Firefly for Java by Example

Reactive Microservices with Spring Boot, WebFlux & the Firefly Framework.

The canonical, build-it-yourself guide to the Java Firefly Framework — a metaframework on top of Spring Boot (WebFlux · Project Reactor · R2DBC). You build one real application, a personal-loan origination service (Lumen Lending), from an empty folder to a secured, observable, event-driven, three-tier system — making every concept concrete before the next.

Published by the Firefly Software Foundation under Apache-2.0, in American English and Spain Spanish (es-ES).

What makes this book different

Every code listing in these pages is a verbatim slice of the companion Maven reactor under samples/lumen-lending, which is built and tested in CI. When prose drifts from working code, the build fails. What you read is what actually compiles and passes its tests.

Building the book

The book is a print-grade EPUB + PDF produced by a self-contained Python (WeasyPrint) pipeline.

# one-time
python3 -m venv build/.venv
./build/.venv/bin/pip install -r build/requirements.txt

# build (macOS — uses the DYLD shim for cairo/pango)
./build/run.sh --config book.yaml        # English  -> dist/firefly-java-by-example.{epub,pdf}
./build/run.sh --config book.es.yaml     # Spanish  -> dist/firefly-java-by-example-es.{epub,pdf}

# build (Linux / CI)
./build/build-book.sh --config book.yaml

Use JDK 25 for the companion application and verify that mvn -version reports that JDK. The published framework dependencies used by this sample target Java 25.

Verify the sample reactor, book tooling, and both editions’ listings:

mvn -f samples/lumen-lending/pom.xml verify
./build/.venv/bin/python -m pytest tests -q
./build/.venv/bin/python build/verify_code.py manuscript samples/lumen-lending
./build/.venv/bin/python build/verify_code.py manuscript-es samples/lumen-lending

Releases

Built editions (EN + ES, EPUB + PDF) are attached to each GitHub Release on a CalVer (YY.MM.PATCH) tag. The release workflow prepares a draft with all four files; publish the draft after reviewing the rendered editions.

Scope

This book is about the Java Firefly Framework only. The Python (PyFly), Rust, Go, and .NET ports, the Python agentic metaframework, the agentic-bridge, and the frontend frameworks are parallel projects and are not covered here.

---

Copyright © 2026 Firefly Software Foundation. Licensed under Apache-2.0.