Upgrade to Java 25 and Spring Boot 3.5; modernize code

[devin-ai-integration]

Summary

Upgrades the project from Java 8 / Spring Boot 2.0.2 (2018) to Java 25 (LTS) / Spring Boot 3.5.0, and batches in a set of correctness, modernization, and hygiene improvements.

Build & dependencies

• pom.xml: parent spring-boot-starter-parent 2.0.2.RELEASE → 3.5.0; <java.version> 1.8 → 25; maven.compiler.release=25.
• Drop obsolete spring-boot-properties-migrator (it was a 1.x→2.x aid).
• Add spring-boot-starter-actuator, spring-boot-starter-test, springdoc-openapi-starter-webmvc-ui (Swagger UI at /swagger-ui.html).
• Remove the duplicate Gradle build (build.gradle, gradle/, gradlew, gradlew.bat) — Maven is now the single build tool.
• Project coordinates changed: org.springframework:gs-spring-boot:0.1.0 (pom) → hello:springboot-java-modern:1.0.0 (jar).

Correctness / behavior

• Remove the hard-coded call to http://gturnquist-quoters.cfapps.io/api/random in Application.main and the old CommandLineRunner (PWS was retired in 2023; this would fail on startup).
• TopicService.getTopicWithId now returns Optional<Topic>; controller throws ResponseStatusException(NOT_FOUND) → proper 404 instead of a 500 NoSuchElementException.
• Guard the H2-seeding CommandLineRunner with @ConditionalOnBean(JdbcTemplate.class) so slice tests don't need a DataSource.
• Replace the mutable ArrayList of topics with CopyOnWriteArrayList for thread-safety; getAllTopics() returns an immutable copy.

Modernization

• DTOs → record: Topic, Greeting, Quote, Value, Customer.
• Controllers: @RequestMapping → @GetMapping/@PostMapping/@PutMapping/@DeleteMapping; @RequestMapping("/topic") hoisted to class level; constructor injection replaces @Autowired fields; @ResponseStatus on create/delete.
• TopicService uses List.of, method references, Pattern#asMatchPredicate, Stream#toList.
• HelloController uses a text block + String#formatted.
• Adjusted demo file-ops endpoint to operate on pom* / README.md now that grad* and temp.txt no longer exist.

Tests & CI

• Added TopicServiceTest (11 unit tests), TopicControllerTest (5 @WebMvcTest MockMvc tests), ApplicationTests context-load smoke test. Local mvn verify: 17/17 passing.
• New .github/workflows/ci.yml running ./mvnw -B -ntp verify on Temurin JDK 25.

Hygiene

• Expand .gitignore (target/, build/, .idea/, *.iml, .vscode/, …).
• Untrack target/, .idea/, gs-spring-boot.iml, stray .gitignore.txt, root-level application.properties, temp.txt.
• README refreshed (prereqs, ./mvnw commands, Swagger/Actuator URLs).

Review & Testing Checklist for Human

• Wire format parity for Topic — the model is now a record, so Jackson serializes id/subjectName/subjectDescription via record accessors. POST a topic and GET it back; the JSON field names should match the pre-PR shape (I did not find any consumers that relied on get* setters, but please double-check).
• Spring Boot 3 namespace / behavior changes — no explicit javax.* usages were found in this codebase, but please skim the runtime logs after ./mvnw spring-boot:run for any deprecation/config-migration warnings.
• Behavior of demo endpoints that read files — /topic/file/operation now searches for pom* instead of grad* and reads README.md instead of the deleted temp.txt. Confirm that's acceptable (these are demo endpoints, not user-facing).
• Artifact coordinates change — groupId/artifactId/version went from org.springframework:gs-spring-boot:0.1.0 to hello:springboot-java-modern:1.0.0. Confirm nothing external consumes the published artifact under the old coords.
• CI run on JDK 25 — Temurin 25 is selected via actions/setup-java; verify the runner has it available (this is new enough that it's worth a sanity check on the first green run).

Notes

• Spring Boot 3.4 was tried first but its bundled ASM does not support class-file major version 69 (Java 25); bumped to 3.5.0 which does.
• Out of scope (flagged as possible follow-ups): replace the in-memory topics list with JPA + H2 (the deps are already pulled in), containerize with a Dockerfile/buildpacks, Micrometer/Prometheus observability.

Link to Devin session: https://app.devin.ai/sessions/36c4f507879e4903a73800cd780af6bd
Requested by: @bnob-git

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment and CI monitoring