You can open this sample in an IDE that supports Gradle.

This guide demonstrates how to create a Kotlin library with Gradle using gradle init. You can follow the guide step-by-step to create a new project from scratch or download the complete sample project using the links above.

What you’ll build

You’ll generate a Kotlin library that follows Gradle’s conventions.

What you’ll need

Create a project folder

Gradle comes with a built-in task, called init, that initializes a new Gradle project in an empty folder. The init task uses the (also built-in) wrapper task to create a Gradle wrapper script, gradlew.

The first step is to create a folder for the new project and change directory into it.

$ mkdir demo
$ cd demo

Run the init task

From inside the new project directory, run the init task using the following command in a terminal: gradle init. When prompted, select the 2: library project type and 2: Kotlin as the implementation language. Next you can choose the DSL for writing buildscripts - 1 : Kotlin or 2: Groovy. For the other questions, press enter to use the default values.

The output will look like this:

$ gradle init

Select type of build to generate:
  1: Application
  2: Library
  3: Gradle plugin
  4: Basic (build structure only)
Enter selection (default: Application) [1..4] 2

Select implementation language:
  1: Java
  2: Kotlin
  3: Groovy
  4: Scala
  5: C++
  6: Swift
Enter selection (default: Java) [1..6] 2

Enter target Java version (min: 7, default: 21):

Project name (default: demo):

Select application structure:
  1: Single application project
  2: Application and library project
Enter selection (default: Single application project) [1..2] 1

Select build script DSL:
  1: Kotlin
  2: Groovy
Enter selection (default: Kotlin) [1..2]

Select test framework:
  1: JUnit 4
  2: TestNG
  3: Spock
  4: JUnit Jupiter
Enter selection (default: JUnit Jupiter) [1..4]

Generate build using new APIs and behavior (some features may change in the next minor release)? (default: no) [yes, no]

BUILD SUCCESSFUL
1 actionable task: 1 executed

The init task generates the new project with the following structure:

├── gradle (1)
│   ├── libs.versions.toml (2)
│   └── wrapper
│       ├── gradle-wrapper.jar
│       └── gradle-wrapper.properties
├── gradlew (3)
├── gradlew.bat (3)
├── settings.gradle.kts (4)
└── lib
    ├── build.gradle.kts (5)
    └── src
        ├── main
        │   └── kotlin (6)
        │       └── demo
        │           └── Library.kt
        └── test
            └── kotlin (7)
                └── demo
                    └── LibraryTest.kt
├── gradle (1)
│   ├── libs.versions.toml (2)
│   └── wrapper
│       ├── gradle-wrapper.jar
│       └── gradle-wrapper.properties
├── gradlew (3)
├── gradlew.bat (3)
├── settings.gradle (4)
└── lib
    ├── build.gradle (5)
    └── src
        ├── main
        │   └── kotlin (6)
        │       └── demo
        │           └── Library.kt
        └── test
            └── kotlin (7)
                └── demo
                    └── LibraryTest.kt
1 Generated folder for wrapper files
2 Generated version catalog
3 Gradle wrapper start scripts
4 Settings file to define build name and subprojects
5 Build script of lib project
6 Default Kotlin source folder
7 Default Kotlin test source folder

You now have the project setup to build a Kotlin library.

Review the project files

The settings.gradle(.kts) file has two interesting lines:

settings.gradle.kts
rootProject.name = "demo"
include("lib")
settings.gradle
rootProject.name = 'demo'
include('lib')

  • rootProject.name assigns a name to the build, which overrides the default behavior of naming the build after the directory it’s in. It’s recommended to set a fixed name as the folder might change if the project is shared - e.g. as root of a Git repository.

  • include("lib") defines that the build consists of one subproject called lib that contains the actual code and build logic. More subprojects can be added by additional include(…​) statements.

Our build contains one subproject called lib that represents the Kotlin library we are building. It is configured in the lib/build.gradle(.kts) file:

lib/build.gradle.kts
plugins {
    alias(libs.plugins.kotlin.jvm) (1)
    `java-library` (2)
}

repositories {
    mavenCentral() (3)
}

dependencies {
    testImplementation("org.jetbrains.kotlin:kotlin-test-junit5") (4)

    testImplementation(libs.junit.jupiter.engine) (5)

    testRuntimeOnly("org.junit.platform:junit-platform-launcher")

    api(libs.commons.math3) (6)

    implementation(libs.guava) (7)
}

tasks.named<Test>("test") {
    useJUnitPlatform() (8)
}
lib/build.gradle
plugins {
    alias(libs.plugins.kotlin.jvm) (1)
    id 'java-library' (2)
}

repositories {
    mavenCentral() (3)
}

dependencies {
    testImplementation 'org.jetbrains.kotlin:kotlin-test-junit5' (4)

    testImplementation libs.junit.jupiter.engine (5)

    testRuntimeOnly 'org.junit.platform:junit-platform-launcher'

    api libs.commons.math3 (6)

    implementation libs.guava (7)
}

tasks.named('test') {
    useJUnitPlatform() (8)
}
1 Apply the org.jetbrains.kotlin.jvm Plugin to add support for Kotlin.
2 Apply the java-library plugin for API and implementation separation.
3 Use Maven Central for resolving dependencies.
4 Use the Kotlin JUnit 5 integration.
5 Use the JUnit 5 integration.
6 This dependency is exported to consumers, that is to say found on their compile classpath.
7 This dependency is used internally, and not exposed to consumers on their own compile classpath.
8 Use JUnit Platform for unit tests.

