Unable to load class Plugin

I'm trying to generate first plugin but have this error:
Could not find implementation class 'CommonPluginClass' for plugin 'common-plugin' specified in jar:file
org.gradle.api.plugins.InvalidPluginException: An exception occurred applying plugin request [id: 'common-plugin']

this is my CommonPluginClass:

class CommonPluginClass: Plugin<Project> {

    override fun apply(project: Project) {
        project.task("hello") {
            doLast {
                println ("Hello from the CommonPluginClass")
            }
        }
    }
}

its very simple. My build.gradle (convention)

plugins {
    id 'groovy-gradle-plugin'
}

gradlePlugin {
    plugins {
        commonPlugin {
            id = "common-plugin"
            implementationClass = "CommonPluginClass"
        }
    }
}

and into settings.gradle (build-logic)

dependencyResolutionManagement {
    repositories {
        google()
        mavenCentral()
    }
}

rootProject.name = "build-logic"
include(":convention")

Into build.gradle (app) i call plugin in this mode:
plugins { id 'common-plugin' }
my exeption:

What went wrong: An exception occurred applying plugin request [id: 'common-plugin']
Could not find implementation class 'CommonPluginClass.kt' for plugin 'common-plugin' specified in jar:file:/C:/Users/User/.gradle/caches/jars-9/******/convention.jar!/META-INF/gradle-plugins/common-plugin.properties.
I would like to keep this tree, what am I doing wrong?

Solution

In short

You should basically get this to work by changing the plugins block of your build-logic/convention/build.gradle file to something like the following:

plugins {
    id 'java-gradle-plugin'
    id 'org.jetbrains.kotlin.jvm' version '1.6.21'
}

In more detail

I have replicated the relevant bits of your setup as follows (not showing Gradle Wrapper files):

├── app
│   └── build.gradle
├── build-logic
│   ├── convention
│   │   ├── build.gradle
│   │   └── src
│   │       └── main
│   │           └── kotlin
│   │               └── CommonPluginClass.kt
│   └── settings.gradle
└── settings.gradle

The files have the same content as given in your question, except that I’ve changed the files below.

With that, I could successfully run: ./gradlew :app:hello

`build-logic/convention/build.gradle`

plugins {
    id 'java-gradle-plugin'
    id 'org.jetbrains.kotlin.jvm' version '1.6.21'
}

gradlePlugin {
    plugins {
        commonPlugin {
            id = "common-plugin"
            implementationClass = "CommonPluginClass"
        }
    }
}

`build-logic/convention/src/main/kotlin/CommonPluginClass.kt`

import org.gradle.api.Project
import org.gradle.api.Plugin

class CommonPluginClass: Plugin<Project> {

    override fun apply(project: Project) {
        project.task("hello") { t ->
            t.doLast {
                println ("Hello from the CommonPluginClass")
            }
        }
    }
}

`settings.gradle`

pluginManagement {
    includeBuild('build-logic')
}

include ':app'