How to add Kotlin as a second language to a SpringBoot app using Maven multimodule
The official Kotlin Documentation has a section on how to configure Maven so that it compiles Kotlin code next to Java code. But that setup assumes a configuration without Maven modules. Maybe it’s just me, but it took me quite some time to adapt that solution.
So this is what I learned:
Assumptions
You have a Java Project using Spring Boot which is seperated into several Maven modules. The parent pom of all modules is the pom-file in the base directory and the parent of that pom is spring-boot-starter-parent. Here’s a demo project configured just like that.
How to modify the main pom-file
- add the following properties:
<properties>
...
<java.version>17</java.version>
<kotlin.version>1.9.20</kotlin.version>
<kotlin.compiler.incremental>true</kotlin.compiler.incremental> <!-- optional: for faster builds -->
...
</properties>
The properties maven.compiler.source and maven.compiler.target can be removed - should they be present in your configuration -
since we’ll also update the configuration of the maven-compiler-plugin.
- prepare the dependencies for kotlin-reflect and kotlin-stdlib:
<dependencyManagement>
<dependencies>
<dependency>
<groupId>org.jetbrains.kotlin</groupId>
<artifactId>kotlin-reflect</artifactId>
<version>${kotlin.version}</version>
<scope>runtime</scope>
</dependency>
<dependency>
<groupId>org.jetbrains.kotlin</groupId>
<artifactId>kotlin-stdlib</artifactId>
<version>${kotlin.version}</version>
</dependency>
</dependencies>
</dependencyManagement>
- prepare the following configuration of the kotlin-maven-plugin and the maven-compiler-plugin:
<build>
<pluginManagement>
<plugins>
<plugin>
<groupId>org.jetbrains.kotlin</groupId>
<artifactId>kotlin-maven-plugin</artifactId>
<version>${kotlin.version}</version>
<extensions>true</extensions>
<executions>
<execution>
<id>compile</id>
<!--<goals><goal>compile</goal></goals> not needed because of <extensions>true<.../>-->
<configuration>
<sourceDirs>
<sourceDir>${project.basedir}/src/main/kotlin</sourceDir>
<sourceDir>${project.basedir}/src/main/java</sourceDir>
</sourceDirs>
</configuration>
</execution>
<execution>
<id>test-compile</id>
<!--<goals><goal>test-compile</goal></goals> not needed because of <extensions>true<.../>-->
<configuration>
<sourceDirs>
<sourceDir>${project.basedir}/src/test/kotlin</sourceDir>
<sourceDir>${project.basedir}/src/test/java</sourceDir>
</sourceDirs>
</configuration>
</execution>
</executions>
<configuration>
<args>
<arg>-Xjsr305=strict</arg>
</args>
<compilerPlugins>
<plugin>spring</plugin>
</compilerPlugins>
</configuration>
<dependencies>
<dependency>
<groupId>org.jetbrains.kotlin</groupId>
<artifactId>kotlin-maven-allopen</artifactId>
<version>${kotlin.version}</version>
</dependency>
</dependencies>
</plugin>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<version>3.11.0</version>
<executions>
<!-- Replacing default-compile as it is treated specially by maven -->
<execution>
<id>default-compile</id>
<phase>none</phase>
</execution>
<!-- Replacing default-compile 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>
<configuration>
<source>${java.version}</source>
<target>${java.version}</target>
</configuration>
</plugin>
</plugins>
</pluginManagement>
</build>
How to modify all the module pom-files
first add kotlin-maven-plugin and the maven-compiler-plugin to all modules dependency list
<dependencies>
<dependency>
<groupId>org.jetbrains.kotlin</groupId>
<artifactId>kotlin-reflect</artifactId>
</dependency>
<dependency>
<groupId>org.jetbrains.kotlin</groupId>
<artifactId>kotlin-stdlib</artifactId>
</dependency>
...
</dependencies>
and then if you’re in a module which uses the spring-boot-maven-plugin, use this order:
<build>
<plugins>
<plugin>
<groupId>org.jetbrains.kotlin</groupId>
<artifactId>kotlin-maven-plugin</artifactId>
</plugin>
<plugin>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-maven-plugin</artifactId>
</plugin>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
</plugin>
...
</plugins>
</build>
if you’re in a module without that plugin the order of the other two remains:
<build>
<plugins>
<plugin>
<groupId>org.jetbrains.kotlin</groupId>
<artifactId>kotlin-maven-plugin</artifactId>
</plugin>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
</plugin>
...
</plugins>
</build>
The order of these plugins is important. Otherwise, your Kotlin code might see your Java code but not the other way around.
Now you can add a src/main/kotlin-Path in your project and convert some of your Java classes to Kotlin.
And that’s already all there is to do to change your Java project to a Java and Kotlin project. You can check the Java and Kotlin config branch of my demo project.
Bonus: pure Kotlin configuration
So once all the source files have all been converted to Kotlin, can we simplify the Java&Kotlin configuration? Yes we can:
- remove the maven-compiler-plugin-config from all the pom-files
- remove the
<executions>-block from the kotlin-maven-plugin in the main pom-file - configure
sourceDirectoryandtestSourceDirectoryat the start of thebuild-block in the main pom-file
<build>
<sourceDirectory>${project.basedir}/src/main/kotlin</sourceDirectory>
<testSourceDirectory>${project.basedir}/src/test/kotlin</testSourceDirectory>
<pluginManagement>
...
</pluginManagement>
</build>
- keep the
<java.version>in the properties block, the spring-boot-maven-plugin will still use it to compile the Kotlin code to the configured JVM bytecode version.
Here’s the Kotlin config branch of my demo project to double-check.
What about JDK21 support
When you check my demo project, you’ll see that I’ve used <java.version>17</java.version> for all the different branches/configurations.
So assuming a JDK21 is installed on your system, can you simply set that property to 21 so that your Kotlin code can use Java 21 APIs
and your code gets compiled to JVM 21 bytecode?
pure Java config
Yes
mixed Java and Kotlin config
Yes, as long as your kotlin.version is at least 1.9.20,
since Kotlin 1.9.20-Beta1 was the first version to support a jvmTarget of 21.
pure Kotlin config
No, or more precisely, not before Kotlin 2.0 which will be released in early 2024.
Background
Apparently a CommandLine class, that Kotlin uses, was moved/removed in Java 21, so Kotlin currently doesn’t find it.
The error, which is thrown, does look like this:
Caused by: java.lang.IllegalAccessError: superclass access check failed: class org.jetbrains.kotlin.kapt3.base.javac.KaptJavaCompiler (in unnamed module @0x17de6b) cannot access class com.sun.tools.javac.main.JavaCompiler (in module jdk.compiler) because module jdk.compiler does not export com.sun.tools.javac.main to unnamed module @0x17de6b
The bug
has already been fixed, but only in Kotlin version 2.0.0-Beta1 and onwards.
Your project might not trigger this bug, if it doesn’t use Spring or Maven. E.g. I’ve got a demo project showcasing Kotlin using Java 21 APIs using Gradle and not using Spring that works without a problem. So give it a try.
Maybe there’s a pure Kotlin config that does work even with SpringBoot, but I didn’t find it. The workaround I see, if you want to use Java 21 and Spring Boot from Kotlin right now, is to us the Java & Kotlin config, even though you don’t have any Java source files in your project.
But luckily, Kotlin 2.0 isn’t far off :)