Reducing code duplication when testing a KtorClient - unit-testing

Question

I am creating a service on top of a Ktor client. My payload is XML, and as such a simplified version of my client looks like this :
class MavenClient(private val client : HttpClient) {
private suspend fun getRemotePom(url : String) =
try{ MavenClientSuccess(client.get<POMProject>(url)) }catch (e: Exception) { MavenClientFailure(e)
}
companion object {
fun getDefaultClient(): HttpClient {
return HttpClient(Apache) {
install(JsonFeature) {
serializer = JacksonSerializer(jackson = kotlinXmlMapper)
accept(ContentType.Text.Xml)
accept(ContentType.Application.Xml)
accept(ContentType.Text.Plain)
}
}
}
}
}
Note the use of a custom XMLMapper, attached to a custom data class.
I want to test this class, and follow the documentation.
I end up with the following code for my test client :
private val mockClient = HttpClient(MockEngine) {
engine {
addHandler { request ->
when (request.url.fullUrl) {
"https://lengrand.me/minimal/1.2/minimal-1.2.pom" -> {
respond(minimalResourceStreamPom.readBytes()
, headers = headersOf("Content-Type" to listOf(ContentType.Application.Xml.toString())))
}
"https://lengrand.me/unknown/1.2/unknown-1.2.pom" -> {
respond("", HttpStatusCode.NotFound)
}
else -> error("Unhandled ${request.url.fullUrl}")
}
}
}
// TODO : How do I avoid repeating this again ? That's my implementation?!
install(JsonFeature) {
serializer = JacksonSerializer(jackson = PomParser.kotlinXmlMapper)
accept(ContentType.Text.Xml)
accept(ContentType.Application.Xml)
accept(ContentType.Text.Plain)
}
}
private val Url.hostWithPortIfRequired: String get() = if (port == protocol.defaultPort) host else hostWithPort
private val Url.fullUrl: String get() = "${protocol.name}://$hostWithPortIfRequired$fullPath"
private val mavenClient = MavenClient(mockClient)
Now, I am not worried about the Mapper itself, because I test it directly.
However what bothers me is that I essentially have to duplicate the complete logic of my client to test behaviour?
This seems very brittle, because for example it will cause my tests to fail and have to be updated if I move to Json tomorrow. Same if I start using Response Validation for example.
This is even more true for another client where I am using a defaultRequest, which I have to completely copy over as well:
private val mockClient = HttpClient(MockEngine) {
install(JsonFeature) {
serializer = JacksonSerializer(mapper)
accept(ContentType.Application.Json)
}
defaultRequest {
method = HttpMethod.Get
host = "api.github.com"
header("Accept", "application/vnd.github.v3+json")
if (GithubLogin().hasToken()) header("Authorization", GithubLogin().authToken)
}
Am I doing things wrong? Am I testing too much ? I am curious as to how I can improve this.
Thanks a lot for your input!
P.S : Unrelated but the page about testing on Ktor mentions adding the dependency to the implementation. Sounds like I should use testImplementation instead to avoid shipping the lib with my application ?

Answer 1

The MockEngine is designed for stubbing real HTTP client implementation to test objects that use it. The duplication problem, you encounter, lies in the fact that transforming response body responsibility belongs to the client. So I suggest either use Jackson directly to transform a response body (in this case you don't need to use JsonFeature) or extract common configuration in a extension function and call it for both engines.