Recently, I migrated my project's Gradle build script from Groovy to Kotlin DSL. As part of that change, I converted the configuration for the Transmode gradle-docker plugin as follows:

// Groovy DSL
task buildDocker(type: Docker) {
    baseImage = 'develar/java:latest'
    push = project.hasProperty('push')
    tag = 'geowarin/sout-chuck-norris'
    addFile {
        from jar
        rename {'app.jar'}
    }
    entryPoint(['java', '-Djava.security.egd=file:/dev/./urandom', '-jar', '/app.jar'])
    exposePort(8080)
}
buildDocker.dependsOn(build)

// Kotlin DSL
import se.transmode.gradle.plugins.docker.DockerTask

buildscript {
    dependencies {
        classpath("se.transmode.gradle:gradle-docker:1.2")
    }
    repositories {
        jcenter()
    }
}

tasks {
    "buildDocker"(DockerTask::class) {
        val appJarName = "app.jar"
        dependsOn(tasks["build"])
        baseImage = "develar/java:latest"
        push = project.hasProperty("push")
        tag = "geowarin/sout-chuck-norris"
        addFile(closureOf<CopySpec> {
            from(tasks["jar"] as CopySpec)
            rename { appJarName }
        })
        entryPoint(mutableListOf("java", "-Djava.security.egd=file:/dev/./urandom", "-jar", "/$appJarName"))
        exposePort(if (project.hasProperty("server.port")) project.property("server.port") as Int else 8080)
    }
}

However, when I run the buildDocker task I receive the following error on the console:

What went wrong: Execution failed for task ':buildDocker'. se.transmode.gradle.plugins.docker.DockerTask$_createTarArchive_closure3_closure11_closure12 cannot be cast to org.gradle.api.file.CopySpec

I have worked around it by copying the file using doFirst and then invoking the form of addFile that accepts a path String instead of a Closure but would like to get to the bottom of this issue as it might help others in the future.

well, you're trying to cast a

JarTask

to

CopySpec

, that can't work. Also from what I understand

from(...)

expects not a task at all, but a directory or a file Another problem is you're trying to make a closure into

CopySpec

, which is printing the error you mention. Take a look at this example of configuring copy: https://github.com/gradle/kotlin-dsl/blob/master/samples/copy/build.gradle.kts

AFAIK

Jar

is a Gradle task that implements `CopySpec`: I have modified the

from

in the CopySpec to a

with

but the build fails with the following error (same as before):

Caused by: java.lang.ClassCastException: se.transmode.gradle.plugins.docker.DockerTask$_createTarArchive_closure3_closure11_closure12 cannot be cast to org.gradle.api.file.CopySpec
        at Build_gradle$$special$$inlined$invoke$lambda$4.invoke(build.gradle.kts:19)
        at org.gradle.kotlin.dsl.KotlinClosure1.doCall(GroovyInteroperability.kt:93)
        at se.transmode.gradle.plugins.docker.DockerTask$_createTarArchive_closure3_closure11_closure12.doCall(DockerTask.groovy:141)

The frame in question points to the following code in `DockerTask.groovy`: https://github.com/Transmode/gradle-docker/blob/1.2/src/main/groovy/se/transmode/gradle/plugins/docker/DockerTask.groovy#L126-L151 The issue is likely to do with how Kotlin passes that

copySpec

referenced in line 141 above.