A service in OSGi is defined by a standard Java class or interface. Typically a Java interface is used to define the service interface.

A bundle can register and use OSGi services. OSGi provides the central OSGi service registry for this purpose.

A service can be dynamically started and stopped, and bundles which use services must be able to handle this dynamic. The bundles can register listeners to be informed, if a service is started or stopped.

It is common practice to define a service via a bundle which only contains the interface definition. Another bundle would provide the implementation for this service. This allows you to change the implementation of the service via a different bundle.

If several service are available and valid then the OSGi runtime will by default select the service with the lowest bundle ID. You can also set a service ranking for your services via a property. OSGi assigns by default a value of zero as the service ranking and selects the service with the highest ranking.

The Constants class from the org.osgi.framework package defines via the Constants.SERVICE_RANKING constant the service.ranking constant. This constant can be used to set the integer property of the service ranking.

In the registerService() method from the BundleContext class you can specify arbitrary properties in the Dictionary.

You can use the getProperty() method of the org.osgi.framework.ServiceReference class, to access a specific property.

The OSGi declarative services (DS) functionality allows you to define and consume services via metadata (XML). Via DS you can define OSGi services without extending or implementing OSGi classes. This allows for these services to be tested independently of the OSGi runtime.

The OSGi service component is responsible for starting the service (service component). For the service consumer it is not visible if the service has been created via declarative service or via other means.

Service Components consists of an XML description (Component Description) and an object (Component Instance). The component description contains all information about the service component, e.g. the class name of the component instance and the service interface.

A reference to the component description file is entered in the MANIFEST.MF file. This file is read by the OSGi runtime. If a component description is found, the corresponding service is created. The component description in the MANIFEST.MF file looks for example like the following:

Service-Component: OSGI-INF/component.xml 

To run declarative services you require the following bundles.

Typically component definitions are created in the project OSGI-INF folder via New → Other... → Plug-in Development → Component Definition. The wizard will also add the Service-Component entry to the MANIFEST.MF file.

On the next page of the wizard, you can maintain the name of the component definition file, a name and the class which implements the services (or consumes the services).

If you press finish, the service editor opens.

On the Services tab you can enter the provided services or the referred services. For example to provide a service you would press the Add button under Provided Services and select the service interface you want to implement.

As a final step you would implement the class which would provide the service.

A correctly maintained component.xml XML file would look like the following.

<?xml version="1.0" encoding="UTF-8"?>
<scr:component xmlns:scr="http://www.osgi.org/xmlns/scr/v1.1.0" 
  name="de.vogella.osgi.ds.quoteservice">
<implementation class="de.vogella.osgi.ds.quoteservice.QuoteService"/>
   <service>
      <provide interface="de.vogella.osgi.quote.IQuoteService"/>
   </service>
</scr:component> 

This means that there is a component called de.vogella.osgi.ds.quoteservice which provides a service to the OSGi Service Registry under the IQuoteService interface. This component is implemented by the QuoteService class.

After the definition of the component your MANIFEST.MF file would contain an entry to the service component.

Manifest-Version: 1.0
Bundle-ManifestVersion: 2
Bundle-Name: Quoteservice
Bundle-SymbolicName: de.vogella.osgi.ds.quoteservice
Bundle-Version: 1.0.0.qualifier
Bundle-RequiredExecutionEnvironment: JavaSE-1.6
Import-Package: de.vogella.osgi.quote
Service-Component: OSGI-INF/component.xml 

The org.eclipse.core.runtime bundle defined the OSGi runtime.

The org.eclipse.equinox.ds bundle is the bundle which is responsible for reading the component meta data and registering the services based on the component definition files.

Therefore these two bundle needs to start before the service becomes available.

You can ensure this in your run configuration by setting .ds bundle to auto start and using a start level lower than 4, which is the default.

This requires that the service plug-in has the Activate this plug-in when one of its classes is loaded flag set in the MANIFEST.MF file. Via this flag it is ensured that the service is avaiable after the org.eclipse.equinox.ds has started.

