About Outerstellar
About Outerstellar
Outerstellar is a collection of open-source Kotlin projects for building JVM applications. The focus is on libraries and tools that solve recurring problems in server-side development: content management, internationalisation, theming, and application scaffolding.
All projects are Apache 2.0 or MIT licensed.
What does Outerstellar build?
fragments4k
A Markdown-based content engine for Kotlin. Drop .md files with YAML front matter into a directory and get routing, pagination, RSS feeds, XML sitemaps, and full-text search via Lucene. Framework adapters exist for HTTP4k, Spring Boot, Javalin, Quarkus, and Micronaut.
This website is built with fragments4k — every page you see is a Markdown file rendered by the library.
Outerstellar Framework
Headless, framework-agnostic Kotlin utility modules:
- i18n —
ResourceBundle-backed translation service with parameter substitution, locale fallback, and hot reload. Includes a Maven plugin and CLI that validate translations at build time (missing keys, unused keys, parameter mismatches). - Theming — JSON-based color definitions that produce CSS custom properties or raw values for programmatic use. Supports shade generation and hot reload.
- Plugin engine —
ServiceLoader-based plugin discovery with lifecycle hooks and caching.
Outerstellar Platform
A multi-module Kotlin project template with HTTP4k + JTE web frontend, jOOQ/JDBI persistence, OpenTelemetry tracing, Micrometer metrics, RBAC security, and a Swing desktop client. Intended as a starting point for new applications.
Runary
A self-hosted book, audiobook, and comic manager with built-in readers for EPUB, PDF, CBZ/CBR, FB2, DjVu, and audiobooks. Supports metadata fetching from OpenLibrary, Google Books, Hardcover, ComicVine, and Audible. Available as a Docker image, fat JAR, or native binary.
Maven Cleaner
A tool to clean up your local Maven repository and Gradle caches. Scans for old artifact versions, checks Maven Central for upstream availability, and protects local-only builds. Available as a GUI (Swing) or CLI.
AI Usage Tracker
A Windows dashboard and tray utility to monitor AI API usage, costs, and quotas across multiple providers from a single compact window. Supports Anthropic Claude, GitHub Copilot, Mistral, DeepSeek, and more.
Needlecast
A Swing-based project launcher for developers. Organizes projects into groups, auto-detects build tools across 14 language ecosystems (Maven, Gradle, npm, Cargo, Go, .NET, and more), runs commands, opens terminals, and tracks git status from one window.
sparkle4j
An in-app update library for Java and Kotlin desktop applications. Handles background update checking, Ed25519 signature verification, and silent installation across Windows, macOS, and Linux.
What technology is used?
The projects are built in Kotlin on the JVM (JDK 21+), using:
- HTTP4k — lightweight, functional HTTP toolkit
- JTE — type-safe server-side templates
- Koin — dependency injection
- Hoplite — type-safe configuration from YAML, environment variables, and system properties
- Tailwind CSS — utility-first CSS framework
- Lucene — full-text search (used inside fragments4k)
- PostgreSQL — production database (H2 for development)
Where is the source code?
All code is on GitHub. Issues, pull requests, and contributions are welcome.
Community channels: Discord · X / Twitter · Substack · Reddit