How to set mainClassName in Gradle 6.6 with modular Java

In Gradle 6.3 within build.gradle, mainClassName may be set like this with no complaints:

mainClassName = 'mod/app.Main'

In Gradle 6.61, the above line results in this:

> java.lang.module.InvalidModuleDescriptorException: Package mod.app not
> found in module

This can be resolved by removing the module portion of mainClassName:

mainClassName = 'app.Main'

While the exception is resolved, Gradle still states:

> No module was provided for main class, assuming the current module.
> Prefer providing 'mainClassName' in the following format:
> '$moduleName/a.b.Main'

I have experimented with variations of this:

ext.moduleName = 'mod'
mainClassName = '${ext.moduleName}/app.Main'

So far I have been unable to get any of these to work. I could work with the setting as 'app.Main', but I would prefer to use the variation that Gradle prefers. How should this be done?

Gradle 6.3 doesn't support JPMS, but 6.4+ does. So I assume you are using the 3rd party gradle-modules-plugin. For this plugin, you need to use version 1.7.0 or later for supporting newer versions of Gradle.

Alternatively, remove the plugin and use the native support in Gradle. Right now it looks like you are a mix of the two at the same time.

work in 6.3 mainClassName = "$moduleName/app.Main"

work in 6.4+

run {
    main = "$moduleName/app.Main"
}