Allow serialization/deserialization of enums using `kebab-case`.

[garretwilson]

Is your feature request related to a problem? Please describe.

The Java world (and many other languages) uses mostly camelCase identifiers, except for constants (in particular enums), which are identified with UPPER_SNAKE_CASE. However the modern web world uses (lowercase) kebab-case for identifiers. You can see this in:

• URI path segments
• URI path slugs
• CSS classes
• HTML values
• etc.

Modern web frameworks such as Angular and Vue translate between the two worlds (in this case, the worlds of JavaScript/TypeScript and the web) transparently. Similarly I'm sure you're aware that JavaScript-based DOM methods use camelCase identifiers when accessing kebab-case data, both for standard attributes and actual custom data attributes.

I'm creating an entire framework that needs to convert Java enum UPPER_SNAKE_CASE to kebab-case and back transparently when serializing/deserializing.

Issue #3053 got us half the way there, but really that feature request was specific to the requestor's use case. I'm not saying that functionality is never useful. I'm saying that the usage of kebab-case (this request) in the modern web world is overwhelmingly more needed than lower_snake_case (which is effectively what #3053 produces).

Describe the solution you'd like

We need a feature to tell Jackson to serialize/deserialize an enum value such as FOO_BAR as foo-bar. Perhaps EnumFeature.WRITE_ENUMS_TO_KEBAB_CASE would work. (I assume/hope that the implementation of #3053 handles deserialization as well? I haven't tried it.)

Usage example

No response

Additional context

As I workaround, I've thrown together part of a proof of concept to override serialization. I could use some help on this so I can complete it and use it in the interim:

.addModule(new SimpleModule() //
    .addSerializer(Enum.class, new JsonSerializer<Enum>() {
      @Override
      public void serialize(Enum value, JsonGenerator gen, SerializerProvider serializers) throws IOException {
        gen.writeString(value.name().toLowerCase().replace('_', '-'));
      }
    })
    .addKeySerializer(Enum.class, new JsonSerializer<Enum>() {
      @Override
      public void serialize(Enum value, JsonGenerator gen, SerializerProvider serializers) throws IOException {
        gen.writeFieldName(value.name().toLowerCase().replace('_', '-'));
      }
    })

This works for serialization, although it's a shame to use raw Enum. I couldn't immediately figure out a way to prevent the warning, even using Enum<?> and the like.

However I don't see how to create a deserializer that will override the behavior for all enums. The com.fasterxml.jackson.databind.deser.std.EnumDeserializer class is huge! Is there some way to override that and do a pre-transformation on the token or something? Or is there a simpler way to add a new custom enum serializer with this functionality?

How best to add a quick-and-dirty workaround until (and if) Jackson adds this feature natively? Thanks.

[garretwilson]

I've added Stack Overflow question Change Jackson deserialization of all enums without using annotations to help find a temporary workaround.

[JooHyukKim]

There are 7 naming stratgies for Enum atm. There may be other users that want different source-target naming strategy.
Would be nice if we could just pull off some global default like

mapper.customEnumNamingStrategy(
     YourCustomEnum.class
    /* source= */ UPPER_SNAKE_CASE,
    /* target= */ KEBAB_CASE
)

[garretwilson]

I wasn't aware there were these existing naming strategies. Is there a way to choose one of them in my object mapper? Or are you saying that is not way at the moment?

[JooHyukKim]

Those Naming strategies are real. You may find them in test cases. And...

mapper.customEnumNamingStrategy(
     YourCustomEnum.class
    /* source= */ UPPER_SNAKE_CASE,
    /* target= */ KEBAB_CASE
)

... this is non-existent feature. Sorry if I confused u :-0

[garretwilson]

Thanks for that information, @JooHyukKim . Do you know of a quick-and-dirty way I can add a custom enum deserializer at the the object mapper level for all enum types as a temporary workaround?

[cowtowncoder]

Enum deserializers can be registered like any other, by registering Deserializers implementation, overriding this callback:

public JsonDeserializer<?> findEnumDeserializer(Class<?> type,
        DeserializationConfig config, BeanDescription beanDesc)
    throws JsonMappingException;

and registration via Module. If you have a closed set of Enums, then SimpleDeserializers via SimpleModule would also work; otherwise callback let's you create deserializer for any and all Enum types.

[cowtowncoder]

On EnumNamingStrategy; 2.19 will add:

    /**
     * Method for setting custom enum naming strategy to use.
     *
     * @since 2.19
     */
    public ObjectMapper setEnumNamingStrategy(EnumNamingStrategy s) {
        _serializationConfig = _serializationConfig.with(s);
        _deserializationConfig = _deserializationConfig.with(s);
        return this;
    }

but while 2.19.0-rc2 is out, final 2.19.0 not yet.
But once that is out, EnumNamingStrategies.KEBAB_CASE will be an option.

Also: @EnumNaming can be attached to Enum types via mix-ins so that's another option before 2.19.0.

[garretwilson]

Well my framework is still in development, so if I can use v2.19.0-rc2 right now and call setEnumNamingStrategy(EnumNamingStrategies.KEBAB_CASE) at the object mapper level, that would be awesome! Thank you.

[cowtowncoder]

Excellent! I hope it works -- being a newer feature there can be rough edges but conceptually it should work.

Plus a good way for us to get feedback

[garretwilson]

For future reference it seems the naming strategies were added in issue #2667, but I didn't see where the kebab-case stuff was added.

[cowtowncoder]

@garretwilson Hmmh. I think #4676 would be the issue, PR #4728.

[garretwilson]

I don't see any deserialization tests of kebab-case in PR #4728. But I'll give it a spin.

[garretwilson]

I haven't tried v2.19.0-rc2 yet—I'm exploring some other avenues. But in my exploration and from looking at the code, there's a potential problem you might want to look at.

Let me ask you a question: with default enum deserialization, if Jackson is deserializing the enum value FOO_BAR and it finds Foo_Bar in the stream, will Jackson accept the value and return TestEnum.FOO_BAR? I haven't tested this, but I would guess the answer is "no", because Jackson doesn't consider values in a case-insensitive manner. And that would be the correct approach!

But what if I turn on EnumNamingStrategies.KEBAB_CASE? What if Foo-Bar is in the stream? Would Jackson blindly change it to uppercase FOO_BAR before trying to create the enum instance, ignoring the fact that the value should have been foo-bar instead of Foo-Bar? If so, EnumNamingStrategies.KEBAB_CASE in effect would change Jackson behavior to be case-insensitive!! (One easy but perhaps not the most efficient way to guard against this would be, after deserialization, to do the reverse transformation from the enum name and verify that the original value matched what was expected.)

But I don't even see any deserialization code for kebab-case enums in PR #4728, or any tests, so I'm guessing about behavior you might want to check for.

[cowtowncoder]

I did not implement this feature so I don't know off-hand, but one thing to note on all Naming Strategies -- they work statically, mapping class defined names, without considering possible input. So it's all about going from implied name (from Enum constant) to expected/produced output name; and there can only be a single mapping for each Enum constant.

From that, incoming String is never changed to anything; but it is expected to match "external name" produced from Enum name.

@lbenedetto Might be better able to answer here, having provided the PR.