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{{Ecl_menu}}<h2>OSGi Services</h2>
Service Layer
The Service Layer provides a dynamic, concise and consistent programming
model for Java bundle developers, simplifying the development and deployment of service bundles by de-coupling the service’s specification (Java
interface) from its implementations. This model allows bundle developers
to bind to services only using their interface specifications. The selection of
a specific implementation, optimized for a specific need or from a specific
vendor, can thus be deferred to run-time (see [http://www.osgi.org/Release4/HomePage OSGi Service Platform]).
:A bundles can register and use services in OSGi. OSGi provides therefore a central registry for this purpose. A service is defined by a Java interface (POJI - Plain Old Java Interface) [http://www.vogella.de/articles/OSGi/article.html#OSGiintro_services].
}
</source>
An implementation of this interface could be:
<source lang="java">
package cs.ecl.osgi.simple.declarativeservice.say.internals;
import java.util.Date;
import cs.ecl.osgi.simple.declarativeservice.say.Sayable;
public class TodaySay implements Sayable {
public String say() {
return " Declarative Service: Today is " + new Date();
}
}
</source>
The relationships between the interface that is exposed to the client and the implementation that is hidden, must be defined in a xml file:
<source lang="xml">
<?xml version="1.0"?>
<component name="sayable">
<implementation class="cs.ecl.osgi.simple.declarativeservice.say.internals.TodaySay"/>
<service>
<provide interface="cs.ecl.osgi.simple.declarativeservice.say.Sayable"/>
</service>
</component>
</source>
Thus, the bundle that provides the service could register its services to an OSGi registry, while the bundle that needs the service could query the registry. The model is Service Oriented Architecture (SOA) inside the JVM (Java Virtual Machine).
[[Image:OSGi-DS.png|thumb|300px| ]]
The bundle consumer must implement the CommandProvider interface
<source lang="java">
package cs.ecl.osgi.simple.declarativeservice.consumer;
import org.eclipse.osgi.framework.console.CommandInterpreter;
import org.eclipse.osgi.framework.console.CommandProvider;
import cs.ecl.osgi.simple.declarativeservice.say.Sayable;
public class SaySingleConsumer implements CommandProvider {
private Sayable s;
public synchronized void bindSayable(Sayable s) {
this.s = s;
}
public synchronized void unbindSayable(Sayable s) {
this.s = null;
}
public synchronized void _run(CommandInterpreter ci) {
if (s != null) {
System.out.println(s.say());
} else {
ci.println("Error, No Service of type Sayable available");
}
}
@Override
public String getHelp() {
return "\n\t run - EXECUTE the Sayable service";
}
}
</source>
