Kotlin Moshi adapter return null when library throw JsonDataException

We are using old backend. Sometimes list of objects is returned when contract says it should be text (and other ways).

How/where can we handle JsonDataException Expected a string but was BEGIN_OBJECT so we can remove object with null value for wrong parsed data?

{
...
   "data":{
      "value1":"example",
      "value2":2,
      "value3":"example",
      "value4":"example"
   },
   "other_data":"example",
   "data2":"only string"
}

{
...
   "data":"", //how to ignore this part -> set null?
   "other_data":"example",
   "data2":{ //how to ignore this part?
      "value":2
   }
}

Should this be handled in adapter, if yes is there a need to parse all json manually?

Yes, you can make a Moshi adapter for this. You'll just need to peek ahead, see if it's the value you expect, and skip the value if it's not.

Here's a generalized adapter in Kotlin.

@Retention(AnnotationRetention.RUNTIME)
@JsonQualifier
internal annotation class SpecifiedTypeOrNull(val token: JsonReader.Token) {
  class Adapter(
    private val delegate: JsonAdapter<Any?>,
    private val token: JsonReader.Token
  ) : JsonAdapter<Any?>() {
    override fun fromJson(reader: JsonReader): Any? {
      if (reader.peek() != token) {
        reader.skipValue()
        return null
      }
      return delegate.fromJson(reader)
    }

    override fun toJson(writer: JsonWriter, value: Any?) {
      delegate.toJson(writer, value)
    }

    class Factory : JsonAdapter.Factory {
      override fun create(
        type: Type,
        annotations: Set<Annotation>,
        moshi: Moshi
      ): JsonAdapter<*>? {
        if (annotations.isEmpty()) {
          return null
        }
        var token: JsonReader.Token? = null
        var nextAnnotations: MutableSet<Annotation>? = null
        for (annotation in annotations) {
          if (annotation is SpecifiedTypeOrNull) {
            token = annotation.token
            nextAnnotations = annotations.toMutableSet()
            nextAnnotations -= annotation
          }
        }
        if (token == null) {
          return null
        }
        val delegate = moshi.nextAdapter<Any?>(this, type, nextAnnotations!!)
        return Adapter(delegate, token)
      }
    }
  }
}

Here's how you can use it for your above code.

@JsonClass(generateAdapter = true)
data class Foo(
  @SpecifiedTypeOrNull(JsonReader.Token.BEGIN_OBJECT) val data: Data?,
  val other_data: String,
  @SpecifiedTypeOrNull(JsonReader.Token.STRING) val data2: String?
) {
  @JsonClass(generateAdapter = true)
  data class Data(val value1: String)
}

val encoded1 = """
  {
     "data":{
        "value1":"example",
        "value2":2,
        "value3":"example",
        "value4":"example"
     },
     "other_data":"example",
     "data2":"only string"
  }
"""

val encoded2 = """
  {
     "data":"",
     "other_data":"example",
     "data2":{
        "value":2
     }
  }
"""

fun main() {
  val moshi = Moshi.Builder()
    .add(SpecifiedTypeOrNull.Adapter.Factory())
    .build()
  val adapter = moshi.adapter(Foo::class.java)
  val decoded1 = adapter.fromJson(encoded1)
  val decoded2 = adapter.fromJson(encoded2)
  println(decoded1)
  println(decoded2)
}