Ensure routes are loaded only after the initialization process finishes, ensuring all configuration options were applied.

actionpack/lib/action_dispatch/railtie.rb

@@ -8,10 +8,5 @@ module ActionDispatch
     config.action_dispatch.ip_spoofing_check = true
     config.action_dispatch.show_exceptions = true
     config.action_dispatch.best_standards_support = true
-
-    # Prepare dispatcher callbacks and run 'prepare' callbacks
-    initializer "action_dispatch.prepare_dispatcher" do |app|
-      ActionDispatch::Callbacks.to_prepare { app.routes_reloader.execute_if_updated }
-    end
   end
 end

railties/lib/rails/application/finisher.rb

@@ -46,6 +46,13 @@ module Rails
         ActiveSupport.run_load_hooks(:after_initialize, self)
       end
 
+      # Force routes to be loaded just at the end and add it to to_prepare callbacks
+      initializer :set_routes_reloader do |app|
+        reloader = lambda { app.routes_reloader.execute_if_updated }
+        reloader.call
+        ActionDispatch::Callbacks.to_prepare(&reloader)
+      end
+
       # Disable dependency loading during request cycle
       initializer :disable_dependency_loading do
         if config.cache_classes && !config.dependency_loading

railties/test/application/routing_test.rb

@@ -215,6 +215,23 @@ module ApplicationTests
       end
     end
 
+    test 'routes are loaded just after initialization' do
+      require "#{app_path}/config/application"
+
+      ActiveSupport.on_load(:after_initialize) do
+        ::InitializeRackApp = lambda { |env| [200, {}, ["InitializeRackApp"]] }
+      end
+
+      app_file 'config/routes.rb', <<-RUBY
+        AppTemplate::Application.routes.draw do |map|
+          match 'foo', :to => ::InitializeRackApp
+        end
+      RUBY
+
+      get '/foo'
+      assert_equal "InitializeRackApp", last_response.body
+    end
+
     test 'resource routing with irrigular inflection' do
       app_file 'config/initializers/inflection.rb', <<-RUBY
         ActiveSupport::Inflector.inflections do |inflect|