Gradle Kotlin DSL: UPDATE Using the Maven Publish Plugin

This post is an update of the old one on using the Maven Publish Plugin which is not incubating anymore.
The main reason for this update is the introduction of the new lazy API for task configuration with Gradle 4.9 and the change of the Strings invoke() function in Gradle Kotlin DSL with Gradle 4.10. Invoke() is now built on top of this new API and won't work the way it is described in the old post anymore. Also, the way source sets are accessed was changed.

The recommended way to declare the task for creating the Jar file with the sources is:

val sourcesJar by tasks.registering(Jar::class) {
    classifier = "sources"
    from(sourceSets["main"].allSource)
}

We will use the new API and its new delegate registering() to lazily create the task.
The variable for the task is declared outside of the tasks block to make it accessible inside the publishing block.
Also note that the java prefix for accessing source sets is gone.

With the new API the variable sourcesJar is not of type Task anymore but of type TaskProvider. By calling get() on the provider, the task can be accessed:

publishing {
    publications {
        register("mavenJava", MavenPublication::class) {
            from(components["java"])
            artifact(sourcesJar.get())
        }
    }
}

Analogous to the creation of the sourcesJar task we use the new API and its new register() function to create the publishing task. And because of that, the publications block can be used as intended without braces.