Migrate from Java 11 to Java 21 LTS with dependency updates and Docker optimization

This migration upgrades the project to Java 21 LTS while maintaining backward compatibility with the jDisco library (Java 6). The primary motivation was to resolve deprecated Observable/Observer APIs that were blocking future Java version upgrades.

Key changes:

**API Modernization:**
- Replace deprecated java.util.Observable/Observer with PropertyChangeSupport
- Add ContextChangeListener interface extending PropertyChangeListener
- Refactor DefaultContext, StatusBar, RailwayNetGridCanvas to use new pattern
- Add 6 PropertyChangeTest unit tests (242 tests total, all passing)

**Dependency Updates:**
- Java: 11 → 21 LTS
- JUnit: 5.10.1 → 5.11.4
- AssertJ: 3.24.2 → 3.27.6 (required for Java 21)
- Mockito: 5.7.0 → 5.21.0
- SLF4J: 2.0.9 → 2.0.17
- Logback: 1.4.11 → 1.5.23
- Gradle: 8.5 → 8.12
- Shadow Plugin: 8.1.1 → 8.3.8 (plugin ID changed to com.gradleup.shadow)

**Docker Optimization:**
- Replace Debian Buster + manual Java install with Eclipse Temurin official images
- Use eclipse-temurin:21-jdk for builder stage
- Use eclipse-temurin:21-jre for runtime stage
- Simplify Dockerfile, improve maintainability

**CI/CD Updates:**
- Rename .github/workflows/ant-java11.yml → gradle-java21.yml
- Update JAVA_VERSION environment variable to '21'
- Update build badges and references

**Documentation:**
- Update CLAUDE.md with Java 21 requirements and dependencies
- Update README.md with current build system and Docker setup
- Add JAVA21-MIGRATION-SUMMARY.md with comprehensive migration details

All 242 tests pass with Java 21 (236 existing + 6 new PropertyChange tests).
Docker build and runtime verified successfully.
GUI functionality confirmed working with X11 forwarding.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: bedaHovorka

.github/workflows/ant-java11.yml .github/workflows/gradle-java21.yml.github/workflows/ant-java11.yml renamed to .github/workflows/gradle-java21.yml

@@ -1,4 +1,4 @@
-name: Gradle Build with Java 11
+name: Gradle Build with Java 21
 
 on:
   push:
@@ -16,7 +16,7 @@ concurrency:
   cancel-in-progress: true
 
 env:
-  JAVA_VERSION: '11'
+  JAVA_VERSION: '21'
   MAVEN_OPTS: '-Xmx512m'
   GRADLE_OPTS: '-Xmx512m -XX:MaxMetaspaceSize=512m'
 

.github/workflows/sonarqube.yml

@@ -17,7 +17,7 @@ concurrency:
   cancel-in-progress: true
 
 env:
-  JAVA_VERSION: '11'
+  JAVA_VERSION: '21'
   MAVEN_OPTS: '-Xmx512m'
   GRADLE_OPTS: '-Xmx512m -XX:MaxMetaspaceSize=512m'
 

CLAUDE.md

@@ -6,22 +6,26 @@ This file provides guidance to Claude Code (claude.ai/code) when working with co
 
 Railway Interlocking Simulator - A BSc thesis project (2006/2007) from Brno University of Technology that simulates railway interlocking systems with a graphical editor and discrete event simulation engine.
 
-[![Gradle Build with Java 11](https://github.com/bedavs/interlockSim/actions/workflows/ant-java11.yml/badge.svg)](https://github.com/bedavs/interlockSim/actions/workflows/ant-java11.yml)
+[![Gradle Build with Java 21](https://github.com/bedavs/interlockSim/actions/workflows/gradle-java21.yml/badge.svg)](https://github.com/bedavs/interlockSim/actions/workflows/gradle-java21.yml)
 [![SonarQube Analysis](https://github.com/bedavs/interlockSim/actions/workflows/sonarqube.yml/badge.svg)](https://github.com/bedavs/interlockSim/actions/workflows/sonarqube.yml)
 
 ## Build System
 
-This project uses Gradle with Kotlin DSL for building. Java 11 bytecode compatibility is maintained (can be built with Java 11+). The jDisco library module remains at Java 6 for compatibility and uses Maven.
+This project uses Gradle with Kotlin DSL for building. Java 21 LTS is the minimum required version. The jDisco library module remains at Java 6 for compatibility and uses Maven.
 
