Retaining null element deserialization in enum "set" after jackson 3 migration

[Dragas]

While migrating to jackson 3 I've encountered an issue where the following json block

{"set": ["DOESNT_EXIST", "DOES_EXIST"]}

fails to deserialize as following java record

enum Foo { DOES_EXIST; }
record Container(Set<Foo> set){};

The gist is that jackson used to deserialize sets regardless of values they contained as hash sets if interface type was selected, which in turn permitted those sets to contain nulls for enumerated values. In jackson 3, on the other hand, this got changed and if a set containing enums is identified jackson opts to deserialize to EnumSet<T extends Enum<T>> which doesn't permit null values. I see a note in EnumSetDeserializer confirming this (https://github.com/FasterXML/jackson-databind/blob/jackson-databind-3.1.4/src/main/java/tools/jackson/databind/deser/jdk/EnumSetDeserializer.java#L212-L214)

                        // EnumSet does not accept nulls, so we need to report an error
                        // 05-Nov-2025, tatu: In case of no explicit Nulls handling; with 2.x,
                        //   let's just skip for backward compatibility; with 3.x, FAIL

Is there a way to nudge jackson to retain the old behavior without having to explicitly declare that a hash set is required in such contexts?

Full reproduction code

public class Main {
    public static void main(String[] args) {
        //language=JSON
        String json = "{ \"set\": [\"DOESNT_EXIST\", \"DOES_EXIST\"] }\n";
        Container jackson3Result = jackson3(json); // throws
        Container jackson2Result = jackson2(json); // works as expected; set contains two values: null and DOES_EXIST
    }

    private static Container jackson2(String json) {
        com.fasterxml.jackson.databind.json.JsonMapper mapper = com.fasterxml.jackson.databind.json.JsonMapper.builder()
                .configure(com.fasterxml.jackson.databind.DeserializationFeature.READ_UNKNOWN_ENUM_VALUES_AS_NULL, true)
                .build();
        try {
            Container container = mapper.readValue(json, Container.class);
            return container;
        } catch (IOException e) {
            e.printStackTrace();
            return null;
        }
    }

    public static Container jackson3(String json) {
        tools.jackson.databind.json.JsonMapper.Builder builder = tools.jackson.databind.json.JsonMapper.builder();
        builder.datatypeFeatures()
                .with(tools.jackson.databind.cfg.EnumFeature.READ_UNKNOWN_ENUM_VALUES_AS_NULL);
        tools.jackson.databind.json.JsonMapper mapper = builder
                .build();
        try {
            Container container = mapper.readValue(json, Container.class);
            return container;
        } catch (tools.jackson.core.JacksonException e) {
            e.printStackTrace();
            return null;
        }
    }

    public static enum Foo {
        DOES_EXIST;
    }

    public record Container(Set<Foo> set){

    };
}

Self nitpick: apparently Builder#configure accepts DatatypeFeature as argument, which i never noticed.

[JooHyukKim]

@Dragas did u find answers to your problem?

Your last statement made me hope so.

Self nitpick: apparently Builder#configure accepts DatatypeFeature as argument, which i never noticed.

[Dragas]

That nitpick was more about me misusing the configuration API, but yes, I just forgot to post it.

Gist is you can use @tools.jackson.databind.annotation.JsonDeserialize(as = java.util.HashSet.class) to retain jackson 2 behavior, meaning your "Enum Set" field is defined like so

    public record Container(
            @JsonDeserialize(as = HashSet.class)
            Set<Foo> set
    )

I suggest including this into migration guide.