The Apache Felix Service Component Runtime described by the OSGi Desclarative 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.
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 theactivate
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 Maven SCR Plugin may be used. This helps keeping the descriptor and the code in sync especially during development.
Management
The OSGi Compendium specification defines no management API for Declarative Services. As of version 0.9.0-20071123.131249-8 a simple management 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.
Shell Command
The management API is made available to the Felix Shell as the scr
command with a short list of subcommands:
Synopsis |
Description |
---|---|
|
Show help of the specific |
|
List registered components of the bundle specified by |
|
Show a complete information dump of the given component. This dump includes the name, status, provided services and information on the service references |
|
Enable the given component if not already enabled. If the component is already destroyed or enabled, this command has no effect. |
|
Disable the given component if not already disabled. If the component is already destroyed or disabled, this command has no effect. |
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 !