HatJBoss

HatJBoss is an open-source framework that extends the JBoss Application Server with a modular plugin system. It aims to enable dynamic feature composition for Java EE and Jakarta EE applications without modifying the core server. The project introduces a lightweight runtime that runs inside the JBoss process and loads units called Hat modules through a dedicated module loader. Each Hat module provides a self-contained capability such as security, auditing, observability, or rate limiting, and can be enabled or disabled per deployment.

HatJBoss originated from a community effort in the 2022–2023 period to explore modular server architectures for cloud-native Java applications. The first public release appeared in 2023, with major stable releases following in 2024. The project is hosted on an open-source repository and is distributed under a permissive license. Documentation covers installation steps, module development guidelines, and a compatibility matrix with various WildFly/JBoss versions.

Architecturally, HatJBoss relies on a pluggable classloader, a compact configuration DSL (hat.yaml) or JSON, and an administrative CLI for managing modules. Hat modules can declare dependencies, provide lifecycle hooks, and report metrics via standard monitoring interfaces such as JMX or Micrometer. The framework is designed to minimize changes to application code while offering a clear pathway to feature toggling and localized capability upgrades.

Common use cases include rapid feature composition for enterprise applications, cloud deployments on Kubernetes, and enhanced observability and security without altering business logic. Community reception has been mixed, with praise for modularity and extensibility and concerns about added complexity and potential versioning conflicts.