Upgrade to Java 21: Dockerfile, Spring Boot 3.4.3, and Maven

[devin-ai-integration]

Upgrade to Java 21: Dockerfile, Spring Boot 3.4.3, and Maven

Summary

Adds a new Dockerfile.java21 and upgrades pom.xml to fully target Java 21. The original Dockerfile remains unchanged for Java 17 fallback.

Dockerfile.java21 (new file):

• Build stage: maven:3.8.3-openjdk-17 → maven:3.9-eclipse-temurin-21
• Runtime stage: openjdk:17-alpine → eclipse-temurin:21-jre-alpine (openjdk images are deprecated past JDK 17)
• Removed deprecated MAINTAINER directive

pom.xml updates:

• Spring Boot 3.3.3 → 3.4.3
• <java.version> 17 → 21
• maven-compiler-plugin 3.8.0 → 3.13.0 with <source>/<target> changed from 1.8 to 21
• MySQL connector migrated from mysql:mysql-connector-java:8.0.33 → com.mysql:mysql-connector-j (version now managed by Spring Boot BOM)

Review & Testing Checklist for Human

• Run mvn clean install: Neither the Maven build nor Docker build was tested. Verify all dependencies resolve and the project compiles successfully against Java 21.
• Build and run the Docker image: Run docker build -f Dockerfile.java21 -t bankapp:java21 . and verify the app starts and connects to MySQL.
• Verify Spring Boot 3.3.3 → 3.4.3 compatibility: Review the Spring Boot 3.4 release notes for any breaking changes affecting this app (Security, JPA, Thymeleaf).
• Confirm mysql-connector-j BOM version: The explicit version pin was removed in favor of Spring Boot's dependency management. Verify the resolved version is compatible with your MySQL server.
• Verify Docker image tags exist on Docker Hub: Confirm maven:3.9-eclipse-temurin-21 and eclipse-temurin:21-jre-alpine are valid published tags.

Suggested test plan: Build with Maven → build Docker image → run with docker-compose (updating image reference to use -f Dockerfile.java21) → verify login, registration, deposit, withdrawal, and transfer flows against MySQL.

Notes

• The existing Dockerfile for Java 17 is untouched.
• Downstream references (Jenkinsfile, docker-compose.yml, K8s manifests) still point at the original Dockerfile; those would need updating separately if this becomes the primary build.
• No application source code was changed — the Java code is already compatible with modern Spring Boot 3.x / Jakarta EE APIs.

Requested by: @matthewguerra-cog
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