default_form_builder
-    @@default_form_builder = ::ActionView::Helpers::FormBuilder
+  ActionView.base_hook do
+    class ActionView::Base
+      cattr_accessor :default_form_builder
+      @@default_form_builder = ::ActionView::Helpers::FormBuilder
+    end
   end
 end

actionpack/lib/action_view/railtie.rb

@@ -10,7 +10,9 @@ class Railtie < Rails::Railtie
 
     initializer "action_view.cache_asset_timestamps" do |app|
       unless app.config.cache_classes
-        ActionView::Helpers::AssetTagHelper.cache_asset_timestamps = false
+        ActionView.base_hook do
+          ActionView::Helpers::AssetTagHelper.cache_asset_timestamps = false
+        end
       end
     end
   end

actionpack/lib/action_view/test_case.rb

@@ -1,4 +1,5 @@
 require 'action_controller/test_case'
+require 'action_view'
 
 module ActionView
   class Base

actionpack/test/abstract_unit.rb

@@ -16,7 +16,6 @@
 require 'abstract_controller'
 require 'action_controller'
 require 'action_view'
-require 'action_view/base'
 require 'action_dispatch'
 require 'fixture_template'
 require 'active_support/dependencies'

activerecord/lib/active_record.rb

@@ -30,16 +30,15 @@
 
 require 'active_support'
 require 'active_model'
-require 'arel'
 
 module ActiveRecord
   extend ActiveSupport::Autoload
 
   eager_autoload do
     autoload :VERSION
 
-    autoload :ActiveRecordError, 'active_record/base'
-    autoload :ConnectionNotEstablished, 'active_record/base'
+    autoload :ActiveRecordError, 'active_record/errors'
+    autoload :ConnectionNotEstablished, 'active_record/errors'
 
     autoload :Aggregations
     autoload :AssociationPreload
@@ -106,12 +105,16 @@ module ConnectionAdapters
 
     eager_autoload do
       autoload :AbstractAdapter
+      autoload :ConnectionManagement, "active_record/connection_adapters/abstract/connection_pool"
     end
   end
 
   autoload :TestCase
   autoload :TestFixtures, 'active_record/fixtures'
+
+  base_hook do
+    Arel::Table.engine = Arel::Sql::Engine.new(self)
+  end
 end
 
-Arel::Table.engine = Arel::Sql::Engine.new(ActiveRecord::Base)
-I18n.load_path << File.dirname(__FILE__) + '/active_record/locale/en.yml'
+I18n.load_path << File.dirname(__FILE__) + '/active_record/locale/en.yml'