moshi custom qualifier annotation to serialise null on one property only - kotlin

Question

I'd like to serialise null for only one property in my JSON body that is going on a PUT. I don't want to serialize null for any other types in the object. Model class is like this
#Parcel
class User #ParcelConstructor constructor(var college: College?,
var firstname: String?,
var lastname: String?,
var email: String?,
var active: Boolean = true,
var updatedAt: String?,
var gender: String?,
var picture: String?,
var id: String?,
#field: [CollegeField] var collegeInput: String?,
#field: [CollegeField] var otherCollege: String?,)
I only want to serialise collegeInput and otherCollege fields if either of them are null. For example
val user = User(firstname = "foo", lastname=null, collegeInput="abcd", otherCollege = null)
Json will look something like this:
{"user":{
"firstname": "foo",
"collegeInput": "abcd",
"otherCollege": null
}}
Where otherCollege is null, lastname is omitted from the object as by default moshi does not serialise nulls which is what I want, but qualifer fields should be serialized with null values
I tried using
class UserAdapter {
#FromJson
#CollegeField
#Throws(Exception::class)
fun fromJson(reader: JsonReader): String? {
return when (reader.peek()) {
JsonReader.Token.NULL ->
reader.nextNull()
JsonReader.Token.STRING -> reader.nextString()
else -> {
reader.skipValue() // or throw
null
}
}
}
#ToJson
#Throws(IOException::class)
fun toJson(#CollegeField b: String?): String? {
return b
}
#Retention(AnnotationRetention.RUNTIME)
#JsonQualifier
annotation class CollegeField
I added the adapter to moshi but it never gets called
#Provides
#Singleton
fun provideMoshi(): Moshi {
return Moshi.Builder()
.add(UserAdapter())
.build()
}
#Provides
#Singleton
fun provideRetrofit(client: OkHttpClient, moshi: Moshi, apiConfig: ApiConfig): Retrofit {
return Retrofit.Builder()
.baseUrl(apiConfig.baseUrl)
.client(client)
.addCallAdapterFactory(RxJava2CallAdapterFactory.create())
.addConverterFactory(ScalarsConverterFactory.create())
.addConverterFactory(MoshiConverterFactory.create(moshi))
.build()
}

Answer 1

Your toJson adapter method will return null when the qualified string value is null, and the JsonWriter will not write the null value.
Here is a qualifier and adapter factory to install that will work.
#Retention(RUNTIME)
#JsonQualifier
public #interface SerializeNulls {
JsonAdapter.Factory JSON_ADAPTER_FACTORY = new JsonAdapter.Factory() {
#Nullable #Override
public JsonAdapter<?> create(Type type, Set<? extends Annotation> annotations, Moshi moshi) {
Set<? extends Annotation> nextAnnotations =
Types.nextAnnotations(annotations, SerializeNulls.class);
if (nextAnnotations == null) {
return null;
}
return moshi.nextAdapter(this, type, nextAnnotations).serializeNulls();
}
};
}
Now, the following will pass.
class User(
var firstname: String?,
var lastname: String?,
#SerializeNulls var collegeInput: String?,
#SerializeNulls var otherCollege: String?
)
#Test fun serializeNullsQualifier() {
val moshi = Moshi.Builder()
.add(SerializeNulls.JSON_ADAPTER_FACTORY)
.add(KotlinJsonAdapterFactory())
.build()
val userAdapter = moshi.adapter(User::class.java)
val user = User(
firstname = "foo",
lastname = null,
collegeInput = "abcd",
otherCollege = null
)
assertThat(
userAdapter.toJson(user)
).isEqualTo(
"""{"firstname":"foo","collegeInput":"abcd","otherCollege":null}"""
)
}
Note that you should use the Kotlin support in Moshi to avoid the #field: oddities.

Answer 2

Try approach from my gist:
https://gist.github.com/OleksandrKucherenko/ffb2126d37778b88fca3774f1666ce66
In my case I convert NULL from JSON into default double/integer value. You can easily modify the approach and make it work for your specific case.
p.s. its JAVA, convert it to Kotlin first.