-**Migration Note**: Migrated from Apache Ant + Ivy to Gradle in January 2026. Gradle can be launched locally for development and builds. The previous Ant build system has been completely removed.
+**Migration Notes**:
+- Migrated from Apache Ant + Ivy to Gradle in January 2026
+- Migrated from Java 11 to Java 21 LTS in January 2026
+- Refactored deprecated Observable/Observer to PropertyChangeSupport
 
 ### Dependency Management
 
 Dependencies are managed via Gradle:
 - **jDisco 1.2.0** - Discrete event simulation library (from Maven local repository, Java 6)
-- **JUnit 5.10.1** - Testing framework (JUnit Jupiter API and Engine)
-- **AssertJ 3.24.2** - Fluent assertion library for better test readability
-- **Mockito 5.7.0** - Mocking framework
+- **JUnit 5.11.4** - Testing framework (JUnit Jupiter API and Engine)
+- **AssertJ 3.27.6** - Fluent assertion library for better test readability
+- **Mockito 5.21.0** - Mocking framework
+- **SLF4J 2.0.17** + **Logback 1.5.23** - Logging framework
 
 Gradle automatically downloads dependencies during the build. Configuration files:
 - `build.gradle.kts` - Build configuration and dependency declarations
@@ -131,7 +135,7 @@ java -ea -jar build/libs/interlockSim.jar example
 
 **Dockerization: 2025** - Complete containerized build and runtime environment with no host dependencies.
 
-The project includes Docker support for both the Java application and LaTeX thesis compilation. This eliminates the need to install Java 11, Ant, or LaTeX tools on the host machine.
+The project includes Docker support for both the Java application and LaTeX thesis compilation. This eliminates the need to install Java 21, Gradle, or LaTeX tools on the host machine.
 
 ### Prerequisites
 
@@ -191,13 +195,13 @@ docker compose build app
 ### Docker Architecture
 
 **Root Dockerfile (multi-stage build):**
-1. **Builder stage** - Uses Debian Buster with OpenJDK 11, Maven, and Ant
+1. **Builder stage** - Uses Eclipse Temurin 21 JDK
    - Builds jDisco dependency (Maven install, Java 6 compatibility)
-   - Resolves dependencies via Apache Ivy (automatic download)
-   - Compiles Java sources (Java 11 target)
+   - Resolves dependencies via Gradle (automatic download)
+   - Compiles Java sources (Java 21 target)
    - Runs all tests with JUnit 5 (build fails if tests fail)
    - Creates uber JAR with all dependencies
-2. **Runner stage** - Debian Buster with OpenJDK 11 JRE and X11 libraries
+2. **Runner stage** - Eclipse Temurin 21 JRE and X11 libraries
    - Minimal runtime environment
    - X11 forwarding for GUI support
    - No build tools in final image
@@ -350,19 +354,19 @@ Follows `.editorconfig` configuration:
 1. **Do not touch Java code unless explicitly requested** - Do not refactor, optimize, or "improve" code that is working
 2. **Tests must exist before modifications** - Any Java source file being modified MUST be covered by tests first. If tests don't exist, they must be written before making any changes
 3. **Minimal changes only** - Make only the specific changes requested, nothing more
-4. **No unsolicited modernization** - While the project now uses Java 11, do not update Java idioms to modern features, do not add new language features, do not restructure working code
+4. **No unsolicited modernization** - While the project now uses Java 21, do not update Java idioms to modern features, do not add new language features, do not restructure working code
 5. **jDisco preservation** - The jDisco module must remain at Java 6 compatibility and should never be modified
 
 This is a working historical codebase from 2007. Stability and preservation are more important than modernization.
 
 ## Testing
 
-Comprehensive JUnit 5.10.1 test suite with AssertJ assertions located in `src/test/java/cz/vutbr/fit/interlockSim/`. All dependencies are managed via Apache Ivy.
+Comprehensive JUnit 5.11.4 test suite with AssertJ assertions located in `src/test/java/cz/vutbr/fit/interlockSim/`. All dependencies are managed via Gradle.
 
 **Test framework:**
 - JUnit 5 (Jupiter API and Engine)
 - JUnit Platform for test execution
-- AssertJ 3.24.2 for fluent assertions
+- AssertJ 3.27.6 for fluent assertions
 
 **Test organization:**
 - **Unit tests** - Fast tests that run by default with `./gradlew test` (excludes integration tests)
@@ -554,16 +558,16 @@ See `.github/workflows/sonarqube.yml` for automated SonarQube analysis on every
 
 The project uses GitHub Actions for automated build, test, and deployment workflows.
 
