Kotlin Help

Maven

Maven is a build system that you can use to build and manage any Java-based project.

Configure and enable the plugin

The kotlin-maven-plugin compiles Kotlin sources and modules. Currently, only Maven v3 is supported.

In your pom.xml file, define the version of Kotlin you want to use in the kotlin.version property:

<properties> <kotlin.version>2.0.0</kotlin.version> </properties>

To enable kotlin-maven-plugin, update your pom.xml file:

<plugins> <plugin> <artifactId>kotlin-maven-plugin</artifactId> <groupId>org.jetbrains.kotlin</groupId> <version>2.0.0</version> </plugin> </plugins>

Use JDK 17

To use JDK 17, in your .mvn/jvm.config file, add:

--add-opens=java.base/java.lang=ALL-UNNAMED --add-opens=java.base/java.io=ALL-UNNAMED

Declare repositories

By default, the mavenCentral repository is available for all Maven projects. To access artifacts in other repositories, specify the ID and URL of each repository in the <repositories> element:

<repositories> <repository> <id>spring-repo</id> <url>https://repo.spring.io/release</url> </repository> </repositories>

Set dependencies

Kotlin has an extensive standard library that can be used in your applications. To use the standard library in your project, add the following dependency to your pom.xml file:

<dependencies> <dependency> <groupId>org.jetbrains.kotlin</groupId> <artifactId>kotlin-stdlib</artifactId> <version>${kotlin.version}</version> </dependency> </dependencies>

If your project uses Kotlin reflection or testing facilities, you need to add the corresponding dependencies as well. The artifact IDs are kotlin-reflect for the reflection library, and kotlin-test and kotlin-test-junit for the testing libraries.

Compile Kotlin-only source code

To compile source code, specify the source directories in the <build> tag:

<build> <sourceDirectory>${project.basedir}/src/main/kotlin</sourceDirectory> <testSourceDirectory>${project.basedir}/src/test/kotlin</testSourceDirectory> </build>

The Kotlin Maven Plugin needs to be referenced to compile the sources:

<build> <plugins> <plugin> <groupId>org.jetbrains.kotlin</groupId> <artifactId>kotlin-maven-plugin</artifactId> <version>${kotlin.version}</version> <executions> <execution> <id>compile</id> <goals> <goal>compile</goal> </goals> </execution> <execution> <id>test-compile</id> <goals> <goal>test-compile</goal> </goals> </execution> </executions> </plugin> </plugins> </build>

Starting from Kotlin 1.8.20, you can replace the whole <executions> element above with <extensions>true</extensions>. Enabling extensions automatically adds the compile, test-compile, kapt, and test-kapt executions to your build, bound to their appropriate lifecycle phases. If you need to configure an execution, you need to specify its ID. You can find an example of this in the next section.

Compile Kotlin and Java sources

To compile projects that include Kotlin and Java source code, invoke the Kotlin compiler before the Java compiler. In Maven terms it means that kotlin-maven-plugin should be run before maven-compiler-plugin using the following method, making sure that the kotlin plugin comes before the maven-compiler-plugin in your pom.xml file:

<build> <plugins> <plugin> <groupId>org.jetbrains.kotlin</groupId> <artifactId>kotlin-maven-plugin</artifactId> <version>${kotlin.version}</version> <extensions>true</extensions> <!-- You can set this option to automatically take information about lifecycles --> <executions> <execution> <id>compile</id> <goals> <goal>compile</goal> <!-- You can skip the <goals> element if you enable extensions for the plugin --> </goals> <configuration> <sourceDirs> <sourceDir>${project.basedir}/src/main/kotlin</sourceDir> <sourceDir>${project.basedir}/src/main/java</sourceDir> </sourceDirs> </configuration> </execution> <execution> <id>test-compile</id> <goals> <goal>test-compile</goal> <!-- You can skip the <goals> element if you enable extensions for the plugin --> </goals> <configuration> <sourceDirs> <sourceDir>${project.basedir}/src/test/kotlin</sourceDir> <sourceDir>${project.basedir}/src/test/java</sourceDir> </sourceDirs> </configuration> </execution> </executions> </plugin> <plugin> <groupId>org.apache.maven.plugins</groupId> <artifactId>maven-compiler-plugin</artifactId> <version>3.5.1</version> <executions> <!-- Replacing default-compile as it is treated specially by Maven --> <execution> <id>default-compile</id> <phase>none</phase> </execution> <!-- Replacing default-testCompile as it is treated specially by Maven --> <execution> <id>default-testCompile</id> <phase>none</phase> </execution> <execution> <id>java-compile</id> <phase>compile</phase> <goals> <goal>compile</goal> </goals> </execution> <execution> <id>java-test-compile</id> <phase>test-compile</phase> <goals> <goal>testCompile</goal> </goals> </execution> </executions> </plugin> </plugins> </build>

Enable incremental compilation

To make your builds faster, you can enable incremental compilation by adding the kotlin.compiler.incremental property:

<properties> <kotlin.compiler.incremental>true</kotlin.compiler.incremental> </properties>

Alternatively, run your build with the -Dkotlin.compiler.incremental=true option.

