Delegate ActiveJob::Base#queue_adapter to class

activejob/lib/active_job/enqueuing.rb

@@ -52,9 +52,9 @@ module ActiveJob
 
       run_callbacks :enqueue do
         if scheduled_at
-          self.class.queue_adapter.enqueue_at self, scheduled_at
+          queue_adapter.enqueue_at self, scheduled_at
         else
-          self.class.queue_adapter.enqueue self
+          queue_adapter.enqueue self
         end
 
         successfully_enqueued = true

activejob/lib/active_job/instrumentation.rb

@@ -18,7 +18,7 @@ module ActiveJob
     private
       def instrument(operation, payload = {}, &block)
         ActiveSupport::Notifications.instrument \
-          "#{operation}.active_job", payload.merge(adapter: self.class.queue_adapter, job: self), &block
+          "#{operation}.active_job", payload.merge(adapter: queue_adapter, job: self), &block
       end
   end
 end

activejob/lib/active_job/queue_adapter.rb

@@ -10,7 +10,10 @@ module ActiveJob
 
     included do
       class_attribute :_queue_adapter_name, instance_accessor: false, instance_predicate: false
-      class_attribute :_queue_adapter, instance_accessor: false, instance_predicate: false
+      class_attribute :_queue_adapter, default: :async, instance_accessor: false, instance_predicate: false
+
+      delegate :queue_adapter, to: self
+
       self.queue_adapter = :async
     end