Boilerplate project configuration in Gradle with Gradle Kotlin DSL

I'm currently trying to improve the way our projects share their configuration. We have lots of different multi-module gradle projects for all of our libraries and microservices (i.e. many git repos).

My main goals are:

To not have my Nexus repository config duplicated in every project (also, I can safely assume that the URL won't change)
To make my custom Gradle plugins (published to Nexus) available to every project with minimal boilerplate / duplication (they should be available to every project, and the only thing the project cares about is the version it's using)
No magic - it should be obvious to developers how everything is configured

My current solution is a custom gradle distribution with an init script that:

adds mavenLocal() and our Nexus repository to the project repos (very similar to the Gradle init script documentation example, except it adds repos as well as validating them)
configures an extension that allows our gradle plugins to be added to the buildscript classpath (using this workaround). It also adds our Nexus repo as a buildscript repo as that's where the plugins are hosted. We have quite a few plugins (built upon Netflix's excellent nebula plugins) for various boilerplate: standard project setup (kotlin setup, test setup, etc), releasing, publishing, documentation, etc and it means our project build.gradle files are pretty much just for dependencies.

Here is the init script (sanitised):

/**
 * Gradle extension applied to all projects to allow automatic configuration of Corporate plugins.
 */
class CorporatePlugins {

    public static final String NEXUS_URL = "https://example.com/repository/maven-public"
    public static final String CORPORATE_PLUGINS = "com.example:corporate-gradle-plugins"

    def buildscript

    CorporatePlugins(buildscript) {
        this.buildscript = buildscript
    }

    void version(String corporatePluginsVersion) {
        buildscript.repositories {
            maven {
                url NEXUS_URL
            }
        }
        buildscript.dependencies {
            classpath "$CORPORATE_PLUGINS:$corporatePluginsVersion"
        }
    }

}

allprojects {
    extensions.create('corporatePlugins', CorporatePlugins, buildscript)
}

apply plugin: CorporateInitPlugin

class CorporateInitPlugin implements Plugin<Gradle> {

    void apply(Gradle gradle) {

        gradle.allprojects { project ->

            project.repositories {
                all { ArtifactRepository repo ->
                    if (!(repo instanceof MavenArtifactRepository)) {
                        project.logger.warn "Non-maven repository ${repo.name} detected in project ${project.name}. What are you doing???"
                    } else if(repo.url.toString() == CorporatePlugins.NEXUS_URL || repo.name == "MavenLocal") {
                        // Nexus and local maven are good!
                    } else if (repo.name.startsWith("MavenLocal") && repo.url.toString().startsWith("file:")){
                        // Duplicate local maven - remove it!
                        project.logger.warn("Duplicate mavenLocal() repo detected in project ${project.name} - the corporate gradle distribution has already configured it, so you should remove this!")
                        remove repo
                    } else {
                        project.logger.warn "External repository ${repo.url} detected in project ${project.name}. You should only be using Nexus!"
                    }
                }

                mavenLocal()

                // define Nexus repo for downloads
                maven {
                    name "CorporateNexus"
                    url CorporatePlugins.NEXUS_URL
                }
            }
        }

    }

}

Then I configure each new project by adding the following to the root build.gradle file:

buildscript {
    // makes our plugins (and any others in Nexus) available to all build scripts in the project
    allprojects {
        corporatePlugins.version "1.2.3"
    }
}

allprojects  {
    // apply plugins relevant to all projects (other plugins are applied where required)
    apply plugin: 'corporate.project'

    group = 'com.example'

    // allows quickly updating the wrapper for our custom distribution
    task wrapper(type: Wrapper) {
        distributionUrl = 'https://com.example/repository/maven-public/com/example/corporate-gradle/3.5/corporate-gradle-3.5.zip'
    }
}

While this approach works, allows reproducible builds (unlike our previous setup which applied a build script from a URL - which at the time wasn't cacheable), and allows working offline, it does make it a little magical and I was wondering if I could do things better.

This was all triggered by reading a comment on Github by Gradle dev Stefan Oehme stating that a build should work without relying on an init script, i.e. init scripts should just be decorative and do things like the documented example - preventing unauthorised repos, etc.

My idea was to write some extension functions that would allow me to add our Nexus repo and plugins to a build in a way that looked like they were built into gradle (similar to the extension functions gradleScriptKotlin() and kotlin-dsl() provided by the Gradle Kotlin DSL.

