Service Event Listener Bundle

This example creates a simple bundle that listens for OSGi service events. This example does not do much at first, because it only prints out the details of registering and unregistering services. In the next example we will create a bundle that implements a service, which will cause this bundle to actually do something. For now, we will just use this example to help us understand the basics of creating a bundle.

A bundle gains access to the OSGi framework using a unique instance of BundleContext. In order for a bundle to get its unique bundle context, it must implement the BundleActivator interface; this interface has two methods, start() and stop(), that both receive the bundle's context and are called when the bundle is started and stopped, respectively. In the following source code, our bundle implements the BundleActivator interface and uses the context to add itself as a listener for service events (the file for our bundle is called Activator.java):

/*
 * Apache Felix OSGi tutorial.
**/

package tutorial.example1;

import org.osgi.framework.BundleActivator;
import org.osgi.framework.BundleContext;
import org.osgi.framework.ServiceListener;
import org.osgi.framework.ServiceEvent;

/**
 * This class implements a simple bundle that utilizes the OSGi
 * framework's event mechanism to listen for service events. Upon
 * receiving a service event, it prints out the event's details.
**/
public class Activator implements BundleActivator, ServiceListener
{
    /**
     * Implements BundleActivator.start(). Prints
     * a message and adds itself to the bundle context as a service
     * listener.
     * @param context the framework context for the bundle.
    **/
    public void start(BundleContext context)
    {
        System.out.println("Starting to listen for service events.");
        context.addServiceListener(this);
    }

    /**
     * Implements BundleActivator.stop(). Prints
     * a message and removes itself from the bundle context as a
     * service listener.
     * @param context the framework context for the bundle.
    **/
    public void stop(BundleContext context)
    {
        context.removeServiceListener(this);
        System.out.println("Stopped listening for service events.");

        // Note: It is not required that we remove the listener here,
        // since the framework will do it automatically anyway.
    }

    /**
     * Implements ServiceListener.serviceChanged().
     * Prints the details of any service event from the framework.
     * @param event the fired service event.
    **/
    public void serviceChanged(ServiceEvent event)
    {
        String[] objectClass = (String[])
            event.getServiceReference().getProperty("objectClass");

        if (event.getType() == ServiceEvent.REGISTERED)
        {
            System.out.println(
                "Ex1: Service of type " + objectClass[0] + " registered.");
        }
        else if (event.getType() == ServiceEvent.UNREGISTERING)
        {
            System.out.println(
                "Ex1: Service of type " + objectClass[0] + " unregistered.");
        }
        else if (event.getType() == ServiceEvent.MODIFIED)
        {
            System.out.println(
                "Ex1: Service of type " + objectClass[0] + " modified.");
        }
    }
}

After implementing the Java source code for the bundle, we must also define a manifest file that contains meta-data needed by the OSGi framework for manipulating the bundle. The manifest is packaged into a JAR file along with the Java class file associated with our bundle; the whole JAR package is actually referred to as a bundle. We create a file called manifest.mf that contains the following:

Bundle-Name: Service listener example
Bundle-Description: A bundle that displays messages at startup and when service events occur
Bundle-Vendor: Apache Felix
Bundle-Version: 1.0.0
Bundle-Activator: tutorial.example1.Activator
Import-Package: org.osgi.framework