Overview of OSGi Application and GlassFish Server

GlassFish Server is fully-compliant with Java EE 6, so it provides the latest Java EE APIs and frameworks. It is built using OSGi technology, and includes as its OSGi module management subsystem the Apache Felix OSGi framework, which is a fully-compliant implementation of the OSGi Service Platform R4 Version 4.2 specification. GlassFish Server supports deployment of OSGi-based applications using this framework. OSGi applications can make use of core as well as enterprise OSGi features. GlassFish Server makes available many of its Java EE platform services, such as the transaction service, HTTP service, JDBC Service and JMS, as OSGi services. It also enables use of Java EE programming model in OSGi applications, so enterprise Java application developers can continue to leverage their existing skills in OSGi-based applications. See Benefits of Using OSGi in Enterprise Java Applications for more information.

OSGi applications are deployed as one or more OSGi bundles, and the GlassFish Server deployment and administration infrastructure enables you to deploy and manage your OSGi bundles. This chapter classifies OSGi bundles into two categories based on the features they use:

• Plain OSGi Application Bundles – bundles that do not contain any Java EE components. See Developing Plain OSGi Bundles.

• Hybrid Application Bundles – bundles that are an OSGi bundle as wells as a Java EE module. At runtime, such modules have both an OSGi bundle context and a Java EE context. GlassFish Server supports the following hybrid application bundles:

  • Web Application Bundles (or WABs) , see Developing Web Application Bundles.

  • EJB Application Bundles, see Developing EJB Application Bundles.

Benefits of Using OSGi in Enterprise Java Applications

Enterprise applications typically need transactional, secured access to data stores, messaging systems and other such enterprise information systems, and have to cater to a wide variety of clients such as web browsers and desktop applications, and so on. Java EE makes development of such applications easier with a rich set of APIs and frameworks. It also provides a scalable, reliable and easy to administer runtime to host such applications.

The OSGi platform complements these features with modularity. It enables applications to be separated into smaller, reusable modules with a well defined and robust dependency specification. A module explicitly specifies its capabilities and requirements. This explicit dependency specification encourages developers to visualize dependencies among their modules and help them make their modules highly cohesive and less coupled. The OSGi module system is dynamic: it allows modules to be added and removed at runtime. OSGi has very good support for versioning: it supports package versioning as well module versioning. In fact, it allows multiple versions of the same package to coexist in the same runtime, thus allowing greater flexibility to deployers. The service layer of the OSGi platform encourages a more service-oriented approach to build a system. The service-oriented approach and dynamic module system used together allow a system to be more agile during development as well as in production. It makes them better suited to run in an Platform-as-a-Service (PaaS) environment.

With GlassFish Server, you do not have to chose one of the two platforms. A hybrid approach like OSGi enabling your Java EE applications allows new capabilities to applications hitherto unavailable to applications built using just one of the two platforms.