So I created my extension functions in a kotlin gradle project:

package com.example

import org.gradle.api.artifacts.dsl.DependencyHandler
import org.gradle.api.artifacts.dsl.RepositoryHandler
import org.gradle.api.artifacts.repositories.MavenArtifactRepository

fun RepositoryHandler.corporateNexus(): MavenArtifactRepository {
    return maven {
        with(it) {
            name = "Nexus"
            setUrl("https://example.com/repository/maven-public")
        }
    }
}

fun DependencyHandler.corporatePlugins(version: String) : Any {
    return "com.example:corporate-gradle-plugins:$version"
}

With the plan to use them in my project's build.gradle.kts as follows:

import com.example.corporateNexus
import com.example.corporatePlugins

buildscript {

    repositories {
        corporateNexus()
    }

    dependencies {
        classpath(corporatePlugins(version = "1.2.3"))
    }
}

However, Gradle was unable to see my functions when used in the buildscript block (unable to compile script). Using them in the normal project repos/dependencies worked fine though (they are visible and work as expected).

If this worked, I was hoping to bundle the jar into my custom distribution , meaning my init script could just do simple validation instead of hiding away the magical plugin and repo configuration. The extension functions wouldn't need to change, so it wouldn't require releasing a new Gradle distribution when plugins change.

What I tried:

adding my jar to the test project's buildscript classpath (i.e. buildscript.dependencies) - doesn't work (maybe this doesn't work by design as it doesn't seem right to be adding a dependency to buildscript that's referred to in the same block)
putting the functions in buildSrc (which works for normal project deps/repos but not buildscript, but is not a real solution as it just moves the boilerplate)
dropping the jar in the lib folder of the distribution

So my question really boils down to:

Is what I'm trying to achieve possible (is it possible to make custom classes/functions visible to the buildScript block)?
Is there a better approach to configuring a corporate Nexus repo and making custom plugins (published to Nexus) available across lots of separate projects (i.e. totally different codebases) with minimal boilerplate configuration?

Solution

I promised @eskatos that I would come back and give feedback on his answer - so here it is!

My final solution consists of:

Gradle 4.7 wrapper per project (pointed at a mirror of http://services.gradle.org/distributions setup in Nexus as a raw proxy repository, i.e. it's vanilla Gradle but downloaded via Nexus)
Custom Gradle plugins published to our Nexus repo along with plugin markers (generated by the Java Gradle Plugin Development Plugin)
Mirroring the Gradle Plugin Portal in our Nexus repo (i.e. a proxy repo pointing at https://plugins.gradle.org/m2)
A settings.gradle.kts file per project that configures our maven repo and gradle plugin portal mirror (both in Nexus) as plugin management repositories.

The settings.gradle.kts file contains the following:

pluginManagement {
    repositories {
        // local maven to facilitate easy testing of our plugins
        mavenLocal()

        // our plugins and their markers are now available via Nexus
        maven {
            name = "CorporateNexus"
            url = uri("https://nexus.example.com/repository/maven-public")
        }

        // all external gradle plugins are now mirrored via Nexus
        maven {
            name = "Gradle Plugin Portal"
            url = uri("https://nexus.example.com/repository/gradle-plugin-portal")
        }
    }
}

This means that all plugins and their dependencies are now proxied via Nexus, and Gradle will find our plugins by id as the plugin markers are published to Nexus as well. Having mavenLocal in there as well facilitates easy testing of our plugin changes locally.

Each project's root build.gradle.kts file then applies the plugins as follows:

plugins {
    // plugin markers for our custom plugins allow us to apply our
    // plugins by id as if they were hosted in gradle plugin portal
    val corporatePluginsVersion = "1.2.3"
    id("corporate-project") version corporatePluginsVersion
    // 'apply false` means this plugin can be applied in a subproject
    // without having to specify the version again
    id("corporate-publishing") version corporatePluginsVersion apply false
    // and so on...
}

And configures the gradle wrapper to use our mirrored distribution, which when combined with the above means that everything (gradle, plugins, dependencies) all come via Nexus):

tasks {
    "wrapper"(Wrapper::class) {
        distributionUrl = "https://nexus.example.com/repository/gradle-distributions/gradle-4.7-bin.zip"
    }
}

I was hoping to avoid the boilerplate in the settings files using @eskatos's suggestion of applying a script from a remote URL in settings.gradle.kts. i.e.

apply { from("https://nexus.example.com/repository/maven-public/com/example/gradle/corporate-settings/1.2.3/corporate-settings-1.2.3.kts" }

I even managed to generate a templated script (published alongside our plugins) that:

configured the plugin repos (as in the above settings script)
used a resolution strategy to apply the version of the plugins associated with the script if the requested plugin id was one of our plugins and the version wasn't supplied (so you can just apply them by id)

However, even though it removed the boilerplate, it meant our builds were reliant on having a connection to our Nexus repo, as it seems that even though scripts applied from a URL are cached, Gradle does a HEAD request anyway to check for changes. It also made it annoying to test plugin changes locally, as I had to point it manually at the script in my local maven directory. With my current config, I can simply publish the plugins to maven local and update the version in my project.

I'm quite happy with the current setup - I think it's far more obvious to developers now how the plugins are applied. And it's made it far easier to upgrade Gradle and our plugins independently now that there's no dependency between the two (and no custom gradle distribution required).