OSGi Declarative Services is way to share services
between the components. OSGi Declarative Service (DS) is shortly we can call it
as DS. We can say DS is framework, which provides implementation that makes components
can share services each other.
According OSGi, application is composed of bundles and
each bundles have set of components. Components consist of services and these services
performs some tasks.
Most of the times one
component is depends on other component or we can say one component uses the
other component services.
DS uses the concept Publish-Find-Bind. DS will use Service
Component Runtime (SCR) to publish and
bind the services.
Register
/ Publish
The component services are
published/registered with SCR. Service Component Runtime will manage the
registered services and it makes available to other components.
Listen
/ Find
Once the component services
published then it will available to other components. Other components will
find the published services with help of SCR. Sometime services may be unregistered
then components will be notified or listen the services unregistered
information.
Get
/ Bind
Once the Services found
then it will bind to the consumer bundles and it will use the services
implementation.
All this concept work through
Service Component Runtime (SCR)
implementation. OSGi implantation provides the SCR implementation. SCR is part
of OSGi environment.
The following is Picture
of Publish-Find-Bind concept.
Bundle
A
registered the service implantation with Service Run Time (SCR) and consumer Bundle B will get the Service Implementation
will use it. SCR it is responsible
to manage the component services registration and reference to other components.
Technically, when we
implements the components using Declarative Services we can find the following bundles.
Service
API Bundle
Service
Provider Bundle
Service
Consumer Bundle
Service
API Bundle
Service API Bundle we
will define the set of interfaces and its methods. We will export bundle in OSGi
way so that it will be available to other bundles.
Service
Provider Bundle
Service Provider Bundle provides
the service implementation for Service API. Here actual implementation
developed as Java classes. We will register this service implementation as components
using Declarative Services framework.
Component is java class,
which provide the implementation for the services, which we declared in the
Service API bundles.
DS framework will provide
xml configuration file or some Annotations such a way we can make
implementation class as Service Component.
Service Component is
simple java class have services implementation and we make service component by
configuring the information in the xml file that followed by the Declarative
Service Framework. We can say its service provider or publisher as per the
concept.
Service
Consumer Bundle
Service Consumer Bundle
will consume the services, which registered in the Service Component Run time. As
per the DS concept bundle will find the component services and bind to the
consumer bundle such a way it will use the implementation provided by the publisher
bundle.
OSGi
in Liferay
Liferay have used
Declarative Services in the portlet development. Liferay applications
development based on OSGi. Most of the Liferay Application Development have
used DS framework.
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