Gson serialize null for specific class or field

Question

I want to serialize nulls for a specific field or class.

In GSON, the option serializeNulls() applies to the whole JSON.

Example:

Answer 1

I took a few ideas from various answers here.

this implementation:

• lets you choose at runtime, whether the JSON is
  • null
    • happens when JsonNullable.isJsonNull() == true
  • not null
    • happens when JsonNullable.isJsonNull() == false
  • omitted from the JSON (useful for HTTP PATCH requests)
    • happens field in Parent containing JsonNullable is null
• no annotations needed
• properly delegates unhandled work to a delegateAdapter by using a TypeAdapterFactory

objects that may need to be serialized to null implement this interface

/**
 * [JsonNullableTypeAdapterFactory] needs to be registered with the [com.google.gson.Gson]
 * serializing implementations of [JsonNullable] for [JsonNullable] to work.
 *
 * [JsonNullable] allows objects to choose at runtime whether they should be serialized as "null"
 * serialized normally, or be omitted from the JSON output from [com.google.gson.Gson].
 *
 * when [isJsonNull] returns true, the subclass will be serialized to a [com.google.gson.JsonNull].
 *
 * when [isJsonNull] returns false, the subclass will be serialized normally.
 */
interface JsonNullable {

    /**
     * return true to have the entire object serialized as `null` during JSON serialization.
     * return false to have this object serialized normally.
     */
    fun isJsonNull(): Boolean
}

type adapter factory that serializes values to null

class JsonNullableTypeAdapterFactory : TypeAdapterFactory {
    override fun  create(gson: Gson, type: TypeToken): TypeAdapter? {
        return object : TypeAdapter() {
            private val delegateAdapter = gson.getDelegateAdapter(this@JsonNullableTypeAdapterFactory, type)
            override fun read(reader: JsonReader): T = delegateAdapter.read(reader)
            override fun write(writer: JsonWriter, value: T?) {
                if (value is JsonNullable && value.isJsonNull()) {
                    val originalSerializeNulls = writer.serializeNulls
                    writer.serializeNulls = true
                    writer.nullValue()
                    writer.serializeNulls = originalSerializeNulls
                } else {
                    delegateAdapter.write(writer, value)
                }
            }
        }
    }
}

register thr type adapter factroy with GSON

new GsonBuilder()
    // ....
    .registerTypeAdapterFactory(new JsonNullableTypeAdapterFactory())
    // ....
    .create();

example object that gets serialized to JSON

data class Parent(
    val hello: Child?,
    val world: Child?
)

data class Child(
    val name: String?
) : JsonNullable {
    override fun isJsonNull(): Boolean = name == null
}