3 files changed, 14 insertions, 12 deletions
diff --git a/actionpack/lib/action_controller/middleware_stack.rb b/actionpack/lib/action_controller/middleware_stack.rb
index dbc2fda41e..ee0ae48ac0 100644
--- a/actionpack/lib/action_controller/middleware_stack.rb
+++ b/actionpack/lib/action_controller/middleware_stack.rb
@@ -27,7 +27,9 @@ module ActionController
       end
 
       def klass
-        if @klass.is_a?(Class)
+        if @klass.respond_to?(:call)
+          @klass.call
+        elsif @klass.is_a?(Class)
           @klass
         else
           @klass.to_s.constantize
@@ -37,6 +39,8 @@ module ActionController
       end
 
       def active?
+        return false unless klass
+
         if @conditional.respond_to?(:call)
           @conditional.call
         else
diff --git a/actionpack/lib/action_controller/middlewares.rb b/actionpack/lib/action_controller/middlewares.rb
index 8ea1b5c7ce..fd70eca1b1 100644
--- a/actionpack/lib/action_controller/middlewares.rb
+++ b/actionpack/lib/action_controller/middlewares.rb
@@ -4,17 +4,8 @@ use "Rack::Lock", :if => lambda {
 
 use "ActionController::Failsafe"
 
-["ActionController::Session::CookieStore",
- "ActionController::Session::MemCacheStore",
- "ActiveRecord::SessionStore"].each do |store|
-   use(store, ActionController::Base.session_options,
-      :if => lambda {
-        if session_store = ActionController::Base.session_store
-          session_store.name == store
-        end
-      }
-    )
-end
+use lambda { ActionController::Base.session_store },
+  ActionController::Base.session_options
 
 use "ActionController::RewindableInput"
 use "ActionController::ParamsParser"
diff --git a/actionpack/test/controller/middleware_stack_test.rb b/actionpack/test/controller/middleware_stack_test.rb
index 2a141697da..612a710fa7 100644
--- a/actionpack/test/controller/middleware_stack_test.rb
+++ b/actionpack/test/controller/middleware_stack_test.rb
@@ -73,4 +73,11 @@ class MiddlewareStackTest < ActiveSupport::TestCase
       end
     end
   end
+
+  test "lazy evaluates middleware class" do
+    assert_difference "@stack.size" do
+      @stack.use lambda { BazMiddleware }
+    end
+    assert_equal BazMiddleware, @stack.last.klass
+  end
 end