Hey all, coming across a very strange bug with a Kotlin data class that's being serialized to a JSON...

# server

[Nick]

Hey all, coming across a very strange bug with a Kotlin data class that's being serialized to a JSON response using Jackson. I have a bool on the data class isMyFeautureEnabled and when the data class gets serialized, there are two booleans serailzed isMyFeatureEnabled and a phantom field myFeatureEnabled (without the is). If I change the name of isMyFeatureEnabled to something like testFieldName, then I don't see the phantom field. Anyone come across this before?

So:

data class MyData(
  val field1: String,
  val field2: String,
  val isMyFeautureEnabled: Boolean,
)

serializes to:

{
  "field1": "test",
  "field2": "test",
  "isMyFeatureEnabled": true,
  "myFeatureEnabled": true
}

but

data class MyData(
  val field1: String,
  val field2: String,
  val testField: Boolean,
)

serializes to:
{
  "field1": "test",
  "field2": "test",
  "testField": true
}

Not sure if it's a Kotlin thing or a Jackson thing but it must be something to do with the is in the name.

[Nick]

UPDATE: It was a Jackson thing, Jackson was trying to do some weird automagical stuff but putting an @get:JsonProperty("isMyFeautreEnabled") annotation on it fixed it.

[dave]

This is Jackson being "helpful". Our solution to this type of thing has just been to use Moshi if possible 🙃

[Klitos Kyriacou]

Interesting. It seems similar to this: https://stackoverflow.com/questions/32270422/jackson-renames-primitive-boolean-field-by-removing-is.

[Tim Mortimer]

Yeah, this is a class Jackson IS GETTER. You can disable it in your mapper configuration without having to explicitly use a @JsonProperty annotation on the affected field: https://github.com/FasterXML/jackson-databind/wiki/Mapper-Features

[dave]

The principle of most surprise strikes again! 😉

[David Kubecka]

Maybe the defaults make sense for Java?

[dave]

I'm not really sure they do - at least for modern java. I suspect the decision comes from waaaaaay back when everyone was doing "java beans"

[Mervyn McCreight]

The naming pattern for a getter of a boolean property is defined as is<PropertyName> for java-beans as per specification (at least the last time I had a look at it). That is most possibly the reason for jackson to interpret the presence of a function like is<PropertyName> that returns boolean as if there was a boolean property with that particular name.

Remember that kotlin autogenerates the function isMyFeatureEnabled() that returns a boolean for your val, that might be why jackson picks that up.

[Mervyn McCreight]

I'd suggest using kotlinx-serialization instead so you don't end up with java convention surprises like this 😄