OSGi

OSGi is a Java technology that provides a dynamic module system and service platform. It enables developers to break applications into bundles, which are Java ARchives with extra metadata defined in their manifest. Bundles declare dependencies through headers such as Import-Package and Export-Package and may require other bundles. The OSGi framework runs these bundles, resolves their dependencies, and offers a service registry where bundles can publish and consume services. Services are plain Java objects registered under a service interface and discovered by other bundles at runtime.

The alliance behind OSGi, originally the Open Services Gateway Initiative, standardizes the core framework and service layer. Since its inception in the late 1990s, OSGi has evolved through multiple releases, providing a stable API and a modular, versioned approach to Java applications. The specifications cover the runtime, lifecycle, service registry, and package import/export semantics, along with security models and lifecycle management.

Architecturally, an OSGi environment consists of the framework, a set of bundles, and the services they expose. Each bundle has its own class loader, enabling strong isolation and conflict-free dependencies. The framework manages bundle installation, activation, deactivation, updating, and uninstallation, and it supports dynamic changes at runtime without restarting the application. The lifecycle states commonly include INSTALLED, RESOLVED, STARTING, ACTIVE, STOPPING, and UNINSTALLED.

OSGi has a broad ecosystem of reference implementations and runtimes, such as Equinox, Apache Felix, and Knopflerfish. It underpins many applications and platforms, including Eclipse-based environments, Karaf-based containers, and embedded systems, by providing modularity, dynamic updates, and robust service-oriented composition.