OSGi Services
- 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) [1].
- Access to the service registry is performed via the class BundleContext. OSGi injects the BundleContext into each bundle during the startup of the bundle. A bundle can also register itself to the BundleContext ServiceEvents which are for example triggered if a new service is installed or de-installed.
For example, let us suppose that one wants to define a service that is capable to define the day and time. For the purpose one defines an interface:
package cs.ecl.osgi.simple.declarativeservice.say;
public interface Sayable {
String say();
}
An implementation of this interface could be:
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();
}
}The relationships between the interface that is exposed to the client and the implementation that is hidden, must be defined in a xml file:
<?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>