In defining and using OSGi you should prefer higher level abstractions as OSGi declarative services as these simplify the handling of OSGi services. This chapter describes the API to work directly with OSGi services. It is included in this tutorial as reference.

Access to the service registry is performed via the BundleContext class.

A bundle can define a Bundle-Activator (Activator) in its declaration. This class must extend the BundleActivator interface.

If defined, OSGi injects the BundleContext into the start() and stop() methods of the implementing Activator class.

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


public class Activator implements BundleActivator {

  public void start(BundleContext context) throws Exception {
    System.out.println("Starting bundle");
    // Do something with the context, e.g. 
    // register services
  }

  public void stop(BundleContext context) throws Exception {
    System.out.println("Stopping bundle");
    serviceTracker.close();
    // Do something with the context, e.g. 
    // deregister service
  }

} 

If you do not have an Activator, you can use the FrameworkUtil class from the OSGi framework which allows you to retrieve the BundleContext for a class.

BundleContext bundleContext = 
        FrameworkUtil.
        getBundle(this.getClass()).
        getBundleContext(); 

A bundle can also register itself for the events (ServiceEvents) of the BundleContext. These are, for example, triggered if a new bundle is installed or de-installed or if a new service is registered.

To publish a service in your bundle use:

public class Activator implements BundleActivator {
  // ...
  public void start(BundleContext context) throws Exception {
    context.
      registerService(IMyService.class.getName(), 
         new ServiceImpl(), null);

  }
  // ...
} 

Once the service is no longer used you must unregister the service with OSGi. OSGi counts the usage of services to enable the dynamic replacement of services.

context.ungetService(serviceReference); 

A bundle can acquire a service via the BundleContext class. The following example demonstrates that.

ServiceReference<?> serviceReference = context.
  getServiceReference(IMyService.class.getName());
IMyService service = (IMyService) context.
  getService(serviceReference); 

The problem with service trackers is that they still obey Java rules. In the case where your service consumer keeps a reference to the service, this service cannot be removed via the OSGi framework. The other disadvantage is that the service tracker requires a lot of boilerplate code. To solve these issues declarative services were developed.

OSGi declarative services simplify the handling of OSGi services.

Create a plug-in project "de.vogella.osgi.quote" and the package "de.vogella.osgi.quote". Do not use a template. You do not need an activator. Afterwards select the MANIFEST.MF and the Runtime tab. Add "de.vogella.osgi.quote" to the exported packages.

Create the following interface "IQuoteService".

package de.vogella.osgi.quote;

public interface IQuoteService {
  String getQuote();
} 

We will now define a bundle which will provide the service.

Create a plug-in project "de.vogella.osgi.quoteservice". Do not use a template.

Select the MANIFEST.MF and dependecy tab. Add "de.vogella.osgi.quote" to the required plugins.

Create the following class "QuoteService".

package de.vogella.osgi.quoteservice.internal;

import java.util.Random;

import de.vogella.osgi.quote.IQuoteService;

public class QuoteService implements IQuoteService {

  @Override
  public String getQuote() {
    Random random = new Random();
    // Create a number between 0 and 2
    int nextInt = random.nextInt(3);
    switch (nextInt) {
    case 0:
      return "Tell them I said something";
    case 1:
      return "I feel better already";
    default:
      return "Hubba Bubba, Baby!";
    }

  }
} 

Register the service in the class Activator.

package de.vogella.osgi.quoteservice;

import java.util.Hashtable;

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

import de.vogella.osgi.quote.IQuoteService;
import de.vogella.osgi.quoteservice.internal.QuoteService;

public class Activator implements BundleActivator {

  public void start(BundleContext context) throws Exception {
    IQuoteService service = new QuoteService();
    // Third parameter is a hashmap which allows to configure the service
    // Not required in this example
    context.registerService(IQuoteService.class.getName(), service,
        null);
    System.out.println("IQuoteService is registered");
  }

  public void stop(BundleContext context) throws Exception {
  }
} 

Export your bundles and install them on your server. Start the service bundle.

Create a new plug-in "de.vogella.osgi.quoteconsumer". Add also a dependency to the package "de.vogella.osgi.quote".

