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kotlinkotlinx.serialization

Serialization omitting nulls

I have a class like this:

@Serializable
data class MyClass(
    val prop1: Int,
    val prop2: Int?
)

When prop2 is null, I want to serialize this class without including the prop2 property. I can do this like this:

val json = Json { explicitNulls = false }
json.encodeToString(MyClass(42, null))  // gives {"prop1": 42}

Unfortunately, there are many places in a large project that serialize this class, and currently they just use Json.encodeToString which includes the nulls explicitly.

How can I enforce the serialization of this class to not serialize nulls? I need this to apply just to this class; other serializable classes in the project need to continue to have explicit nulls.

Solution

  • If you can give the selected fields a default of null, then you can use the EncodeDefault annotation:

    @Serializable
data class MyClass(
    val prop1: Int,
    @EncodeDefault(NEVER) val prop2: Int? = null
)

    This will avoid encoding prop2 if it is null, irrespective of the setting of the encodeDefaults property of the Json serializer you use when you serialize your class instance.