-**Workflow:** `.github/workflows/ant-java11.yml`
+**Workflow:** `.github/workflows/gradle-java21.yml`
 
 **Features:**
 - Builds jDisco library with Java 6 compatibility
-- Compiles main project with Java 11
+- Compiles main project with Java 21
 - Runs all tests with JUnit 5
 - Packages application JAR
 - Uploads JAR as artifact (90-day retention)
 - Smoke test execution
-- Dependency caching (Maven and Ivy) for faster builds
+- Dependency caching (Maven and Gradle) for faster builds
 
 **Triggers:**
 - Push to `main`, `develop`, `feature/**`, `fix/**` branches
@@ -572,7 +576,7 @@ The project uses GitHub Actions for automated build, test, and deployment workfl
 
 **Build environment:**
 - Ubuntu latest
-- Java 11 (Temurin distribution)
+- Java 21 (Temurin distribution)
 - 15-minute timeout
 - Concurrency control (cancels outdated builds)
 

Dockerfile

@@ -7,11 +7,12 @@
 #      Railway Interlocking Simulator
 #
 #      Dockerization: 2025
-#      Optimized: 2026-01 (BuildKit cache mounts, layer optimization)
+#      Gradle migration: 2026-01
+#      Java 21 migration: 2026-01 (Eclipse Temurin + Debian Bookworm)
 #
 #      Multi-stage build for interlockSim with GUI support
 #      Dependency management: Gradle with Kotlin DSL
-#      Migrated from Ant/Ivy: 2026
+#      Build system: Java 21 LTS with Eclipse Temurin
 #
 
 # syntax=docker/dockerfile:1.4
@@ -26,20 +27,7 @@ FROM jdisco:latest AS jdisco-builder
 # ============================================
 # Stage 2: Build interlockSim with Gradle
 # ============================================
-FROM debian:buster-slim AS builder
-
-# Debian Buster is archived - update sources
-RUN sed -i 's|http://deb.debian.org|http://archive.debian.org|g' /etc/apt/sources.list && \
-    sed -i 's|http://security.debian.org|http://archive.debian.org|g' /etc/apt/sources.list && \
-    sed -i '/buster-updates/d' /etc/apt/sources.list
-
-# Install Java 11 JDK (Gradle wrapper handles Gradle itself)
-RUN apt-get update && apt-get install -y \
-    openjdk-11-jdk \
-    && rm -rf /var/lib/apt/lists/*
-
-ENV JAVA_HOME=/usr/lib/jvm/java-11-openjdk-amd64
-ENV PATH=$JAVA_HOME/bin:$PATH
+FROM eclipse-temurin:21-jdk AS builder
 
 WORKDIR /build/interlockSim
 
@@ -82,34 +70,21 @@ RUN ls -lh /build/interlockSim/build/libs/interlockSim.jar && \
 # ============================================
 # Stage 3: Runtime with JRE and X11 support
 # ============================================
-FROM debian:buster-slim AS runner
-
-# Debian Buster is archived - update sources to use archive.debian.org
-RUN sed -i 's|http://deb.debian.org|http://archive.debian.org|g' /etc/apt/sources.list && \
-    sed -i 's|http://security.debian.org|http://archive.debian.org|g' /etc/apt/sources.list && \
-    sed -i '/buster-updates/d' /etc/apt/sources.list
+FROM eclipse-temurin:21-jre AS runner
 
-# Install OpenJDK 11 JRE and X11 libraries for GUI support
+# Install X11 libraries for GUI support
+# Eclipse Temurin already includes Java 21 JRE
 RUN apt-get update && apt-get install -y \
-    openjdk-11-jre \
     libxext6 \
     libxrender1 \
     libxtst6 \
     libxi6 \
     libxrandr2 \
     libxcursor1 \
     libxinerama1 \
-    libxfixes3 \
-    libxdamage1 \
-    libxcomposite1 \
-    libfreetype6 \
-    libfontconfig1 \
+    fontconfig \
     && rm -rf /var/lib/apt/lists/*
 
-# Set Java 11 JRE as default
-ENV JAVA_HOME=/usr/lib/jvm/java-11-openjdk-amd64
-ENV PATH=$JAVA_HOME/bin:$PATH
-
 WORKDIR /app
 
 # Copy compiled uber JAR from builder stage (Gradle output)