build: enforce Java formatting with Spotless

.github/workflows/formatting.yml

@@ -0,0 +1,35 @@
+name: Formatting
+
+on:
+  pull_request:
+    branches:
+      - master
+
+permissions:
+  contents: read
+
+concurrency:
+  group: formatting-${{ github.event.pull_request.number }}
+  cancel-in-progress: true
+
+jobs:
+  spotless:
+    name: Spotless check
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    steps:
+      - name: Check out repository
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1
+        with:
+          persist-credentials: false
+
+      - name: Set up Java 21
+        uses: actions/setup-java@de7274f081f381c8f8158605e0321c36c376e2e6
+        with:
+          distribution: temurin
+          java-version: "21"
+          cache: maven
+          cache-dependency-path: pom.xml
+
+      - name: Check formatting
+        run: mvn --batch-mode --no-transfer-progress spotless:check

pom.xml

@@ -14,6 +14,8 @@
     <properties>
         <maven.compiler.release>21</maven.compiler.release>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+        <spotless.plugin.version>3.10.2</spotless.plugin.version>
+        <spotless.eclipse.version>4.40</spotless.eclipse.version>
     </properties>
 
     <dependencies>
@@ -48,6 +50,35 @@
 
     <build>
         <plugins>
+            <plugin>
+                <groupId>com.diffplug.spotless</groupId>
+                <artifactId>spotless-maven-plugin</artifactId>
+                <version>${spotless.plugin.version}</version>
+                <configuration>
+                    <java>
+                        <eclipse>
+                            <version>${spotless.eclipse.version}</version>
+                            <file>${project.basedir}/config/eclipse-java-formatter.xml</file>
+                        </eclipse>
+                        <removeUnusedImports/>
+                        <importOrder>
+                            <order>module |,jakarta|javax|java,\#</order>
+                        </importOrder>
+                        <forbidWildcardImports/>
+                        <trimTrailingWhitespace/>
+                        <endWithNewline/>
+                    </java>
+                </configuration>
+                <executions>
+                    <execution>
+                        <id>spotless-check</id>
+                        <phase>validate</phase>
+                        <goals>
+                            <goal>check</goal>
+                        </goals>
+                    </execution>
+                </executions>
+            </plugin>
             <plugin>
                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-jar-plugin</artifactId>