Tag: maven

Execute Kotlin Scripts with Gradle

Execute Kotlin Scripts with Gradle

Organize Kotlin Scripts as Gradle tasks

In this article, you will learn how you can organize multiple Kotlin scripts as Gradle tasks and make them easily executable this way. I’ve found a discussion about this here. Somebody wanted to execute Kotlin scripts with Gradle build scripts which is, of course, possible by using kotlinc as shown in this (Groovy) build script. This doesn’t look very pretty though, and, as described in the corresponding thread, isn’t very performant and manageable. Another solution would be to use Gradle scripts written with the Kotlin DSL and define custom tasks within a build.gradle.kts file, which obviously can hold and run Kotlin code naturally:

// build.gradle.kts
// Execute Kotlin task with:  gradle  -q foo

task("foo") {
  group = "com.kotlinexpertise"
  description = "my foo task"
  doLast {
    println("Hello from foo task")

The problem with this approach is that, in my case, I had multiple large Kotlin scripts I wanted to make executable this way. If I had put all of them into tasks, the script would have been too bloated and hard to maintain. Note that in my case, these tasks would not contribute to the build logic directly but rather provide business-relevant tasks, which I wanted to make executable via Gradle.

Gradle buildSrc to the rescue

As described in the Gradle documentation, build logic and especially custom tasks shouldn’t live within the build script directly. Gradle, therefore, offers the possibility to use a so-called buildSrc directory. This directory is treated as an included build, i.e. Gradle automatically compiles and tests this code and makes it available on the build script classpath. The following shows a typical project structure using this special buildSrc directory:

├── build.gradle //main build
├── buildSrc
│   ├── build.gradle //build for buildSrc
│   └── src //custom plugins, taks etc.
│       ├── main
│       │   └── java
│       │       └── com
│       │           └── enterprise
│       │               ├── Deploy.java
│       │               └── DeploymentPlugin.java
│       └── test
│           └── java
│               └── com
│                   └── enterprise
│                       └── DeploymentPluginTest.java
└── settings.gradle

As you can see here, the buildSrc has its own build.gradle, in which we define dependencies and plugins for buildSrc itself. As mentioned, the compiled code will be available for the surrounding build so that you can for instance define custom tasks in the buildSrc and use them in the main build.gradle.

A Kotlin example: Execute Kotlin Scripts with Gradle

Let’s say we have two bigger tasks we want to make available as Gradle tasks, which we call task1 and task2. Both tasks are good fits to be implemented with Kotlin.

The original project build looks like this:

import org.jetbrains.kotlin.gradle.tasks.KotlinCompile

plugins {
    kotlin("jvm") version "1.2.51"

java.sourceSets {

repositories {

dependencies {

tasks.withType<KotlinCompile> {
    kotlinOptions.jvmTarget = "1.8"

This is very basic and there’s nothing special in it. Now, the goal is to define two custom tasks taks1 and task2 within this script. Both tasks are supposed to be executable via gradle -q task1|task2. To make this possible, we create the buildSrc directory structure as shown above on the same level the build script already exists. Within the buildSrc, we now create the custom tasks as Kotlin classes:

package com.kotlinexpertise.tasks

import org.gradle.api.DefaultTask
import org.gradle.api.tasks.TaskAction
import java.sql.DriverManager

open class Task1 : DefaultTask() {

    init {
        group = "com.kotlinexpertise"
        description = "task1"

    fun run() {
        //heavy task implementation

Just like we would define a task within a Gradle build script directly, we define group, description and the task itself in a method annotated with TaskAction, which we call run. Learn more about custom tasks here. To make this a bit more interesting, we want to load a MySQL driver in this task, which requires us to make the corresponding dependency available for the buildSrc build. Let’s take a look at its build script (existing directly in buildSrc):

plugins {

repositories {

dependencies {

As mentioned, we add the dependencies we want to use within the buildSrc. Another thing to note is that the org.gradle.kotlin.kotlin-dsl plugin is applied to this build in order to set up the Kotlin compiler features and dependencies to match the ones shipped with Gradle. We used the nicer alias kotlin-dsl in this example.

Now, after defining both Task1 and Task2 as subclasses of DefaultTask, they can be used in the main build.gradle.kts script like this:

import org.jetbrains.kotlin.gradle.tasks.KotlinCompile
import com.kotlinexpertise.tasks.*




Note that both probably need to be imported as shown here. In the console, gradle tasks will list them, and you can execute them by running gradle -q task1 and gradle -q task2 respectively. You can also see the tasks listed in IntelliJ IDEA:


The source code can be found here: https://github.com/s1monw1/gradle_buildSrc_kotlin

You may want to read another article of mine about the Gradle Kotlin DSL to get more information about the topic. Enjoy.

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Simon is a software engineer based in Germany with 7 years of experience writing code for the JVM and also with JavaScript. He’s very passionate about learning new things as often as possible and a self-appointed Kotlin enthusiast.

Publish Kotlin Library on Bintray using Gradle Kotlin DSL and Travis CI

Publish Kotlin Library on Bintray using Gradle Kotlin DSL and Travis CI

Distribute a Library on Bintray using Gradle Kotlin DSL

In my latest blog post, published a few weeks back, I informed about the usage of the Gradle Kotlin DSL and how it helps with describing build scripts. In another earlier post, I introduced a small library that can be utilized for simplifying the creation of TLS/SSL sockets using a custom Kotlin DSL: SeKurity.

