Gradle Kotlin DSL: Using the Maven Publish Plugin

If you want to publish your libraries built with Gradle to a Maven Repository you need to create the necessary artifacts and a corresponding POM-file. To do that Gradle offers the Maven Plugin and the new incubating Maven Publish Plugin which is used in this post.
Please note that the Maven Publish Plugin does not yet support all the features of the Maven Plugin which it will eventually replace.

plugins {
    ...
    `maven-publish`
}

After declaring the plugin, a publication needs to be defined inside the publishing block. In the Kotlin DSL you have two ways to create an actual publication: as String or as a variable. When using a String to declare a publication the syntax changes slightly:

publishing {
    (publications) {
        "mavenJava"(MavenPublication::class) {
            from(components["java"])
        }
    }
}

The publications function cannot be used since it does not declare the necessary invoke() function on Strings. By putting parenthesis around publications we instead invoke getPublications() which enables the use of Strings as a publication.
When using a variable instead of the String the publications block can be used:

publishing {
    publications {
        val mavenJava by creating(MavenPublication::class) {
            from(components["java"])
        }
    }
}

Additional artifacts can be added to the publication by defining and adding an artifact creating task to it. In this example another JAR file is created containing all the source files from the main source set:

tasks {
    "sourcesJar"(Jar::class) {
        classifier = "sources"
        from(java.sourceSets["main"].allSource)
        dependsOn("classes")
    }  
}

publishing {
    (publications) {
        "mavenJava"(MavenPublication::class) {
            from(components["java"])
            artifact(tasks["sourcesJar"])
        }
    }
}