'Kotlin @Serializable annotation not working in IntelliJ

I'm trying to use the @Serializable annotation in Kotlin. I can build the project with Gradle, but it's showing up red in IntelliJ and when I hover on the @Serializable annotation, there's a message saying:

 kotlinx.serializable compiler plugin is not applied to the module, so this
 annotation would not be processed. Make sure you've setup your buildscript
 correctly and re-import project.

My build.gradle.kts file looks like this:

plugins {
    id("org.jetbrains.kotlin.jvm").version("1.3.50")
    id("org.jetbrains.kotlin.plugin.serialization") version "1.3.50"
    idea
}

repositories {
    jcenter()
}

dependencies {
    implementation("org.jetbrains.kotlin:kotlin-stdlib-jdk8")
    implementation(group = "com.charleskorn.kaml", name = "kaml", version = "0.12.0")

    testImplementation("org.jetbrains.kotlin:kotlin-test")
    testImplementation("org.jetbrains.kotlin:kotlin-test-junit")
}

I have the most recent version of the Kotlin plugin (1.3.50) installed.

intellij-idea kotlin

Solution 1:^[1]

You need to add :

plugins {
    application
    kotlin("jvm") version "1.4.21"
    kotlin("plugin.serialization") version "1.4.21"
}

This is specified by JetBrains here, linked to gradle:

https://github.com/Kotlin/kotlinx.serialization#setup

https://ktor.io/docs/kotlin-serialization.html#add_dependencies

Solution 2:^[2]

So I got this working. The problem was that I was using the idea plugin to generate the IntelliJ project. When I instead imported the project by opening the build.gradle.kts file, everything works as expected.

The reason I didn't import it that way in the first place is because I had disabled Gradle support, so when I opened the build.gradle.kts file, it didn't import the project, it just opened it in an editor. If you have this situation, hit cmd+shift+a and search for Gradle and then toggle that switch to On. After a reboot of IntelliJ you should be able to import the project from the build.gradle.kts file and you should be good to go.

Solution 3:^[3]

In my case, the main and test directories were no longer 'modules' in IntelliJ. Not sure what happened...

You can verify this by going to the menu -> File -> Project Structure...

Under the modules left side navigation the main and test should be modules under the parent app. If not, the easiest way to fix is

Close your project
Delete the .idea and .gradle directories
Select Menu -> File -> New project from existing source... and import as a gradle project

Otherwise, you could also likely recreate your modules by going to Project structure... shown above. Then + New Module under your parent app, then select Java, with Kotlin checked as the module type. Do this for both the main and test dirs (ensure your directories are marked as 'test' source directories in the latter)

Solution 4:^[4]

Have you tried going to File -> Invalidate Caches?

Sources

This article follows the attribution requirements of Stack Overflow and is licensed under CC BY-SA 3.0.

Source: Stack Overflow

Solution	Source
Solution 1	Maximiliano S.
Solution 2	Viktor Nordling
Solution 3	aaronvargas
Solution 4	Rick Tilley

'Kotlin @Serializable annotation not working in IntelliJ

Solution 1:[1]

Solution 2:[2]

Solution 3:[3]

Solution 4:[4]

Sources

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