In the case of a maven project with Kotlin and Java side by side, the Kotlin docu (https://kotlinlang.org/docs/maven.html#compile-kotlin-and-java-sources) suggests this configuration

<configuration>
    <sourceDirs>
        <sourceDir>${project.basedir}/src/main/kotlin</sourceDir>
        <sourceDir>${project.basedir}/src/main/java</sourceDir>
    </sourceDirs>
</configuration>

but what do I do if I have a multi-module project?

${project.basedir}/src/main/kotlin

lacks the module in it's path, doesn't it?

I'm no Maven expert (not at all, ever), but afair

project.basedir

should be the base dir of the according module.

makes sense, but in that case I don't understand why the java-class can't find the Kotlin class

[ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.8.1:compile (java-compile) on project erezept-lib: Compilation failure
[ERROR] /home/stephan/workspace/erp-modul/erezept-lib/src/main/java/de/gematik/titus/erezept/service/PreconditionService.java:[319,27] error: cannot find symbol
[ERROR]   symbol:   variable JwtUtil
[ERROR]   location: class PreconditionService

JwtUtil

is in the same package as

PreconditionService

but in the Kotlin src-root. IntelliJ likes it, but

mvn compile

leads to the error msg. I made sure to have the kotlin-maven-plugin right in front of the maven-compiler-plugin.

Maybe this helps: https://www.baeldung.com/kotlin/maven-java-project Order is not enough

I've also disabled default compile and testCompile on the maven-compiler-plugin. I've done all there is in the baldung example 😕 Maybe it's one of the other plugins maven-source-plugin, spring-boot-maven-plugin, jacoco-maven-plugin, sonar-maven-plugin, maven-surefire-plugin, maven-failsafe-plugin, maven-checkstyle-plugin, versions-maven-plugin that need to be configured to consider kotlin as well!?? 😬

¯\_(ツ)_/¯ Sorry, not touching Maven with a five-foot-pole if I can avoid it any way.

@Stephan Schroeder every module has a POM inside, hasn't it ? Multi-module is only the parent which does not have any source, correct ?

@AndreyVanDenHaag correct

@Stephan Schroeder well, it means every module has its own

sourceDirs

What is the issue ?

The issues / questions were: 1. does

project.basedir

include the module directory or is it the directory of the parent project 2. why do his Java classes not find the Kotlin classes of the same module

@AndreyVanDenHaag the issue is that after the setup the compilation fails:

[ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.8.1:compile (java-compile) on project erezept-lib: Compilation failure
[ERROR] /home/stephan/workspace/erp-modul/erezept-lib/src/main/java/de/gematik/titus/erezept/service/PreconditionService.java:[319,27] error: cannot find symbol
[ERROR]   symbol:   variable JwtUtil
[ERROR]   location: class PreconditionService

PreconditionService

is still a Java-class.

JwtUtil

was refactored to Kotlin, and since my config should be ok (but apparently it isn't), it shouldn't be an unfindable symbol during compilation. When I Strg+click on "JwtUtil" with the the IntelliJ editor window showing the source of

PreconditionSerice

, IntelliJ it jumps to the Kotlin file, no problem.