github workflow can use gradle.

workflow and scripts are duplicated so nothing existing breaks.
ci-gradle triggers are commented apart from the workflow trigger
which allows manual start, so one can compare artifacts.

.github/scripts/release-gradle-snapshot.sh

@@ -0,0 +1,46 @@
+#!/usr/bin/env bash
+set -ex
+
+TAG="snapshot"
+DATE_TIME_UTC=$(date -u +"%F at %T (UTC)")
+RELEASE_DIR="build/github_release"
+
+gh release delete "${TAG}" -y || true
+
+git tag --force "${TAG}"
+
+git push --force origin "${TAG}"
+
+mkdir "${RELEASE_DIR}"
+
+ln -s "../publications/maven/module.json"      "${RELEASE_DIR}/plantuml-SNAPSHOT-module.json"
+ln -s "../publications/maven/pom-default.xml"  "${RELEASE_DIR}/plantuml-SNAPSHOT.pom"
+ln -s "../libs/plantuml.jar"                   "${RELEASE_DIR}/plantuml-SNAPSHOT.jar"
+ln -s "../libs/plantuml-javadoc.jar"           "${RELEASE_DIR}/plantuml-SNAPSHOT-javadoc.jar"
+ln -s "../libs/plantuml-sources.jar"           "${RELEASE_DIR}/plantuml-SNAPSHOT-sources.jar"
+
+if [[ -e "build/publications/maven/module.json.asc" ]]; then
+  # signatures are optional so that forked repos can release snapshots without needing a gpg signing key
+  ln -s "../publications/maven/module.json.asc"     "${RELEASE_DIR}/plantuml-SNAPSHOT-module.json.asc"
+  ln -s "../publications/maven/pom-default.xml.asc" "${RELEASE_DIR}/plantuml-SNAPSHOT.pom.asc"
+  ln -s "../libs/plantuml.jar.asc"                  "${RELEASE_DIR}/plantuml-SNAPSHOT.jar.asc"
+  ln -s "../libs/plantuml-javadoc.jar.asc"          "${RELEASE_DIR}/plantuml-SNAPSHOT-javadoc.jar.asc"
+  ln -s "../libs/plantuml-sources.jar.asc"          "${RELEASE_DIR}/plantuml-SNAPSHOT-sources.jar.asc"
+fi
+
+echo -n "${DATE_TIME_UTC}" > "${RELEASE_DIR}/plantuml-SNAPSHOT.timestamp"
+
+cat <<-EOF >notes.txt
+  This is a pre-release of [the latest development work](https://github.com/plantuml/plantuml/commits/).
+  ⚠️  **It is not ready for general use** ⚠️
+  ⏱  _Snapshot taken the ${DATE_TIME_UTC}_
+EOF
+
+gh release create \
+  --prerelease \
+  --target "${GITHUB_SHA}" \
+  --title "${TAG}" \
+  --notes-file notes.txt \
+  "${TAG}" ${RELEASE_DIR}/*
+
+echo "::notice title=release snapshot::Snapshot released at ${GITHUB_SERVER_URL}/${GITHUB_REPOSITORY}/releases/tag/${TAG} and taken the ${DATE_TIME_UTC}"

.github/scripts/release-gradle.sh

@@ -0,0 +1,18 @@
+#!/usr/bin/env bash
+set -ex
+
+RELEASE_DIR="build/libs/github_release"
+
+mkdir "${RELEASE_DIR}"
+
+ln -s "../libs/plantuml.jar"             "${RELEASE_DIR}/plantuml-${POM_VERSION}.jar"
+ln -s "../libs/plantuml-javadoc.jar"     "${RELEASE_DIR}/plantuml-${POM_VERSION}-javadoc.jar"
+ln -s "../libs/plantuml-sources.jar"     "${RELEASE_DIR}/plantuml-${POM_VERSION}-sources.jar"
+# we do not release the .pom or .asc signature files here, they will be added in a later PR
+
+gh release create \
+  --target "${GITHUB_SHA}" \
+  --title "${TAG}" \
+  "${TAG}" ${RELEASE_DIR}/*
+
+echo "::notice title=::Released at ${GITHUB_SERVER_URL}/${GITHUB_REPOSITORY}/releases/tag/${TAG} 🎉"

