Moshi/Kotlin - How to serialize NULL JSON strings into empty strings instead? - json

Question

I'm trying to write a null-safe String adapter that will serialize this JSON {"nullString": null} into this: Model(nullString = "") so that any JSON with a 'null' value that I expect to be a String will be replaced with "" (assuming there exists a data class like this: data class Model(val nullString: String))
I wrote a custom adapter to try and handle this:
class NullStringAdapter: JsonAdapter<String>() {
#FromJson
override fun fromJson(reader: JsonReader?): String {
if (reader == null) {
return ""
}
return if (reader.peek() == NULL) "" else reader.nextString()
}
#ToJson
override fun toJson(writer: JsonWriter?, value: String?) {
writer?.value(value)
}
}
...in an attempt to solve this parsing error:
com.squareup.moshi.JsonDataException: Expected a name but was NULL at path $.nullString
Moshi parsing code:
val json = "{\"nullString\": null}"
val moshi = Moshi.Builder()
.add(KotlinJsonAdapterFactory())
.add(NullStringAdapter())
.build()
val result = moshi.adapter(Model::class.java).fromJson(configStr)
What am I missing here? Still new to moshi so any help is appreciated!

Answer 1

The immediate problem is the missing reader.nextNull() call to consume the null value.
There are a couple other cleanup things you can do here, too.
With #FromJson, implementing JsonAdapter is unnecessary.
Also, the JsonReader and JsonWriter are not nullable.
object NULL_TO_EMPTY_STRING_ADAPTER {
#FromJson fun fromJson(reader: JsonReader): String {
if (reader.peek() != JsonReader.Token.NULL) {
return reader.nextString()
}
reader.nextNull<Unit>()
return ""
}
}
and use add the adapter:
val moshi = Moshi.Builder()
.add(NULL_TO_EMPTY_STRING_ADAPTER)
.add(KotlinJsonAdapterFactory())
.build()