Figure 1. OSGi overview
This section gives a very brief presentation of OSGi. More
information can be obtained from the OSGi specification.
The Open Services Gateway Initiative OSGi
is an independent, non-profit corporation working to define and promote
open specifications for the delivery of managed services to networked
environments, such as homes and automobiles. These specifications
define
the OSGi Service Platform, which consists of two pieces: the OSGi
framework and a set of standard service definitions. The OSGi
framework,
which sits on top of a Java virtual machine, is the execution
environment for services. The OSGi framework was originally conceived
to
be used inside restricted environments, such as the set-top box in
figure 1. OSGi can however be used in other domains, as for example, an
infrastructure to support underlying release 3.0 of the eclipse IDE.
The framework can be divided in two main elements:
A services platform is defined as a software platform that supports the service orientation interaction depicted in figure 2. This interaction involves three main actors: service providers, service requesters and a service registry, although only the service registry belongs to the services platform. In the service orientation interaction, service providers publish service descriptions, and service requesters discover services and bind to the service providers. Publication and discovery are based on a service description.
In the context of OSGi, a service is described as a Java class or
interface, the service interface,
along with a
variable number of attributes, the service
properties, that are name and value pairs. Service properties
allow different service providers that provide services with the same
service interface to be
differentiated. The
service registry allows service providers to be discovered through
queries formulated in an LDAP syntax. Additionnaly, notification
mechanisms allow service requesters to receive events signalling
changes
in the service registry; these changes include the publication or
retrieval of a particular service.
In OSGi, service providers and requesters are part of an entity
called a bundle that is both a
logical as well as physical entity. Service interfaces are implemented
by objects created by the bundle. In standard OSGi, the bundle is
responsible for run-time service dependency management activities which
include publication, discovery and binding as well as adapting to
changes resulting from dynamic availability (arrival or departure) of
services that are bound to the bundle.
Deployment activities are realized according to a well defined
series of states depicted in Figure 3; these states correspond to the
physical bundle life-cycle. The activation or de-activation of a
physical bundle
results in the creation or destruction of a unique logical
bundle, materialized by an instance from a class inside the bundle
called a bundle activator.
When the instance is created, the execution environment calls an
activation method that signals the logical
bundle that it is active. When the physical bundle is de-activated, the
execution environment calls a de-activation method. When the logical
bundle is active, it can publish or
discover services and bind with other bundles by accessing the
framework's services registry. It can also be notified from changes
that occur in the framework by subscribing as an event listener.
A better understanding of OSGi can be achieved by following this tutorial.
Next : Dependency management