build: migrate Java 17 to Java 21

[devin-ai-integration]

Summary

Migrates the build and runtime from Java 17 to Java 21. Spring Boot spring-boot-starter-parent 3.3.3 already supports Java 21, so no Spring Boot bump is needed.

pom.xml

• <java.version>17</java.version> → 21.
• Removed the explicit maven-compiler-plugin block, which was incorrectly pinning <source>1.8</source>/<target>1.8</target>. The parent POM now manages the compiler plugin and drives --release 21 from the java.version property.

Dockerfile

• Builder stage: maven:3.8.3-openjdk-17 → maven:3.9-eclipse-temurin-21.
• Deployer stage: openjdk:17-alpine → eclipse-temurin:21-jre-alpine (the openjdk Docker Hub repo is deprecated with no 21 tag).

Jenkinsfile and GitOps/Jenkinsfile build via Docker and don't pin a JDK tool, so they need no changes.

Verification

• mvn clean install compiles with --release 21; emitted bytecode is major version 65 (Java 21).
• docker build . succeeds with both new base images.

Link to Devin session: https://app.devin.ai/sessions/91d43ce48dbb41598d9ed0d3727df844

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[devin-ai-integration]

Runtime test — Java 21 migration ✅

Ran the app locally via mvn spring-boot:run under Java 21.0.11 against MySQL 8.0 and exercised the core banking flow end-to-end. All checks passed.

E2E flow (register → login → deposit → transactions)

Step	Expected	Result
Register devintest21	redirect to /login, no error	✅ passed
Login	redirect to /dashboard, balance $0.00	✅ passed
Deposit $100	balance becomes exactly $100.00	✅ passed
Transactions	row: Deposit 100.00 with timestamp	✅ passed

Dashboard after deposit	Transaction history

Build / runtime evidence

• Boot log: Starting BankappApplication using Java 21.0.11
• mvn clean install compiled with --release 21; bytecode major version 65 (Java 21)
• docker build . succeeded with maven:3.9-eclipse-temurin-21 and eclipse-temurin:21-jre-alpine
• CI: Snyk security + license checks passed

Devin session: https://app.devin.ai/sessions/91d43ce48dbb41598d9ed0d3727df844