Apache Felix Service Component Runtime

The Apache Felix Service Component Runtime described by the OSGi Declarative Services Specification is implemented by the org.apache.felix.scr bundle. As specified, the components must be declared in XML-formatted descriptor files which in turn must be listed in the Service-Component header of the declaring bundle.

The component declarations are read when the declaring bundle is started and the respective components are verified and activated depending on their declaration.

The Apache Felix Declarative Services implementation with the OSGi Declarative Services Specification Version 1.1 passes the OSGi CT.

Example

To help you get a head start, here is an example of using Declarative Services. You will find more examples in the trunk/examples folder of the Apache Felix Project.

Component

First of all the component must be implemented in a simple Java class. The Declarative Services Specification basically places no restrictions on the contents of this class. If you make use of advanced functionality such as providing an activate() or deactivate() method or using service loopup by event strategy (see 112.3.1 Accessing Services) you will of course have to provide the respective methods.

For the sake of example, lets define a very simple class, which implements a java.util.Comparator service:

package sample;
import java.util.Comparator;
public class SampleComparator implements Comparator
{
    public int compare( Object o1, Object o2 )
    {
        // TODO: calculate the result
        return o1.equals( o2 ) ? 0 : -1;
    }
}

This is of course a very simple and not very intelligently implemented comparator...

Declaration

The next step consists of writing the declaration. I usually put these files in the OSGI-INF folder of the bundle, but the files may be placed anywhere within the bundle or any of the bundle's fragments as long as its path is listed in the Service-Component bundle manifest header.

So here we go with the file:

<?xml version="1.0" encoding="UTF-8"?>
<component name="sample.component" immediate="true">
  <implementation class="sample.SampleComparator" />
  <property name="service.description" value="Sample Comparator Service" />
  <property name="service.vendor" value="Apache Software Foundation" />
  <service>
    <provide interface="java.util.Comparator" />
  </service>
</component>

There are some noteworthy settings in this descriptor already:

• name - Uniquely identifies this component and is also used to retrieve optional configuration from the Configuration Admin Service (if available).
• immediate - Defines whether the component is to be instantiated immediately (true) or on-demand (false).
• implementation.class - The fully qualified name of the class implementing the component. This class must be public and have a public default constructor for it to be usable by the Service Component Runtime. This class is not required to be exported and may as well be private to the bundle. In fact, you will generally not export the component implementation class.
• property - These elements define configuration properties to the component. These properties are available through the ComponentContext which is presented to the component in the activate method (see below).
• service - If the component is to be registered as a service, the service names are listed in provide elements inside the service element. These names will generally be interfaces and must be visible to other bundles for the service to be usable. In this sample, the service is the Java java.util.Comparator class, which is always visible.

To finalize this declaration, add the following header to the bundle manifest:

Service-Component: OSGI-INF/sample.xml

Activation

It may well be that the component needs to be notified, when it is activated and deactivated. For this, the component may implement an activate method and a deactivate method. Both methods must be public or protected and take a single argument, the org.osgi.service.ComponentContext. It is recommended for this method to the protected as it is only used by the Service Component Runtime and should of course not be part of the public API of the component.

Here is the initial class extended with activation and deactivation methods:

package sample;
import java.util.Comparator;
import org.osgi.service.component.ComponentContext;
public class SampleComparator implements Comparator
{
    public int compare( Object o1, Object o2 )
    {
        // TODO: calculate the result
        return o1.equals( o2 ) ? 0 : -1;
    }

    protected void activate(ComponentContext context)
    {
        // TODO: Do something on activation
    }
    
    protected void deactivate(ComponentContext context)
    {
        // TODO: Do something on deactivation
    }
}

Nothing more needs to be done as the Service Component Runtime automatically recognizes and calls these methods.

Service Binding

The next step would probably be to do some service binding. This is somewhat more overhead, as the referred to services must be declared. On the other hand, you do not have to care to listen for these services. As examples of these strategies we will first use the lookup strategy to access an OSGi HttpService and then we will use the event strategy to access an OSGi LogService (I personally prefer the event strategy, but your mileage may vary).

