"No transformation found: class io.ktor.utils.io.ByteChannelNative" error using Ktor

user_6826124 0
Viewed 7444

I'm trying to fetch and deserialize some data that is being hosted on github.

{
  "Meals": [
    {
      "id": "1598044e-5259-11e9-8647-d663bd870b02",
      "name": "Tomato pasta",
      "quantity": [{
        "quantity": 1 },
        {
          "quantity": 2
        },
        {
          "quantity": 3
        }],
      "availableFromDate": "1605802429",
      "expiryDate": "1905802429",
      "info": "Vegetarian",
      "hot": false,
      "locationLat": 57.508865,
      "locationLong": -6.292,
      "distance": null
    },
    {
      "id": "2be2d854-a067-43ec-a488-2e69f0f2a624",
      "name": "Pizza",
      "quantity": [{
        "quantity": 1 },
        {
        "quantity": 2
        },
        {
          "quantity": 3
        }
      ],
      "availableFromDate": "1605802429",
      "expiryDate": "1905902429",
      "info": "Meat",
      "hot": false,
      "locationLat": 51.509465,
      "locationLong": -0.135392,
      "distance": null
    }
  ]
}

If I spin up a json-server locally then it works perfectly, so I know that my data class is not the problem. However when I try to do the same thing from that github link I get this error:

Error Domain=KotlinException Code=0 "No transformation found: class io.ktor.utils.io.ByteChannelNative -> class kotlin.collections.List

I have a feeling it might be to do with setting a ContentType or something along those lines but I haven't had any success specifying that so far.

Here is my code to make the request:

class MealApi {

    private val httpClient = HttpClient {
        install(JsonFeature) {
            val json = Json { ignoreUnknownKeys = true }
            serializer = KotlinxSerializer(json)
        }
    }

    suspend fun getAllMeals(): List<Meal> {
        return httpClient.get(endpoint)
    }
}

and here is my data class just for completeness:

@Serializable
data class Meal(
    @SerialName("id")
    val id: String,
    @SerialName("name")
    val name: String,
    @SerialName("quantity")
    val quantity: List<Quantity>,
    @SerialName("availableFromDate")
    var availableFromDate: String,
    @SerialName("expiryDate")
    var expiryDate: String,
    @SerialName("info")
    val info: String,
    @SerialName("hot")
    val hot: Boolean,
    @SerialName("locationLat")
    val locationLat: Float,
    @SerialName("locationLong")
    val locationLong: Float,
    @SerialName("distance")
    var distance: Double? = null
)

@Serializable
data class Quantity(
    @SerialName("quantity")
    val quantity: Int
)

UPDATE

I've found that this server https://gitcdn.link/ allows you to serve your raw github files with the right Content-Type.

ktor kotlin-multiplatform kotlin json
5 Answers

I've searched a lot how to change the server response headers (to change the plain/text one to application/json) but it seems that ktor actually doesn't allow to do that:

A nice way should be to allow the ResponseObserver to change the server response headers and pass through the modify response. But you can't actually.

Your problem depends, as you pointed out, from the fact that the raw github page provides an header Content-Type=plain/text instead of ContentType=application/json.

So IRL when you are running your API in a real server this won't occur as you'll take care to put the right content type at server level.

But if you want a workaround to this you could rewrite your api call in this way:

suspend fun getAllMealsWithFallback() {
    var meals: Meals? = null
    try {
        meals = httpClient.get(endpoint)
    } catch (e: NoTransformationFoundException) {
        val mealsString: String = httpClient.get(endpoint)
        val json = kotlinx.serialization.json.Json {
            ignoreUnknownKeys = true
        }
        meals = json.decodeFromString(mealsString)
    } finally {
        println("Meals: ${meals?.meals}")
    }
}

I had to add this class to conform to the json text you have provided in the github link.

@Serializable
data class Meals(
    @SerialName("Meals")
    val meals: List<Meal>,
)
user_46779510

Try this:

install(JsonFeature) {
    serializer = KotlinxSerializer(KotlinJson { ignoreUnknownKeys = true })
    acceptContentTypes = acceptContentTypes + ContentType.Any
}

If you'd like to accept all content types. Or Use ContentType.Text.Any, ContentType.Text.Html if you preferred.

user_96100660

In case the issue is the Content-Type:

You can alter the list of response content types, for which the KotlinxSerializer gets active, by extending the JsonFeature block to:

install(JsonFeature) {
    val json = Json { ignoreUnknownKeys = true }
    serializer = KotlinxSerializer(json)
    receiveContentTypeMatchers += object : ContentTypeMatcher {
        override fun contains(contentType: ContentType): Boolean =
            contentType == ContentType("text", "plain")
    }
}
user_20116220

How would you do this in 2022 ? this JsonFeature plugin isn't available anymore, or please refer me to how to install it

user_88642050

If you're using Ktor 2.0, you would need ContentNegotiation plugin instead of JsonFeature.

For example if you use Gson:

install(io.ktor.client.plugins.contentnegotiation.ContentNegotiation) {
    gson()
}
user_59858530
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