What are Quarkus extensions?

Extensions are Quarkus's mechanism for adding capability — a database driver, a messaging client, security, metrics, and hundreds of other integrations — while keeping both build-time processing and native-image compatibility in mind, unlike a typical library you'd just drop onto the classpath in a traditional framework.

Each extension has two parts: a runtime component (the actual classes used while the application runs) and a deployment/build-time component (code that runs during the build to do things like scan for annotations, generate configuration, or register classes for reflection so the native-image build knows about them).

This split is what lets Quarkus front-load work to build time: rather than an extension scanning the classpath for annotated beans every time the application boots, that scanning happens once during the build, and the result is baked directly into the generated application, so runtime start-up has nothing left to discover.