Lets register directly to the service and use it.

package de.vogella.osgi.quoteconsumer;

import org.osgi.framework.BundleActivator;
import org.osgi.framework.BundleContext;
import org.osgi.framework.ServiceReference;

import de.vogella.osgi.quote.IQuoteService;

public class Activator implements BundleActivator {

  private BundleContext context;
  private IQuoteService service;

  public void start(BundleContext context) throws Exception {
    this.context = context;
    // Register directly with the service
    ServiceReference reference = context
        .getServiceReference(IQuoteService.class.getName());
    service = (IQuoteService) context.getService(reference);
    System.out.println(service.getQuote());
  }

  public void stop(BundleContext context) throws Exception {
    System.out.println(service.getQuote());
  }

} 

Export this bundle, install it and start and stop it. Everything work. But if you stop the service bundle then your receive an error.

The reason for this is that OSGi is a very dynamic environment and service may be registered and de-registered any time. The next chapter will use a service tracker to improve this.

Declare a package dependency to the package "org.osgi.util.tracker" in your bundle.

To use this define the following class "MyQuoteServiceTrackerCustomizer"

package de.vogella.osgi.quoteconsumer;

import org.osgi.framework.BundleContext;
import org.osgi.framework.ServiceReference;
import org.osgi.util.tracker.ServiceTrackerCustomizer;

import de.vogella.osgi.quote.IQuoteService;

public class MyQuoteServiceTrackerCustomizer implements
    ServiceTrackerCustomizer {

  private final BundleContext context;

  public MyQuoteServiceTrackerCustomizer(BundleContext context) {
    this.context = context;
  }

  private MyThread thread;

  @Override
  public Object addingService(ServiceReference reference) {
    IQuoteService service = (IQuoteService) context.getService(reference);
    thread = new MyThread(service);
    thread.start();
    return service;
  }

  @Override
  public void modifiedService(ServiceReference reference, Object service) {
    // removedService(reference, service);
    // addingService(reference);
  }

  @Override
  public void removedService(ServiceReference reference, Object service) {
    context.ungetService(reference);
    System.out.println("How sad. Service for quote is gone");
    thread.stopThread();
  }

  public static class MyThread extends Thread {

    private volatile boolean active = true;
    private final IQuoteService service;

    public MyThread(IQuoteService service) {
      this.service = service;
    }

    public void run() {
      while (active) {
        System.out.println(service.getQuote());
        try {
          Thread.sleep(5000);
        } catch (Exception e) {
          System.out.println("Thread interrupted " + e.getMessage());
        }
      }
    }

    public void stopThread() {
      active = false;
    }
  }

} 

You also need to register a service tracker in your activator of your serviceconsumer.

package de.vogella.osgi.quoteconsumer;

import org.osgi.framework.BundleActivator;
import org.osgi.framework.BundleContext;
import org.osgi.util.tracker.ServiceTracker;

import de.vogella.osgi.quote.IQuoteService;

public class Activator implements BundleActivator {

  private ServiceTracker serviceTracker;

  public void start(BundleContext context) throws Exception {
    System.out.println("Starting quoteconsumer bundles");
    // Register directly with the service
    MyQuoteServiceTrackerCustomizer customer = new MyQuoteServiceTrackerCustomizer(context);
    serviceTracker = new ServiceTracker(context, IQuoteService.class
        .getName(), customer);
    serviceTracker.open();
  }

  public void stop(BundleContext context) throws Exception {
    System.out.println("Stopping quoteconsumer bundles");
    serviceTracker.close();
  }

} 

Export your bundle again. Start the OSGi console. Use the update command or the install command to get the new version of your bundle and start it. Once you start your service the tracker will be called and the consumer bundle will start writing messages to the console. Stop the service and verify that the consumer does not use the service anymore.

Source Code of Examples

http://www.osgi.org OSGi Homepage

http://www.eclipse.org/equinox Equinox Homepage

http://www.eclipse.org/equinox/documents/quickstart.php Equinox Quickstart guide

http://www.ibm.com/developerworks/opensource/library/os-osgiblueprint/ OSGi Blueprint services

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