In this post, we’ll investigate how such a library can be made available to others that actually want to make use of it inside other projects. Ultimately, it should be possible to list the SeKurity library as a simple dependency in a build script like Maven or Gradle. Since the library itself is already backed by Gradle, I’ll show a way of publishing the resulting artifacts at the end of that build.

Where to publish?

In order to make a library available to anybody else, it needs to be published to a publicly accessible e.g. Maven repository like the famous Maven Central. Unless you’re absolutely certain about the quality of your library, it’s recommended to share it on custom repositories rather than THE central one directly as you’ll kinda lose control of it once it’s out there. One alternative is called Bintray and it will also be used in the present article. After creating an account or simply signing in with your GitHub, Google or Twitter account, you can create your own repositories there.

Automating the Build

Before enabling our build to publish the built artifacts, the process should be automated in a CI tool like Travis CI, which interacts nicely with GitHub. To make it work, it must be enabled in the GitHub repo settings by adding it as a service (see Settings -> Integrations & services). Travis expects the repository to incorporate a .travis.yml configuration that tells it what to do. For a simple Java/Kotlin project using Gradle, the following will be sufficient:

language: java

- chmod -R +x src
- chmod +x gradlew

script: ./gradlew clean build
  provider: script
  script: ./gradlew bintrayUpload -PbintrayUser=$BINTRAY_USER
      branch: master

It’s normally not necessary to specify the script command explicitly as long as you’re using the default instructions. In the shown case, Gradle runs the build task first before it deploys via the gradle bintrayUpload taks, which we will learn about later. The deployment only happens for the master branch. The gradle deploy takes two arguments $BINTRAY_USER and $BINTRAY_KEY, which contain private data and are therefore securely stored in Travis directly. When the build is run, Travis takes care of replacing them.
You can finally enable the repository in Travis and it will start a build on every push immediately. For further information on Travis, the documentation is highly recommended.

The Gradle Build

As shown earlier, the bintrayUpload Gradle task gets invoked in the build. Where does it come from? Let’s have a look at the relevant parts of the gradle.build.kts file.

The Plugins

plugins {
    kotlin("jvm") version "1.2.30"
    id("com.github.johnrengelman.shadow") version "2.0.2"
    id("com.jfrog.bintray") version "1.8.0"
  • The shadow plugin is used because we need the library to be bundled with all its dependencies in order to avoid classpath conflicts.
  • The maven-publish plugin helps with creating the relevant artifact and a relevant pom.xml file that will also be published.
  • The bintray plugin does all the heavy work at the end of the build. It refers to the maven-publish result and pushes the artifacts to Bintray.

Plugin Configuration

The build file also adds custom configurations for all previously shown plugins as shown next:

I Shadow

val artifactID = "sekurity"

val shadowJar: ShadowJar by tasks
shadowJar.apply {
    baseName = artifactID
    classifier = null

The shadow plugin uses the project’s name as the artifact’s baseName and the “all” qualifier by default, which is overridden here.

II Maven-Publish

fun MavenPom.addDependencies() = withXml {
    asNode().appendNode("dependencies").let { depNode ->
        configurations.compile.allDependencies.forEach {
            depNode.appendNode("dependency").apply {
                appendNode("groupId", it.group)
                appendNode("artifactId", it.name)
                appendNode("version", it.version)

val publicationName = "tlslib"
publishing {
    publications.invoke {
        publicationName(MavenPublication::class) {
            artifactId = artifactID

This one is a bit tricky. We reuse the result of shadow and add it to the publication which will be published under the artifactId that corresponds to the JAR’s baseName. By default, the generated POM does not list the library dependencies, which is done by mapping each compile dependency to a valid dependency XML node.

III Bintray

fun findProperty(s: String) = project.findProperty(s) as String?

bintray {
    user = findProperty("bintrayUser")
    key = findProperty("bintrayApiKey")
    publish = true
    pkg(delegateClosureOf {
        repo = "SeKurity"
        name = "SeKurity"
        userOrg = "s1m0nw1"
        websiteUrl = "https://kotlinexpertise.com"
        vcsUrl = "https://github.com/s1monw1/TlsLibrary"

The bintray plugin is most important since it takes care of actually uploading the generated artifacts to Bintray itself. The user and key are obtained from the Gradle project properties, which are passed by Travis later on as already shown earlier. By referring to the publication created in maven-publish, all artifacts will be uploaded as specified. The pkg block describes the library’s attributes and where, i.e. to what repository, the publishing happens.

The Result

All these blocks merged result in a pretty nice Gradle build script, which can be observed in this Gist and also the SeKurity repository itself. The resulting artifact is made available here: https://bintray.com/s1m0nw1/SeKurity/SeKurity

Referring to the published Library

Now that the library is being uploaded to Bintray, it can be listed as a dependency in arbitrary build files. If we have another Gradle-backed project, we first need to add the relevant repository to the list of repositories:

repositories {

    maven {

After that, the library can be added as a simple compile dependency like this:

dependencies {
//...other dependencies


As shown, Gradle plugins can be used for setting up a proper build that publishes a custom library to Bintray easily. In order to automate the whole process, CI tools like Travis can be used for executing the build whenever a new change has been made.

I hope the little tutorial helps with your build as well! If you like, have a look at my Twitter account and follow if you’re interested in more Kotlin stuff 🙂 Thanks a lot.

Please follow and like this Blog 🙂

Simon is a software engineer based in Germany with 7 years of experience writing code for the JVM and also with JavaScript. He’s very passionate about learning new things as often as possible and a self-appointed Kotlin enthusiast.