The Open Services Gateway Initiative (OSGi) defines an architecture for developing and deploying modular applications and libraries. In this first article in a three-part introduction to OSGi, Sunil Patil gets you started with OSGi development concepts and shows you how to build a simple Hello World application using the Eclipse OSGi container implementation, Equinox. He also touches briefly on building service-oriented applications using OSGi and introduces OSGi'sServiceFactoryandServiceTrackerclasses.
The Open Services Gateway Initiative (OSGi), also known as the Dynamic Module System for Java, defines an architecture for modular application development. OSGi container implementations such as Knopflerfish, Equinox, and Apache Felix allow you to break your application into multiple modules and thus more easily manage cross-dependencies between them.
Similar to the Java Servlet and EJB specifications, the OSGi specification defines two things: a set of services that an OSGi container must implement and a contract between the container and your application. Developing on the OSGi platform means first building your application using OSGi APIs, then deploying it in an OSGi container. From a developer's perspective, OSGi offers the following advantages:
- You can install, uninstall, start, and stop different modules of your application dynamically without restarting the container.
- Your application can have more than one version of a particular module running at the same time.
- OSGi provides very good infrastructure for developing service-oriented applications, as well as embedded, mobile, and rich internet apps.
Given that you use servlet containers for building Web applications and EJB containers for building transactional applications, you may be wondering why you need yet another type of container. The short answer is that OSGi containers are intended specifically for developing complex Java applications that you want to break up into modules. I'll expand on that short answer throughout this series.
OSGi in enterprise applications
Work on the OSGi specification was started by the OSGi Alliance in March 1999. Its main goal was to create an open specification for delivering managed services to local networks and devices. The basic idea is that once you add an OSGi Service Platform to a networked device (embedded as well as servers), you should be able to manage the lifecycle of software components in that device from anywhere in the network. Software components can be installed, updated, or removed on the fly without ever having to disrupt the operation of the device.
For years, OSGi technology has flourished in the embedded systems and network devices market. Now, thanks in part to Eclipse, OSGi is emerging as a viable and valuable technology for enterprise development.
Growing support for OSGi
In 2003, the Eclipse development team began looking for ways to make Eclipse a more dynamic rich client platform and increase the toolset's modularity. Eventually, the team settled on using the OSGi framework as a runtime component model. Eclipse 3.0, released in June of 2004, was the first version of Eclipse based on OSGi.
Almost all enterprise application servers support or plan to support OSGi. The Spring framework also supports OSGi, via the Spring Dynamic Modules for OSGi Service Platforms project, which provides an infrastructure layer to make it easier to use OSGi in Spring-based Java enterprise application development.
Open source OSGi containers
From an enterprise developer's point of view, the OSGi container has such a low footprint that you can easily embed it into an enterprise application. For example, let's say you're developing a complex Web application. You want to break the application into multiple modules: one module for the view layer, another for the DAO layer, and a third module for the data access layer. Using an embedded OSGi container to manage the cross-dependencies of these modules would enable you to update your DAO layer (say from slow DAO to fast DAO) without restarting your application.
As long as your application is compliant with the OSGi specification it should be able to run in any OSGi-compliant container. Currently, there are three popular open source OSGi containers:
- Equinox is the reference implementation for the framework portion of the OSGi Service Platform Release 4. It is the modular Java runtime at the heart of the Eclipse IDE, and implements all of the mandatory and most of the optional features of the OSGi R4 specification.
- Knopflerfish is an open source implementation of the OSGi R3 and OSGi R4 specifications. Knopflerfish 2 implements all the mandatory features and some of the optional features defined in the R4 specification.
- Apache Felix is the open source OSGi container from the Apache Software Foundation. At the time of writing this container is not fully compliant with the OSGI R4 specification.
In this article we will use Equinox as our OSGi container. See the Resources section for more information about Apache Felix and Knopflerfish.