moshi custom qualifier annotation to serialise null on one property only

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 l

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 annotations, Moshi moshi) {
      Set 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.