Looking up the Service

To use the service, the reference must be declared in the service declaration in an reference element. Here is the respective declaration for a log service to lookup:

<component...>
   ...
    <reference name="http"
        interface="org.osgi.service.http.HttpService"
        cardinality="1..1" 
        policy="static"
    />
   ...
</component>

To use this service you call the ComponentContext.getService(String) method, for example in the activate method:

protected void activate(ComponentContext context)
{
    HttpService http = ( HttpService ) context.locateService( "http" );
}

Receiving the Service

The event strategy works by declaring bind and unbind methods in the component descriptor. These methods take a single parameter of the type defined in the reference.interface attribute and must be declared public or protected. As with the activate and deactive it is recommended for the bind and unbind methods to be declared protected as they are generally not part of the public API of the component.

When using the event strategy, you will want to store the service in a private field of the component for later use.

First here is the reference declaration:

<component...>
   ...
   <reference name="log"
         interface="org.osgi.service.log.LogService"
         cardinality="1..1"
         policy="static"
         bind="bindLog"
         unbind="unbindLog"
   />
   ...
</component>

And here is some code:

private LogService log;

protected void activate(ComponentContext context)
{
    log.log(LogService.LOG_INFO, "Hello Components!");
}

protected void bindLog(LogService log)
{
    this.log = log;
}

protected void unbindLog(LogService log)
{
    this.log = null;
}

Note, that you may refer to the log field in the activate method as we declared the reference as required. In this case the reference is provided to the component in the bind method before the activate method is called.

Maven SCR Plugin

To simplify the tasks of generating the SCR Desriptor and adding the Service-Component header to the bundle manifest, the Apache Felix Maven SCR Plugin may be used. This helps keeping the descriptor and the code in sync especially during development.

Configuration

The Apache Felix Declarative Services implementation can be configured with Framework properties which are read on startup of the implementation bundle and Configuration Admin Service configuraiton which is provided by the Configuration Admin Service to the service PID org.apache.felix.scr.ScrService.

The following properties are supported:

This configuration mechanism is implemented in the ScrConfiguration and its helper classes.

Non-Standard Component Factory Behaviour

Versions of the Apache Felix Declarative Services implementation prior to 1.2.0 supported handling of Component Factory components which is not specification compliant.

This behaviour assumes the component name of the Component Factory component to be Service Factory PID and each configuration with this Service Factory PID causes the service component runtime to actually create and activate an instance of the Component Factory component automatically. This is not foreseen by the specification which defines instantiation of Component Factory components as being purely application controled and not configuration induced.

To have components instantiated with factory configurations, regular components should be used. This case each factory configuration instance will create a component instance.

If you know that you are using Component Factory components depending on this non-standard behaviour you may set the ds.factory.enabled configuration property to true (the default of this property is false thus disabling this functionality for specification compliance).

For details also refer to FELIX-1416

Administration

The OSGi Compendium specification defines no administrative API for Declarative Services. As of version 0.9.0-20071123.131249-8 a simple administrative API is provided the Apache Felix implementation. The bundle itself also has a Felix Shell Command providing easy commands to introspect the states of the registered components.

The Apache Felix Web Console has built-in support for Declarative Services administration based on this API.

Shell Command

The management API is made available to the Felix Shell as the scr command with a short list of subcommands:

The administrative API commands are also available in the Gogo shell where the subcommand names must be prefixed with the name space scr. Thus the list command corresponds to scr:list in the Gogo shell.

API Use

The API consists of the main interface org.apache.felix.scr.ScrService and two helper interfaces org.apache.felix.scr.Component describing a registered component and org.apache.felix.scr.Reference describing a single reference of a registered component. To access the management API, client applications just ask for the ScrService as usual:

....
ServiceReference scrServiceRef = bundleContext.getServiceReference( ScrService.class.getName() );
ScrService scrService = (ScrService) bundleContext.getService(scrServiceRef);
....

Alternatively, you may of course use the ServiceTracker or if you are using the ScrService in a component, you may have the ScrService bound according to the component declaration.

The ScrService allows access to all registered components, to a specific component by component ID or to all registered components of a specific bundle.

Summary

This tutorial just listed some very basic information on Declarative Service. To get more information, for example on hoe the Configuration Admin Service may be used to configure components, refer to the Declarative Services Sepecification in the OSGi Service Platform Service Compendium book.

Have Fun !