Multimap serializer doesn't work with non-String keys, even if it seems like they should.

[stevenschlansker]

Serializing a Multimap<K, ?> fails if K is any non-String type, even if intuitively it should work (e.g. K is a string-like type such as Enum, or K has an @JsonValue annotation).

I couldn't figure out how this is supposed to work, so I have included failing test cases for both the 1.9 branch and 2.x branch and hope that someone else who knows more about how Jackson's typing works can help out.

[pgelinas]

I'll take a look at this, but from memory it should be the same behaviour then Maps: you need a custom key deserializer for this to work. The @JsonValue annotation is ignored in this case and there might be a special case required for Enum (or a built-in key deserializer, not sure).

[stevenschlansker]

Is there any particular reason for that? If you have a to/from String via @JsonValue and @JsonCreator it seems reasonable to expect Map-like types to use that. Or should there be a @JsonKeyValue annotation? Registering custom serializers is a bit of a pain when it feels like a simple annotation should do the job.

[pgelinas]

Indeed it would be, if I was right... I did a quick look and my memory isn't that good. These kind of cases should be handled. I'll take a good look at this instead of spouting nonsense.

[pgelinas]

Alright, I've looked at your test cases and here's what I got:

• The @JsonValue case doesn't work for a simple Map type, so this isn't a guava related issue. This might be by design or a missing feature, I have no idea. However, you can make the case work by having a toString() method return the same thing the @JsonValue would (although this might not always be possible for some complex cases.)
• I've fixed the Enum case, which was caused by defaulting to a String key type when unable to resolve generic type.

[cowtowncoder]

I think this is the related issue for core databind, wrt default key serializer supporting @JsonValue

FasterXML/jackson-databind#47

and as stated, would be a Very Good thing to support.

[stevenschlansker]

Now that the upstream issue is resolved, should we reopen this one?

[cowtowncoder]

Yup.

FWIW, I think key handling might now work without additional changes here. But a test would be good to have.

[ias-dt]

We have a Multimap with Enum keys.
MultimapBuilder.enumKeys(CloudProvider.class).arrayListValues().build()
The Jackson ObjectMapper has the SerializationFeature.WRITE_ENUMS_USING_TO_STRING set to false, as well as WRITE_ENUMS_USING_INDEX set to false.
We therefore would expect the keys of the Multimap to be serialized via Enum.name()
But instead they are serialized as Enum.toString()
(c.f. EnumMapSerializer)

[cowtowncoder]

Could this be same as:

FasterXML/jackson-databind#749

NOTE: should only affect root values, but not values with Enum-key Multimaps.

[cowtowncoder]

Moved to FasterXML/jackson-datatypes-collections#3