How to fix java.lang.NoSuchMethodError: java.nio.ByteBuffer.clear()Ljava/nio/ByteBuffer; when using Ktor on Heroku?

I’m playing with Ktor, and yesterday I tried to serve some static content. According to the documentation, to publish files from the resources directory, you have to do the following:

routing {
    // /static is a remote path under which files from the assets folder will be accessible
    static("/static") {
        // assets is a folder in the resources directory in your project
        resources("assets")
    }
}

It worked locally, but when I pushed my changes to Heroku, I got this error:

java.lang.NoSuchMethodError: java.nio.ByteBuffer.clear()Ljava/nio/ByteBuffer;

I’m using Ktor with Kotlin 1.6.10 without any special config or dependencies. The issue happens due to some incompatibilities between JDK 1.8 and JDK 1.8+. To fix the issue, I decided to set jvmTarget and runtime to Java 11.

To do so add the following code to your build.gradle.kts file:

tasks.withType<org.jetbrains.kotlin.gradle.tasks.KotlinCompile> {
    kotlinOptions.jvmTarget = JavaVersion.VERSION_11.toString()
}

To specify the Java version on Heroku, you need to create the system.properties file with the following content:

java.runtime.version=11

Ref.: https://devcenter.heroku.com/articles/java-support#specifying-a-java-version

Happy building!