See https://github.com/diffplug/spotless/tree/main/plugin-gradle#-spotless-plugin-for-gradle

This issue, unlike #700, is solely focused on using Spotless for copyright headers.

In snippets below, make sure to replace [[[ RELEVANT BRANCH ]]] and [[[ START DATE ]]] with the correct information.

spotless {
	if (project.hasProperty("spotlessFrom")) {
		if (project.spotlessFrom == "ALL") {
			println "[Spotless] Ratchet deactivated"
		}
		else {
			println "[Spotless] Ratchet from $project.spotlessFrom"
			ratchetFrom project.spotlessFrom
		}
	}
	else if (isCiServer) {
		println "[Spotless] CI detected without explicit branch, not enforcing check"
		enforceCheck false
	}
	else {
		String spotlessBranch = "origin/[[[ RELEVANT BRANCH ]]]"
		println "[Spotless] Local run detected, ratchet from $spotlessBranch"
		ratchetFrom spotlessBranch
	}
	java {
		target '**/*.java'
		licenseHeaderFile('codequality/spotless/licenseSlashstarStyle.txt')
		removeUnusedImports()
	}
}

It relies on the -PspotlessFrom to activate spotless on specific CI jobs, since spotless otherwise needs a full checkout of the project (in order to compare the commit with the relevant base branch).

We can update the checkout action of an existing job and add spotless as a separate step in order to make failures stand out. That would avoid an extra job, which counts towards CI limits.

Let's assume we have a check job that has checkout,java,gradle checks steps. We configure the checkout step to now checkout everything and add a spotless step configured to "ratchet from" the base branch of the PR right before the existing gradle step:

#ASSUMING THIS JOB EXISTS:
  check:
    name: other checks
    runs-on: ubuntu-latest
    steps:
    - uses: actions/checkout@v2
#ADD:

      with:
        fetch-depth: 0

#ASSUMING NOW THERE'S A STEP TO CONFIGURE JAVA LIKE:
    - uses: actions/setup-java@v1
      with:
        java-version: 8
#ADD:

    - uses: eskatos/gradle-command-action@v1
      name: gradle
      with:
        arguments: spotlessCheck -PspotlessFrom=origin/${{ github.base_ref }}
        wrapper-cache-enabled: true
        dependencies-cache-enabled: true
        configuration-cache-enabled: true

#ASSUMING NOW THERE IS A check GRADLE STEP LIKE:
    - uses: eskatos/gradle-command-action@v1
      name: other tests
      with:
        arguments: check -x :reactor-core:test --no-daemon
        wrapper-cache-enabled: true
        dependencies-cache-enabled: true
        configuration-cache-enabled: true

The license file includes two line returns, which will enforce a single blank line by Spotless:

/*
 * Copyright (c) $YEAR VMware Inc. or its affiliates, All Rights Reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *   https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

Spotless interprets the $YEAR variable correctly in the face of ranges, updating the end year of the range but not the start year (2017-2018 -> 2017-2021). If a single past year is present, Spotless will add the current year as end of the range (2017 -> 2017-2021).

TODO:

• core: [build] Use Spotless plugin to enforce/update copyright headers reactor-core#2729 and [chores] Revert copyright start year + polish reactor-core#2732
• pool: [build] Use Spotless plugin to enforce/update copyright headers reactor-pool#143 and Refine spotless configuration and copyright ranges reactor-pool#144
• reactor-addons
• netty: [build] Switch to Spotless plugin for license check reactor-netty#1705
• reactor-kafka
• reactor-rabbitmq
• reactor-kotlin-extensions
• mention spotless in CONTRIBUTING.md
• (bot) Add Spotless and copyright notice headers spring-attic/reactor-bot#19
• (BOM)
• (site) Introduce Spotless and update copyright notice headers projectreactor.io#68