5.2. OSGi Framework
The OSGi Alliance is an independent organization responsible for defining the features and capabilities of the OSGi Service Platform Release 4. The OSGi Service Platform is a set of open specifications that simplify building, deploying, and managing complex software applications.
OSGi technology is often referred to as the dynamic module system for Java. OSGi is a framework for Java that uses bundles to modularly deploy Java components and handle dependencies, versioning, classpath control, and class loading. OSGi's lifecycle management allows you to load, start, and stop bundles without shutting down the JVM.
OSGi provides the best runtime platform for Java, a superior class loading architecture, and a registry for services. Bundles can export services, run processes, and have their dependencies managed. Each bundle can have its requirements managed by the OSGi container.
JBoss Fuse uses Apache Felix as its default OSGi implementation. The framework layers form the container where you install bundles. The framework manages the installation and updating of bundles in a dynamic, scalable manner, and manages the dependencies between bundles and services.
As shown in Figure 5.1, “Red Hat JBoss Fuse Architecture”, the OSGi framework contains the following:
- Bundles — Logical modules that make up an application. See Section 5.4, “OSGi Bundles”.
- Service layer — Provides communication among modules and their contained components. This layer is tightly integrated with the lifecycle layer. See Section 5.3, “OSGi Services”.
- Lifecycle layer — Provides access to the underlying OSGi framework. This layer handles the lifecycle of individual bundles so you can manage your application dynamically, including starting and stopping bundles.
- Module layer — Provides an API to manage bundle packaging, dependency resolution, and class loading.
- Execution environment — A configuration of a JVM. This environment uses profiles that define the environment in which bundles can work.
- Security layer — Optional layer based on Java 2 security, with additional constraints and enhancements.
Each layer in the framework depends on the layer beneath it. For example, the lifecycle layer requires the module layer. The module layer can be used without the lifecycle and service layers.