1 files changed, 39 insertions, 11 deletions
diff --git a/activejob/lib/active_job/queue_adapter.rb b/activejob/lib/active_job/queue_adapter.rb
index aa3ebdbc7b..9c4519432d 100644
--- a/activejob/lib/active_job/queue_adapter.rb
+++ b/activejob/lib/active_job/queue_adapter.rb
@@ -1,4 +1,5 @@
 require 'active_job/queue_adapters/inline_adapter'
+require 'active_support/core_ext/class/attribute'
 require 'active_support/core_ext/string/inflections'
 
 module ActiveJob
@@ -7,27 +8,54 @@ module ActiveJob
   module QueueAdapter #:nodoc:
     extend ActiveSupport::Concern
 
+    included do
+      class_attribute :_queue_adapter, instance_accessor: false, instance_predicate: false
+      self.queue_adapter = :inline
+    end
+
     # Includes the setter method for changing the active queue adapter.
     module ClassMethods
-      mattr_reader(:queue_adapter) { ActiveJob::QueueAdapters::InlineAdapter }
+      def queue_adapter
+        _queue_adapter
+      end
 
       # Specify the backend queue provider. The default queue adapter
       # is the :inline queue. See QueueAdapters for more
       # information.
-      def queue_adapter=(name_or_adapter)
-        @@queue_adapter = \
-          case name_or_adapter
-          when Symbol, String
-            load_adapter(name_or_adapter)
-          else
-            name_or_adapter if name_or_adapter.respond_to?(:enqueue)
-          end
+      def queue_adapter=(name_or_adapter_or_class)
+        self._queue_adapter = interpret_adapter(name_or_adapter_or_class)
       end
 
       private
-        def load_adapter(name)
-          "ActiveJob::QueueAdapters::#{name.to_s.camelize}Adapter".constantize
+
+      def interpret_adapter(name_or_adapter_or_class)
+        case name_or_adapter_or_class
+        when Symbol, String
+          ActiveJob::QueueAdapters.lookup(name_or_adapter_or_class).new
+        else
+          if queue_adapter?(name_or_adapter_or_class)
+            name_or_adapter_or_class
+          elsif queue_adapter_class?(name_or_adapter_or_class)
+            ActiveSupport::Deprecation.warn "Passing an adapter class is deprecated " \
+              "and will be removed in Rails 5.1. Please pass an adapter name " \
+              "(.queue_adapter = :#{name_or_adapter_or_class.name.demodulize.remove('Adapter').underscore}) " \
+              "or an instance (.queue_adapter = #{name_or_adapter_or_class.name}.new) instead."
+              name_or_adapter_or_class.new
+          else
+            raise ArgumentError
+          end
         end
+      end
+
+      QUEUE_ADAPTER_METHODS = [:enqueue, :enqueue_at].freeze
+
+      def queue_adapter?(object)
+        QUEUE_ADAPTER_METHODS.all? { |meth| object.respond_to?(meth) }
+      end
+
+      def queue_adapter_class?(object)
+        object.is_a?(Class) && QUEUE_ADAPTER_METHODS.all? { |meth| object.public_method_defined?(meth) }
+      end
     end
   end
 end