spring-depend | Tool for getting a spring bean dependency graph | Dependency Injection library

First off, I would recommend restructuring your project so you don't have cyclic dependencies. As it is now, in order to build starter, you need to build module. And in order to test module, you need to build starter. Gradle can do it, but it is usually a smell.

In terms of troubleshooting: when you get a test failure like this, look at the test report as that has the full stack trace. You should see that it complains that it can't find the Starter class (Caused by: java.lang.ClassNotFoundException: de.kramhal.multi.Starter), which is of cause in the starter module.

You mentioned the spring-dependency-management plugin, but that is only relevant for managing Maven dependencies, and not project dependencies like this. So it is not helpful here.

I am not entirely sure if this is Windows specific or not as I remember there were some discussions around performance a while back when having a lot of classes. But I believe the java-library plugin will look for jar files in other projects, and not the folder for compiled classes. This is a problem for you since the spring-boot plugin will by default disable the standard jar task and instead create "fat" a jar file through the bootJar task. Because you need both the fat jar for packaging the application to run stand-alone but also the normal jar for consuming it as a dependency, you need to do some tweaks to the starter project (Kotlin DSL):