Moshi Adapter. Fields annotated with @JsonNullable will be serialized as `null` instead of being ignored.

JsonNullableAdapter.kt

@Retention(AnnotationRetention.RUNTIME)
@JsonQualifier
annotation class JsonNullable

/**
 * Fields annotated with @JsonNullable will be serialized as `null` instead of being ignored.
 */
class JsonNullableAdapter : JsonAdapter.Factory {

    override fun create(type: Type, annotations: MutableSet<out Annotation>, moshi: Moshi): JsonAdapter<Any>? {
        val nextAnnotations = nextAnnotations(annotations, JsonNullable::class.java) ?: return null
        return moshi.nextAdapter<Any>(this, type, nextAnnotations).serializeNulls()
    }
}

Setup:

val moshi = Moshi.Builder()
    .add(JsonNullableAdapter())
    .build()