Configure annotation processing

See kapt – Using in Maven.

Create JAR file

To create a small JAR file containing just the code from your module, include the following under build->plugins in your Maven pom.xml file, where main.class is defined as a property and points to the main Kotlin or Java class:

<plugin> <groupId>org.apache.maven.plugins</groupId> <artifactId>maven-jar-plugin</artifactId> <version>2.6</version> <configuration> <archive> <manifest> <addClasspath>true</addClasspath> <mainClass>${main.class}</mainClass> </manifest> </archive> </configuration> </plugin>

Create a self-contained JAR file

To create a self-contained JAR file containing the code from your module along with its dependencies, include the following under build->plugins in your Maven pom.xml file, where main.class is defined as a property and points to the main Kotlin or Java class:

<plugin> <groupId>org.apache.maven.plugins</groupId> <artifactId>maven-assembly-plugin</artifactId> <version>2.6</version> <executions> <execution> <id>make-assembly</id> <phase>package</phase> <goals> <goal>single</goal> </goals> <configuration> <archive> <manifest> <mainClass>${main.class}</mainClass> </manifest> </archive> <descriptorRefs> <descriptorRef>jar-with-dependencies</descriptorRef> </descriptorRefs> </configuration> </execution> </executions> </plugin>

This self-contained JAR file can be passed directly to a JRE to run your application:

java -jar target/mymodule-0.0.1-SNAPSHOT-jar-with-dependencies.jar

Specify compiler options

Additional options and arguments for the compiler can be specified as tags under the <configuration> element of the Maven plugin node:

<plugin> <groupId>org.jetbrains.kotlin</groupId> <artifactId>kotlin-maven-plugin</artifactId> <version>${kotlin.version}</version> <extensions>true</extensions> <!-- If you want to enable automatic addition of executions to your build --> <executions>...</executions> <configuration> <nowarn>true</nowarn> <!-- Disable warnings --> <args> <arg>-Xjsr305=strict</arg> <!-- Enable strict mode for JSR-305 annotations --> ... </args> </configuration> </plugin>

Many of the options can also be configured through properties:

<project ...> <properties> <kotlin.compiler.languageVersion>1.9</kotlin.compiler.languageVersion> </properties> </project>

The following attributes are supported:

Attributes specific to JVM

Name

Property name

Description

Possible values

Default value

nowarn

Generate no warnings

true, false

false

languageVersion

kotlin.compiler.languageVersion

Provide source compatibility with the specified version of Kotlin

"1.3" (DEPRECATED), "1.4" (DEPRECATED), "1.5", "1.6", "1.7", "1.8", "1.9", "2.0" (EXPERIMENTAL), "2.1" (EXPERIMENTAL)

apiVersion

kotlin.compiler.apiVersion

Allow using declarations only from the specified version of bundled libraries

"1.3" (DEPRECATED), "1.4" (DEPRECATED), "1.5", "1.6", "1.7", "1.8", "1.9", "2.0" (EXPERIMENTAL), "2.1" (EXPERIMENTAL)

sourceDirs

The directories containing the source files to compile

The project source roots

compilerPlugins

Enabled compiler plugins

[]

pluginOptions

Options for compiler plugins

[]

args

Additional compiler arguments

[]

jvmTarget

kotlin.compiler.jvmTarget

Target version of the generated JVM bytecode

"1.8", "9", "10", ..., "22"

"1.8"

jdkHome

kotlin.compiler.jdkHome

Include a custom JDK from the specified location into the classpath instead of the default JAVA_HOME

Use BOM

To use a Kotlin Bill of Materials (BOM), write a dependency on kotlin-bom:

<dependencyManagement> <dependencies> <dependency> <groupId>org.jetbrains.kotlin</groupId> <artifactId>kotlin-bom</artifactId> <version>2.0.0</version> <type>pom</type> <scope>import</scope> </dependency> </dependencies> </dependencyManagement>

Generate documentation

The standard Javadoc generation plugin (maven-javadoc-plugin) doesn't support Kotlin code. To generate documentation for Kotlin projects, use Dokka. Dokka supports mixed-language projects and can generate output in multiple formats, including standard Javadoc. For more information about how to configure Dokka in your Maven project, see Maven.

Enable OSGi support

Learn how to enable OSGi support in your Maven project.

Last modified: 28 May 2024