Made it to work under Java 21

.github/workflows/maven.yml

@@ -0,0 +1,32 @@
+name: Java Build
+
+on:
+  push:
+    branches: [ "master" ]
+  pull_request:
+    branches: [ "master" ]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    timeout-minutes: 15
+
+    steps:
+    - name: Checkout code
+      uses: actions/checkout@v4
+
+    - name: Set up JDK 17
+      uses: actions/setup-java@v3
+      with:
+        java-version: '17'
+        distribution: 'temurin'
+        cache: maven
+
+    - name: Build with Maven
+      run: mvn -B package --file pom.xml
+
+    - name: Upload
+      uses: actions/upload-artifact@v4
+      with:
+        name: All JAR packages
+        path: target/*.jar

.gitignore

@@ -1,2 +1,4 @@
 /classes/
 /deploy/
+/target
+/.idea