Serializer for class '...' is not found. Mark the class as @Serializable or provide the serializer explicitly

Question

I have a problem with the serialization of a class in Kotlin.

build.gradle.kt

...
plugins {
    application
    kotlin("jvm") version "1.6.21"
    kotlin("plugin.serialization").version("1.6.21")
}
...
dependencies{
...
    implementation("io.ktor:ktor-server-content-negotiation:$ktor_version")
    implementation("io.ktor:ktor-serialization-kotlinx-json:$ktor_version")
} 

Response.kt

import kotlinx.serialization.*
...
interface BaseResponse<T>

@Serializable
data class PaginatedResponse<T>(
    val prev: Int?,
    val next: Int?,
    val totalCount: Int = 0,
    val totalPages: Int = 0,
    val data: T? = null,
    val message: String? = null
) : BaseResponse<T>

usage

...
  return PaginatedResponse<List<User>>(
                prev,
                next,
                totalCount,
                totalPages,
                users
            )

The data I am passing looks quite healthy and the error that is thrown when the return is reached is:

kotlinx.serialization.SerializationException: Serializer for class 'PaginatedResponse' is not found. Mark the class as @Serializable or provide the serializer explicitly.

Doing call.respond(User(...)) will not throw any error

so if I remove generic and make the PaginatedResponse non generic it will work but is not reusable anymore

@Serializable
data class PaginatedResponse(
    val prev: Int?,
    val next: Int?,
    val totalCount: Long = 0,
    val totalPages: Long = 0,
    val data: List<User>? = null,
    val message: String? = null
) : BaseResponse<User>

Answer 1

after reading some documentation I found that the error in my case was mainly due to two misconfigurations that I had:

first: I needed to add the plugin in the gradle at the app and project level, I solved this as follows: adding in gradle.project the next line: id 'org.jetbrains.kotlin.plugin.serialization' version '1.6.21'

As is seen in the following image:

and after that, adding in the gradle.app the next line in the plugins block : id 'kotlinx-serialization'

As is seen in the following image:

secondly, the problem was that when the classes were serialized at runtime, the proguard generated conflicts, for this I had to add a few lines to the file proguard-rules.pro

And add the following lines into the file:

### your config ....

# Keep `Companion` object fields of serializable classes.
# This avoids serializer lookup through `getDeclaredClasses` as done for named companion objects.
-if @kotlinx.serialization.Serializable class **
-keepclassmembers class <1> {
   static <1>$Companion Companion;
}

# Keep `serializer()` on companion objects (both default and named) of serializable classes.
-if @kotlinx.serialization.Serializable class ** {
   static **$* *;
}
-keepclassmembers class <2>$<3> {
   kotlinx.serialization.KSerializer serializer(...);
}

# Keep `INSTANCE.serializer()` of serializable objects.
-if @kotlinx.serialization.Serializable class ** {
   public static ** INSTANCE;
}
-keepclassmembers class <1> {
   public static <1> INSTANCE;
   kotlinx.serialization.KSerializer serializer(...);
}

# @Serializable and @Polymorphic are used at runtime for polymorphic serialization.
-keepattributes RuntimeVisibleAnnotations,AnnotationDefault

As is seen in the following image:

I finally synced the gradle, compiled again and everything worked fine.

Reference documentation: https://github.com/Kotlin/kotlinx.serialization#gradle-with-plugins-block