Compile and package a Maven project

You can set up your Maven project to compile Kotlin-only or mixed Kotlin and Java sources, configure the Kotlin compiler, specify compiler options, and package your application into JARs.

Configure source code compilation

To ensure that your source code is compiled correctly, adjust project configuration. Your Maven project can be set up to compile Kotlin-only sources or a combination of Kotlin and Java sources.

Compile Kotlin-only sources

To compile your Kotlin source code:

Specify the source directories in the <build> section:
<build> <sourceDirectory>src/main/kotlin</sourceDirectory> <testSourceDirectory>src/test/kotlin</testSourceDirectory> </build>
Ensure that the Kotlin Maven plugin is applied:
<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>

You can replace the whole <executions> section 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 a project with both Kotlin and Java source files, make sure the Kotlin compiler runs before the Java compiler. The Java compiler can't see Kotlin declarations until they are compiled into .class files. If your Java code uses Kotlin classes, those classes must be compiled first to avoid cannot find symbol errors.

Maven determines plugin execution order based on two main factors:

The order of plugin declarations in the pom.xml file.
Built-in default executions, such as default-compile and default-testCompile, which always run before user-defined executions, regardless of their position in the pom.xml file.

To control the execution order:

Declare kotlin-maven-plugin before maven-compiler-plugin.
Disable the Java compiler plugin's default executions.
Add custom executions to control the compile phases explicitly.

You can simplify the configuration of mixed Kotlin/Java compilation using extensions. It allows skipping the Maven compiler plugin configuration:

<build>
    <plugins>
        <!-- Kotlin compiler plugin configuration -->
        <plugin>
            <groupId>org.jetbrains.kotlin</groupId>
            <artifactId>kotlin-maven-plugin</artifactId>
            <version>${kotlin.version}</version>
            <extensions>true</extensions>
            <executions>
                <execution>
                    <id>default-compile</id>
                    <phase>compile</phase>
                    <configuration>
                        <sourceDirs>
                            <sourceDir>src/main/kotlin</sourceDir>
                            <!-- Ensure Kotlin code can reference Java code -->
                            <sourceDir>src/main/java</sourceDir>
                        </sourceDirs>
                    </configuration>
                </execution>
                <execution>
                    <id>default-test-compile</id>
                    <phase>test-compile</phase>
                    <configuration>
                        <sourceDirs>
                            <sourceDir>src/test/kotlin</sourceDir>
                            <sourceDir>src/test/java</sourceDir>
                        </sourceDirs>
                    </configuration>
                </execution>
            </executions>
        </plugin>
        <!-- No need to configure Maven compiler plugin with extensions -->
    </plugins>
</build>

If your project previously had a Kotlin-only configuration, you also need to remove the following lines from the <build> section:

<build>
    <sourceDirectory>src/main/kotlin</sourceDirectory>
    <testSourceDirectory>src/test/kotlin</testSourceDirectory>
</build>

It ensures that both Kotlin code can reference Java code and vice versa with the extensions setup.

<build>
    <plugins>
        <!-- Kotlin compiler plugin configuration -->
        <plugin>
            <groupId>org.jetbrains.kotlin</groupId>
            <artifactId>kotlin-maven-plugin</artifactId>
            <version>${kotlin.version}</version>
            <executions>
                <execution>
                    <id>kotlin-compile</id>
                    <phase>compile</phase>
                    <goals>
                        <goal>compile</goal>
                    </goals>
                    <configuration>
                        <sourceDirs>
                            <sourceDir>src/main/kotlin</sourceDir>
                            <!-- Ensure Kotlin code can reference Java code -->
                            <sourceDir>src/main/java</sourceDir>
                        </sourceDirs>
                    </configuration>
                </execution>
                <execution>
                    <id>kotlin-test-compile</id>
                    <phase>test-compile</phase>
                    <goals>
                        <goal>test-compile</goal>
                    </goals>
                    <configuration>
                        <sourceDirs>
                            <sourceDir>src/test/kotlin</sourceDir>
                            <sourceDir>src/test/java</sourceDir>
                        </sourceDirs>
                    </configuration>
                </execution>
            </executions>
        </plugin>

        <!-- Maven compiler plugin configuration -->
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-compiler-plugin</artifactId>
            <version>3.14.0</version>
            <executions>
                <!-- Disable default executions -->
                <execution>
                    <id>default-compile</id>
                    <phase>none</phase>
                </execution>
                <execution>
                    <id>default-testCompile</id>
                    <phase>none</phase>
                </execution>

                <!-- Define custom executions -->
                <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>

This configuration ensures that:

Kotlin code is compiled first.
Java code is compiled after Kotlin and can reference Kotlin classes.
Default Maven behavior doesn't override the plugin order.

For more details on how Maven handles plugin executions, see Guide to default plugin execution IDs in the official Maven documentation.

Configure Kotlin compiler

Choose execution strategy

The Kotlin compiler execution strategy defines where the Kotlin compiler runs. There are two available strategies:

Strategy	Where the Kotlin compiler is executed
Kotlin daemon (default)	Inside its own daemon process
In process	Inside the Maven process

By default, the Kotlin daemon is used. You can switch to the "in process" strategy by setting the following property in your pom.xml file:

<properties>
    <kotlin.compiler.daemon>false</kotlin.compiler.daemon>
</properties>

Regardless of the compiler execution strategy that you use, you still need to explicitly configure incremental compilation.

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.

Specify compiler options

Additional options and arguments for the compiler can be specified as elements in the <configuration> section 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>2.3</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	"1.9", "2.0", "2.1", "2.2", "2.3", "2.4" (EXPERIMENTAL)
`apiVersion`	kotlin.compiler.apiVersion	Allow using declarations only from the specified version of bundled libraries	"1.9", "2.0", "2.1", "2.2", "2.3", "2.4" (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", ..., "25"	"1.8"
`jdkHome`	`kotlin.compiler.jdkHome`	Include a custom JDK from the specified location into the classpath instead of the default JAVA_HOME

Package your project

Create JAR files

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>3.5.0</version>
    <configuration>
        <archive>
            <manifest>
                <addClasspath>true</addClasspath>
                <mainClass>${main.class}</mainClass>
            </manifest>
        </archive>
    </configuration>
</plugin>

Create self-contained JAR files

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>3.8.0</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

19 December 2025