如何定义Data Class使Moshi序列化出指定JSON结构？

The issue you're hitting comes down to two key quirks with Moshi's default behavior:

1. Moshi doesn't serialize classes with no properties (your FormValues is an empty class), so it gets completely omitted from the output JSON.
2. Your formValues property is declared as a class member instead of part of the primary constructor, which can make Moshi's code generator overlook it (even with the @Json annotation, the empty class problem is the bigger blocker here).

Here are a few clean, practical solutions to get your desired JSON structure {"id":"abcdef","formValues":{}}:

Solution 1: Replace FormValues with an Empty Map (Simplest Approach) #

If you don't strictly need a dedicated FormValues class, swapping it for an empty Map is the fastest way to get the empty object in your output:

@JsonClass(generateAdapter = true)
class MyDataClass(
    @Json(name = "id") val id: String,
    @Json(name = "formValues") val formValues: Map<String, Any?> = emptyMap()
)

When serialized, Moshi automatically converts the empty Map to {} in JSON, giving you exactly the structure you want without extra code.

Solution 2: Add a Custom JsonAdapter for FormValues #

If you need to keep the FormValues class for your code structure, create a custom adapter to force Moshi to serialize it as an empty object:

First, define the adapter:

class FormValuesAdapter {
    @ToJson
    fun serialize(writer: JsonWriter) {
        // Explicitly write an empty JSON object
        writer.beginObject().endObject()
    }

    @FromJson
    fun deserialize(reader: JsonReader): FormValues {
        // Skip the empty object during deserialization
        reader.beginObject()
        reader.endObject()
        return FormValues()
    }
}

Then update MyDataClass to include formValues in the primary constructor (this ensures Moshi's generator reliably picks it up):

@JsonClass(generateAdapter = true)
class MyDataClass(
    @Json(name = "id") val id: String,
    @Json(name = "formValues") val formValues: FormValues = FormValues()
) {
    @JsonClass(generateAdapter = true)
    class FormValues
}

Finally, register the adapter when building your Moshi instance:

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

val json = moshi.adapter(MyDataClass::class.java).toJson(MyDataClass("abcdef"))
// Output: {"id":"abcdef","formValues":{}}

Solution 3: Add a Dummy Property to FormValues (Quick Hack) #

For a minimal change without writing a custom adapter, add a hidden dummy property to FormValues to trick Moshi into serializing it as an object:

@JsonClass(generateAdapter = true)
class MyDataClass(
    @Json(name = "id") val id: String,
    @Json(name = "formValues") val formValues: FormValues = FormValues()
) {
    @JsonClass(generateAdapter = true)
    class FormValues {
        // Dummy property to force Moshi to recognize this as a serializable class
        @Json(ignore = true) private val placeholder: Unit? = null
    }
}

This works because Moshi now sees FormValues as a class with properties (even though the only one is ignored) and will serialize it as an empty object {}.

内容的提问来源于stack exchange，提问作者Egis