[CI] Add Java Format CI check for pull requests

.github/workflows/javaCodestyle.yml

@@ -21,14 +21,23 @@
 
 name: Java Codestyle
 
+# Two Java style gates share this workflow:
+#   * Checkstyle    -- whole-tree rule check (dev/checkstyle), on push and PR.
+#   * Java Format   -- Eclipse formatter (dev/CodeStyle_eclipse.xml) applied to
+#                      ONLY the lines a PR edits; fails if any edited line would
+#                      change. The tree is not yet fully formatter-clean, so
+#                      scoping to edited lines keeps it actionable and lets the
+#                      codebase converge line-by-line. This gate needs the PR
+#                      base commit, so it runs on pull_request only.
+# They are separate jobs so each reports its own pass/fail status.
+
 on:
   push:
     paths-ignore:
       - 'docs/**'
       - '*.md'
       - '*.html'
       - 'src/main/python/**'
-      - 'dev/**'
     branches:
       - main
   pull_request:
@@ -37,16 +46,18 @@ on:
       - '*.md'
       - '*.html'
       - 'src/main/python/**'
-      - 'dev/**'
     branches:
       - main
 
+permissions:
+  contents: read
+
 concurrency:
   group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
   cancel-in-progress: ${{ github.event_name == 'pull_request' }}
 
 jobs:
-  java_codestyle:
+  java_checkstyle:
     name: Java Checkstyle
     runs-on: ubuntu-latest
     steps:
@@ -62,3 +73,47 @@ jobs:
 
     - name: Run Checkstyle
       run: mvn -ntp -B -Dcheckstyle.skip=false checkstyle:check
+
+  java_format:
+    name: Java Format Check
+    # line-scoped to the PR diff -> needs the pull_request base commit
+    if: github.event_name == 'pull_request'
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout Repository
+      uses: actions/checkout@v7
+      with:
+        fetch-depth: 0
+
+    - name: Setup Java 17 adopt
+      uses: actions/setup-java@v5
+      with:
+        distribution: adopt
+        java-version: '17'
+        cache: 'maven'
+
+    - name: Setup Python
+      uses: actions/setup-python@v5
+      with:
+        python-version: '3.11'
+
+    - name: Unit-test the format checker
+      run: |
+        python -m pip install --quiet pytest
+        python -m pytest dev/tests -q
+
+    - name: Check formatting of PR-edited lines
+      env:
+        BASE_SHA: ${{ github.event.pull_request.base.sha }}
+      run: |
+        # Fails only if the Eclipse formatter would change a line this PR edited.
+        # See dev/format_changed.py for the line-scoping logic.
+        if ! python3 dev/format_changed.py --check "$BASE_SHA"; then
+          echo "::error::Some lines edited by this PR are not formatted per dev/CodeStyle_eclipse.xml."
+          echo "Fix only your edited lines locally and commit the result:"
+          echo ""
+          echo "  dev/format-changed.sh"
+          echo ""
+          echo "(Do NOT run a bare 'mvn formatter:format' -- it reformats the whole tree.)"
+          exit 1
+        fi

dev/format-changed.sh

@@ -0,0 +1,48 @@
+#!/usr/bin/env bash
+#-------------------------------------------------------------
+#
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you 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
+#
+#   http://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.
+#
+#-------------------------------------------------------------
+#
+# Apply the Eclipse formatter (dev/CodeStyle_eclipse.xml) to ONLY the lines you
+# edited -- the exact scope the "Java Format" CI check enforces.
+#
+# A bare `mvn formatter:format` would reformat EVERY .java file under the source
+# roots, and even scoping to changed *files* would reformat their pre-existing
+# (not-yet-clean) lines. The existing tree is not fully formatter-clean, so both
+# would produce a large unrelated diff. This delegates to dev/format_changed.py,
+# which formats each changed file but keeps only the changes that land on the
+# lines you actually edited.
+#
+# "Changed" = lines that differ from the base branch (the merge target),
+# including your committed-on-branch, staged, unstaged and untracked edits. Run
+# `git fetch upstream main` first so the diff is accurate, or pass an explicit,
+# current base ref if the default is stale/behind your branch point. The base-ref
+# fallback order lives in one place: see resolve_base() in dev/format_changed.py.
+#
+# Usage:
+#   dev/format-changed.sh [base-ref]
+#     base-ref  branch/commit to diff against (default: resolved by
+#               dev/format_changed.py)
+#
+set -euo pipefail
+
+cd "$(git rev-parse --show-toplevel)"
+
+exec python3 dev/format_changed.py --fix "$@"

dev/format-exclude.txt

@@ -0,0 +1,35 @@
+#-------------------------------------------------------------
+#
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you 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
+#
+#   http://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.
+#
+#-------------------------------------------------------------
+
+# Files exempt from the Java Format check (dev/format_changed.py).
+#
+# One pattern per line; blank lines and lines starting with '#' are ignored.
+# Each pattern is matched (glob / fnmatch) against the repo-relative path AND
+# against the bare file name, so both of these work:
+#
+#   src/main/java/org/apache/sysds/conf/DMLConfig.java
+#   *DMLConfig.java
+#
+# Use this only for files whose non-standard layout is intentional and must not
+# be reformatted (e.g. hand-aligned tables of constants).
+
+# Hand-aligned configuration constants; intentional non-standard formatting.
+src/main/java/org/apache/sysds/conf/DMLConfig.java