.github/workflows/ci-gradle.yml

@@ -0,0 +1,163 @@
+name: CI gradle
+
+on:
+#  create:
+#  pull_request:
+#    types: [ opened, synchronize, reopened ]
+#    paths-ignore:
+#      - '*.md'
+#      - 'docs/**'
+#  push:
+#    branches:
+#      - master
+#    paths-ignore:
+#      - '*.md'
+#      - 'docs/**'
+  workflow_dispatch:
+
+defaults:
+  run:
+    shell: bash
+
+jobs:
+  workflow_config:
+    runs-on: ubuntu-latest
+    outputs:
+      do_release: ${{ steps.config.outputs.do_release }}
+      do_snapshot_release: ${{ steps.config.outputs.do_snapshot_release }}
+      pom_version: ${{ steps.config.outputs.pom_version }}
+    steps:
+      - name: Configure workflow
+        id: config
+        env:
+          ACTOR: ${{ github.actor }}
+          EVENT_ACTION: ${{ github.event.action }}
+          REF_TYPE: ${{ github.event.ref_type }}
+          REF: ${{ github.event.ref }}
+        run: |
+          cat <<-EOF
+            ::group::Debug Info
+            GITHUB_EVENT_NAME       : '${GITHUB_EVENT_NAME}'
+            EVENT_ACTION            : '${EVENT_ACTION}'
+            REF_TYPE                : '${REF_TYPE}'
+            REF                     : '${REF}'
+            ACTOR                   : '${ACTOR}'
+            GITHUB_REPOSITORY_OWNER : '${GITHUB_REPOSITORY_OWNER}'
+            ::endgroup::
+          EOF
+
+          # Do a release when a git tag starting with 'v' has been created by a suitable user.
+          # (We match against github.repository_owner as a kludge so that forked repos can release themselves when testing the workflow)
+
+          if [[ "${GITHUB_EVENT_NAME}" == "create" && "${REF_TYPE}" == "tag" && "${REF}" == v* && \
+                ( "${ACTOR}" == "arnaudroques" || "${ACTOR}" == "${GITHUB_REPOSITORY_OWNER}" ) \
+             ]]; then
+            echo "::notice title=::This run will release '${REF}'"
+            echo "::set-output name=do_release::true"
+            echo "::set-output name=pom_version::${REF#v}"  # pom_version is the tag without the 'v' prefix
+
+          elif [[ "${GITHUB_EVENT_NAME}" =~ push|workflow_dispatch && "${REF}" == "refs/heads/master" ]]; then
+            echo "::notice title=::This run will release a snapshot"
+            echo "::set-output name=do_snapshot_release::true"
+
+          else
+            echo "This run will NOT make a release"
+          fi
+
+  # We run the tests on many OS / Java combinations but also the Compile step because some users build
+  # their own jars from source, so it is good for CI to check that is working on all combinations.
+  build:
+    needs: workflow_config
+    strategy:
+      fail-fast: false
+      matrix:
+        java_version: [ 8, 11, 17 ]
+        os: [ macos-10.15,  macos-11, ubuntu-18.04, ubuntu-20.04, windows-2019, windows-2022 ]
+        include:
+          - release_from_this_build: true
+            os: ubuntu-20.04
+            java_version: 8
+    runs-on: ${{ matrix.os }}
+    env:
+      SIGN_ARTIFACTS: ${{ secrets.ARTIFACT_SIGNING_KEY != '' }}
+    steps:
+      - name: Checkout the repository
+        uses: actions/checkout@v2
+
+      - name: Set up java
+        uses: actions/setup-java@v2.3.1
+        with:
+          java-version: ${{ matrix.java_version }}
+          distribution: temurin
+          cache: gradle
+
+      # Compile / Test / Package are separate steps so the reason for any failure is more obvious in GitHub UI
+      - name: Compile
+        run: gradle -q compileJava
+
+      - name: Test
+        run: gradle -q test
+
+      # The repeated "matrix.release_from_this_build" checks are messy, but I have not found a simple way to avoid them
+      # See https://github.com/actions/runner/issues/662
+
+      - name: Setup gpg
+        if: matrix.release_from_this_build && env.ARTIFACT_SIGNING_KEY
+        id: gpg
+        env:
+          ARTIFACT_SIGNING_KEY: ${{ secrets.ARTIFACT_SIGNING_KEY }}
+        run: |
+          echo "Importing key ..."
+          echo "${ARTIFACT_SIGNING_KEY}" | gpg --batch --import --import-options import-show
+
+          echo "Getting key id ..."
+          key_id="$(echo "${ARTIFACT_SIGNING_KEY}" | gpg --batch --show-keys --with-colons | awk -F: '$1 == "sec" { print $5 }')"
+          echo "::set-output name=key_id::${key_id}"
+
+      - name: Create artifacts
+        if: matrix.release_from_this_build
+        env:
+          GPG_KEYNAME: ${{ steps.gpg.outputs.key_id }}
+          GPG_PASSPHRASE: ${{ secrets.ARTIFACT_SIGNING_PASSPHRASE }}
+        run: |
+          gradle sign "-Pversion=${POM_VERSION}"\
+            "-Psigning.gnupg.keyName=${GPG_KEYNAME}" \
+            "-Psigning.gnupg.passphrase=${GPG_PASSPHRASE}"
+
+      - name: Upload artifacts
+        if: matrix.release_from_this_build
+        uses: actions/upload-artifact@v2
+        with:
+          # Using github.run_number here to reduce confusion when downloading & comparing artifacts from several builds
+          name: ${{ github.run_number }}-artifacts
+          path: |
+            build/libs/*
+            build/publications/maven/*
+
+  release:
+    needs: [ workflow_config, build ]
+    if: needs.workflow_config.outputs.do_release == 'true' || needs.workflow_config.outputs.do_snapshot_release == 'true'
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout the repository
+        uses: actions/checkout@v2
+
+      - name: Download artifacts
+        uses: actions/download-artifact@v2
+        with:
+          name: ${{ github.run_number }}-artifacts
+          path: build
+
+      - name: Create snapshot release
+        if: needs.workflow_config.outputs.do_snapshot_release == 'true'
+        env:
+          GITHUB_TOKEN: ${{ github.token }}
+        run: .github/scripts/release-gradle-snapshot.sh
+
+      - name: Create release in GitHub
+        if: needs.workflow_config.outputs.do_release == 'true'
+        env:
+          GITHUB_TOKEN: ${{ github.token }}
+          POM_VERSION: ${{ needs.workflow_config.outputs.pom_version }}
+          TAG: ${{ github.event.ref }}
+        run: .github/scripts/release-gradle.sh