Deprecate usage of custom ActiveJob serializers without public #klass methods

activejob/lib/active_job/serializers.rb

@@ -76,11 +76,20 @@ def index_serializers
           @serializers_index.clear
           serializers.each do |s|
             if s.respond_to?(:klass)
-              @serializers_index[s.klass] = s
+              begin
+                @serializers_index[s.klass] = s
+              rescue NotImplementedError => e
+                ActiveJob.deprecator.warn(
+                  <<~MSG.squish
+                    #{e.message}. This will raise an error in Rails 8.2.
+                  MSG
+                )
+              end
             elsif s.respond_to?(:klass, true)
               klass = s.send(:klass)
               ActiveJob.deprecator.warn(<<~MSG.squish)
                 #{s.class.name}#klass method should be public.
+                This will raise an error in Rails 8.2.
               MSG
               @serializers_index[klass] = s
             end

activejob/test/cases/serializers_test.rb

@@ -30,6 +30,8 @@ def klass
     end
   end
 
+  class TestSerializerWithoutKlass < ActiveJob::Serializers::ObjectSerializer; end
+
   setup do
     @value_object = DummyValueObject.new 123
     @original_serializers = ActiveJob::Serializers.serializers
@@ -93,4 +95,12 @@ def klass
       ActiveJob::Serializers.add_serializers DummySerializer
     end
   end
+
+  test "raises a deprecation warning if the klass method doesn't exist" do
+    expected_message = "TestSerializerWithoutKlass should implement a public #klass method. This will raise an error in Rails 8.2"
+
+    assert_deprecated(expected_message, ActiveJob.deprecator) do
+      ActiveJob::Serializers.add_serializers TestSerializerWithoutKlass
+    end
+  end
 end

guides/source/8_1_release_notes.md

@@ -179,6 +179,8 @@ Please refer to the [Changelog][active-job] for detailed changes.
 
 ### Deprecations
 
+*   Custom Active Job serializers must have a public `#klass` method.
+
 ### Notable changes
 
 Action Text