Gradle multi project build with empty projects

When applying a multi-project Gradle structure to our project, my settings.gradle looks like this:

include "source:compA:api"
include "source:compA:core"
include "source:compB"

gradle projects give me

Root project 'tmp'
\--- Project ':source'
     +--- Project ':source:compA'
     |    +--- Project ':source:compA:api'
     |    \--- Project ':source:compA:core'
     \--- Project ':source:compB'

This is exactly the directory structure!
In my root directory I have a build.gradle which applies the java plugin to all subprojects:

subprojects {
    apply plugin: 'java'
}

When building I end up having artifacts for :source:compA which are empty because this is actually not a project just the subdirectories api and core are proper Java projects.

What's the best way to avoid having an empty artifact?

You can try using the trick they use in Gradle's own settings.gradle file. Note how each of the sub projects are located in the 'subprojects/${projectName}' folder, but the subprojects folder itself is not a project.

So in your case you'd do something like:

include "source:compA-api"
include "source:compA-core"
include "source:compB"

project(':source:compA-api').projectDir = new File(settingsDir, 'source/compA/api')
project(':source:compA-core').projectDir = new File(settingsDir, 'source/compA/core')

I have intentionally omitted the colon between compA and api to make sure source:compA does not get evaluated as a project container.

Alternatively, you can try excluding the source:compA project from having the java plugin applied to it, by doing something like:

def javaProjects() { 
  return subprojects.findAll { it.name != 'compA' }
} 

configure(javaProjects()) { 
  apply plugin: 'java' 
} 

Edit: Alternatively you can try something like this (adjust to your liking):

def javaProjects() { 
  return subprojects.findAll { new File(it.projectDir, "src").exists() }
} 

configure(javaProjects()) { 
  apply plugin: 'java' 
}