The file src/main/kotlin/demo/Library.kt is shown here:

Generated src/main/kotlin/demo/Library.kt
/*
 * This source file was generated by the Gradle 'init' task
 */
package demo

class Library {
    fun someLibraryMethod(): Boolean {
        return true
    }
}

The generated test, src/test/kotlin/demo/Library.kt is shown next:

Generated src/test/kotlin/demo/LibraryTest.kt
/*
 * This source file was generated by the Gradle 'init' task
 */
package demo

import kotlin.test.Test
import kotlin.test.assertTrue

class LibraryTest {
    @Test fun someLibraryMethodReturnsTrue() {
        val classUnderTest = Library()
        assertTrue(classUnderTest.someLibraryMethod(), "someLibraryMethod should return 'true'")
    }
}

The generated test class has a single kotlin.test test. The test instantiates the Library class, invokes a method on it, and checks that it returns the expected value.

More information about the features the java-library plugin adds to any JVM library project, such as API and implementation separation, can be found in the Java Library Plugin documentation.

Assemble the library JAR

To build the project, run the build task. You can use the regular gradle command, but when a project includes a wrapper script, it is considered good form to use it instead.

$ ./gradlew build

BUILD SUCCESSFUL in 0s
5 actionable tasks: 5 executed
The first time you run the wrapper script, gradlew, there may be a delay while that version of gradle is downloaded and stored locally in your ~/.gradle/wrapper/dists folder.

The first time you run the build, Gradle will check whether or not you already have the required dependencies in your cache under your ~/.gradle directory. If not, the libraries will be downloaded and stored there. The next time you run the build, the cached versions will be used. The build task compiles the classes, runs the tests, and generates a test report.

You can view the test report by opening the HTML output file, located at lib/build/reports/tests/test/index.html.

You can find your newly packaged JAR file in the lib/build/libs directory with the name lib.jar. Verify that the archive is valid by running the following command:

$ jar tf lib/build/libs/lib.jar
META-INF/
META-INF/MANIFEST.MF
lib/
lib/Library.class

You should see the required manifest file —MANIFEST.MF— and the compiled Library class.

All of this happens without any additional configuration in the build script because Gradle’s java-library plugin assumes your project sources are arranged in a conventional project layout. You can customize the project layout if you wish as described in the user manual.

Congratulations, you have just completed the first step of creating a Kotlin library! You can now customize this to your own project needs.

Customize the library JAR

You will often want the name of the JAR file to include the library version. This is achieved by setting a top-level version property in the build script:

build.gradle.kts
version = "0.1.0"
build.gradle
version = '0.1.0'

Next to the version, other important identity properties of a library are it’s name and group. The name is directly derived from the subproject name that represents the library. It’s lib in the example so you probably want to adjust it by changing the name of the lib folder and the corresponding include(…​) statement in the settings.gradle(.kts) file. The group is used to give your library full coordinates when published. You can define it directly in the build script by setting the group property similar to how you set the version (shown above).

Now run the jar task:

$ ./gradlew jar

BUILD SUCCESSFUL
2 actionable tasks: 1 executed, 1 up-to-date

You’ll notice that the resulting JAR file at lib/build/libs/lib-0.1.0.jar contains the version as expected.

Another common requirement is customizing the manifest file, typically by adding one or more attributes. Let’s include the library name and version in the manifest file by configuring the jar task. Add the following to the end of your build script:

build.gradle.kts
tasks.jar {
    manifest {
        attributes(mapOf("Implementation-Title" to project.name,
                         "Implementation-Version" to project.version))
    }
}
build.gradle
tasks.named('jar') {
    manifest {
        attributes('Implementation-Title': project.name,
                   'Implementation-Version': project.version)
    }
}

To confirm that these changes work as expected, run the jar task again, and this time also unpack the manifest file from the JAR:

$ ./gradlew jar
$ jar xf lib/build/libs/lib-0.1.0.jar META-INF/MANIFEST.MF

Now view the contents of the META-INF/MANIFEST.MF file and you should see the following:

META-INF/MANIFEST.MF
Manifest-Version: 1.0
Implementation-Title: lib
Implementation-Version: 0.1.0

Generating Sources JAR

You can easily generate a sources JAR for your library:

build.gradle.kts
java {
    withSourcesJar()
}
build.gradle
java {
    withSourcesJar()
}

The additional JAR will be produced as part of the assemble or build lifecycle tasks and will be part of the publication. The resulting file is found in lib/build/libs, with a name using the conventional classifier -sources.

Publish a Build Scan

The best way to learn more about what your build is doing behind the scenes, is to publish a build scan. To do so, just run Gradle with the --scan flag.

$ ./gradlew build --scan

BUILD SUCCESSFUL in 0s
5 actionable tasks: 5 executed

Publishing a build scan to scans.gradle.com requires accepting the Gradle Terms of Service defined at https://gradle.com/terms-of-service.
Do you accept these terms? [yes, no] yes

Gradle Terms of Service accepted.

Publishing build scan...
https://gradle.com/s/5u4w3gxeurtd2

Click the link and explore which tasks where executed, which dependencies where downloaded and many more details!

Summary

That’s it! You’ve now successfully configured and built a Kotlin library project with Gradle. You’ve learned how to:

  • Initialize a project that produces a Kotlin library

  • Run the build and view the test report

  • Customize the Jar files the build produces

Now you could complete this exercise by trying to compile some Kotlin code that uses the library you just built.

Next steps

Building a library is just one aspect of reusing code across project boundaries. From here, you may be interested in: