lazily instantiate application subclasses

this means we can meaningfully override methods in the subclass

railties/lib/rails.rb

@@ -29,7 +29,13 @@ module Rails
   autoload :WelcomeController
 
   class << self
-    attr_accessor :application, :cache, :logger
+    @application = @app_class = nil
+
+    attr_writer :application
+    attr_accessor :app_class, :cache, :logger
+    def application
+      @application ||= (app_class.instance if app_class)
+    end
 
     delegate :initialize!, :initialized?, to: :application
 

railties/lib/rails/application.rb

@@ -87,7 +87,7 @@ module Rails
     class << self
       def inherited(base)
         super
-        base.instance
+        Rails.app_class = base
       end
 
       # Makes the +new+ method public.
@@ -117,8 +117,6 @@ module Rails
       @railties          = nil
       @message_verifiers = {}
 
-      Rails.application ||= self
-
       add_lib_to_load_path!
       ActiveSupport.run_load_hooks(:before_configuration, self)
 

railties/test/engine_test.rb

@@ -11,4 +11,14 @@ class EngineTest < ActiveSupport::TestCase
 
     assert !engine.routes?
   end
+
+  def test_application_can_be_subclassed
+    klass = Class.new(Rails::Application) do
+      attr_reader :hello
+      def initialize
+        @hello = "world"
+      end
+    end
+    assert_equal "world", klass.instance.hello
+  end
 end