Kotlin Help

Maven

Plugin and versions

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

Define the version of Kotlin you want to use via a kotlin.version property:

<properties> <kotlin.version>1.6.0</kotlin.version> </properties>

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 in the pom file:

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

If you're targeting JDK 7 or JDK 8, you can use extended versions of the Kotlin standard library. They contain additional extension functions for APIs added in new JDK versions. Instead of kotlin-stdlib, use kotlin-stdlib-jdk7 or kotlin-stdlib-jdk8, depending on your JDK version.

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>

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 that 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>${kotlin.version}</version> <executions> <execution> <id>compile</id> <goals> <goal>compile</goal> </goals> <configuration> <sourceDirs> <sourceDir>${project.basedir}/src/main/kotlin</sourceDir> <sourceDir>${project.basedir}/src/main/java</sourceDir> </sourceDirs> </configuration> </execution> <execution> <id>test-compile</id> <goals> <goal>test-compile</goal> </goals> <configuration> <sourceDirs> <sourceDir>${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>

Incremental compilation

To make your builds faster, you can enable incremental compilation for Maven by defining 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.

Annotation processing

See the description of Kotlin annotation processing tool (kapt).

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>

Self-contained Jar file

To create a self-contained Jar file containing the code from your module along with 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

Specifying 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> <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>1.0</kotlin.compiler.languageVersion> </properties> </project>

The following attributes are supported:

Attributes common to JVM and JS

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

"1.4" (DEPRECATED), "1.5", "1.6", "1.7" (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" (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

[]

Attributes specific to JVM

Name

Property name

Description

Possible values

Default value

jvmTarget

kotlin.compiler.jvmTarget

Target version of the generated JVM bytecode

"1.6" (DEPRECATED), "1.8", "9", "10", "11", "12", "13", "14", "15", "16", "17"

"1.8"

jdkHome

kotlin.compiler.jdkHome

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

Attributes specific to JS

Name

Property name

Description

Possible values

Default value

outputFile

Destination *.js file for the compilation result

metaInfo

Generate .meta.js and .kjsm files with metadata. Use to create a library

true, false

true

sourceMap

Generate source map

true, false

false

sourceMapEmbedSources

Embed source files into source map

"never", "always", "inlining"

"inlining"

sourceMapPrefix

Add the specified prefix to paths in the source map

moduleKind

The kind of JS module generated by the compiler

"umd", "commonjs", "amd", "plain"

"umd"

Generating documentation

The standard JavaDoc generation plugin (maven-javadoc-plugin) does not support Kotlin code. To generate documentation for Kotlin projects, use Dokka; please refer to the Dokka README for configuration instructions. Dokka supports mixed-language projects and can generate output in multiple formats, including standard JavaDoc.

OSGi

For OSGi support see the Kotlin OSGi page.

Last modified: 16 November 2021