Hello everyone! I’m having an issue with Ktor de-serializing json in a post request. I’m using `kotl...

# ktor

[David Gethers]

Hello everyone! I’m having an issue with Ktor de-serializing json in a post request. I’m using kotlinx.serialization and the exception occurs with call.receive<MyMode>. I’m curious if it a similar issue is the same as @nrobi. I don’t get the java.lang.NoClassDefFoundError but I do get io.ktor.features.CannotTransformContentToTypeException: Cannot transform this request's content to. I followed the instructions in https://ktor.io/servers/features/content-negotiation/serialization-converter.html and https://ktor.io/servers/calls/requests.html. I have the correct grade entry in my build.gradle file:

implementation "io.ktor:ktor-serialization:$ktor_version"

I install the ContentNegotiator:

fun Application.module() {
    install(ContentNegotiation) {
        serialization(
            contentType = ContentType.Application.Json,
            json = Json(DefaultJsonConfiguration.copy(prettyPrint = true))
        )
    }

    val dao = DAOFacadeDatabase(Database.connect("jdbc:h2:mem:test;DB_CLOSE_DELAY=-1", driver = "org.h2.Driver"))
    val server = embeddedServer(Netty, port = 8080) {

I have my simple data class:

data class UrlMap(val id: String, val url: String, val userId: String)

The exception is thrown here:

val receive = call.receive<UrlMap>()

I follow multiple examples on website and the official Ktor documentation and I can’t get it to work. I’m using Ktor 1.3.0 with Kotlin 1.3.61.

Any help would be greatly appreciated

[Tim Malseed]

Cannot transform this request

It sounds like your request data doesn’t match what the content negotiator expects

[Tim Malseed]

Maybe the request body doesn’t contain the json data you’re expecting?

[David Gethers]

@Tim Malseed I double checked the payload and nothing obvious stands out.
My payload:

{
  "id": "7",
  "url": "<https://kotlinlang.org/>",
  "userId": "user1"
}

Data Class:

data class UrlMap(val id: String, val url: String, val userId: String)

Also I doublechecked the headers as well, making sure this was present:

Content-Type: application/json

[nrobi]

@jetbrains could someone look into this please? @Artyom Degtyarev [JB] @e5l 🙏🏼

[e5l]

@sandwwraith

[louiscad]

Isn't the data class missing the @Serializable annotation?

[David Gethers]

@louiscad - I went through several examples on various blogs and from the official Ktor website. I didn't see any data class with @Serializable.

[nrobi]

@David Gethers could you try out with the annotations? Let’s see if it helps, I’m using the annotations but still having the problem

[David Gethers]

@nrobi I added the annotation and I'm still getting the same error. I did notice that IntelliJ is displaying the following warning: kotlinx.serialization compiler plugin is not applied to the module, so this annotation would not be processed. Make sure that you've setup your buildscript correctly and re-import project. Let me see how I can get rid of this warning.

[David Gethers]

@nrobi The interesting thing is that I tried several different Content Negotiations and I get the same error.

[nrobi]

try adding

id("kotlinx-serialization")

plugin, let’s see if it helps

[David Gethers]

@nrobi I added the plugin and I'm still getting the same error

[louiscad]

Could any of you set up a reproducing project and give the GitHub/Gitlab/Bitbucket/Whatever link?

[David Gethers]

@louiscad https://github.com/dgethers/url_shortener

[David Gethers]

@louiscad - the changes are on a branch called: adding-url-mapping

[louiscad]

@David Gethers Hopefully, this information can help @sandwwraith or one of his colleagues help you 🙂

I initially had a few ideas, but I don't really have experience with ktor yet, I'm only getting started, so JB folks or other users can probably help further with the sample on the branch you specified.

[sandwwraith]

It looks fine from the serialization point of view. @e5l maybe ktor misconfiguration?

[nrobi]

@sandwwraith on my end probably yes, since it works well on the ktor-client side, but fails on the server side

[e5l]

@cy

[David Gethers]

@cy - any thoughts?

[christoph.pickl]

@David Gethers i'm having my lazy day today and managed to fix it for you 🙇🏻
so what happened here was that ktor was started up twice: once through Netty's EngineMain and then the second time in the Application.module() definition through calling embeddedServer().

after solving that, there was a simple transaction {} missing in your DAO.

my patch attached here also contains the proper test (test first 🙌🏻 ).

hope that helps, all the best and enjoy ktor, christoph

fix_server_startup_and_post.patch

[David Gethers]

Thank you @christoph.pickl