OSGi Cookbook: #1. A Simple Bean

This is something that I have been meaning to do for some time now, i.e. an OSGi cookbook. So this is the first in a series of recipes, with each one building on the next in usefulness and complexity. This recipe is really for newbies, just to take some of the mystery out of OSGi and to introduce some of the emerging tools for OSGi development.  If you have never done anything with OSGi before, then I recommend that you work through Neil Bartlett’s excellent series on getting started with OSGi.

Getting ready

 Ensure that you have maven andthe pax-construct scripts installed.The installation is straight forward, just follow the instructions on their respective web sites.

#1. A Simple Bean

This recipe creates a service out of a simple POJO. The service doesn’t do anything useful, but the recipe is a simple way to getyour development environment up and running.

  1. Create a project using pax-construct.The pax-construct scripts uses maven and the maven-pax-plugin.Using the familiar maven terminology of groupId and artifactId, create the project with a groupId of net.aslamkhan and artifactId of osgi-simplebean.
    /> pax-create-project -g net.aslamkhan -a osgi-simplebean
    ...
    [INFO] Archetype created in dir: /Users/aslam/projects/osgi/osgi-simplebean
    [INFO] ------------------------------------------------------------------------
    [INFO] BUILD SUCCESSFUL
    [INFO] ------------------------------------------------------------------------
  2. Have a look at what’s in the project and make sure that it does build cleanly (well, there’s actually nothing worthwhile to build here, but let’s make sure that everything is still ok).
    /> cd osgi-simplebean		
    /> ls -l
    -rw-r--r--   1 aslam  aslam  2354 Jan  9 10:26 pom.xml
    drwxr-xr-x   5 aslam  aslam   170 Jan  9 10:26 poms
    drwxr-xr-x   3 aslam  aslam   102 Jan  9 10:26 provision			
    /> mvn package
    ...
    [INFO] ------------------------------------------------------------------------
    [INFO] Reactor Summary:
    [INFO] ------------------------------------------------------------------------
    [INFO] net.aslamkhan.osgi-simplebean (OSGi project) .......... SUCCESS [1.689s]
    [INFO] osgi-simplebean - plugin configuration ................ SUCCESS [0.004s]
    [INFO] osgi-simplebean - wrapper instructions................. SUCCESS [32.144s]
    [INFO] osgi-simplebean - bundle instructions ................. SUCCESS [0.003s]
    [INFO] osgi-simplebean - imported bundles .................... SUCCESS [0.002s]
    ...
  3. We now need to add in the dependencies for Spring Dynamic Modules for OSGi.  Instead of hand crafting our maven pom.xml file(s), we again use a pax-construct script to add the repositoriesfor the Spring distributions.  From this point onwards, I will not show the output of commands unless it helps to illustrate a point.
    /> pax-add-repository -i spring-milestones 
       -u http://s3.amazonaws.com/maven.springframework.org/milestone
    /> pax-add-repository -i spring-snapshots 
       -u http://static.springframework.org/maven2-snapshots 
       -- -Dsnapshots "-Dreleases=false"
  4. Import the spring-osgi-extender bundle, and the slf4j logging service bundle.
    /> pax-import-bundle -g org.springframework.osgi 
       -a spring-osgi-extender -v 1.0-rc1 
       -- -DwidenScope -DimportTransitive

    If you look at the contents of the provision/pom.xml file, you will see a bunch of dependencies that have been pulled inthe moment we imported the spring-osgi-extender bundle.

    <dependency>
    	<groupId>org.springframework.osgi</groupId>
    	<artifactId>spring-osgi-extender</artifactId>
    	<version>1.0-rc1</version>
    	<scope>provided</scope>
    </dependency>
    ...
  5. Now that we have all the infrastructure things sorted out, we can start cooking. The quickest way to get cooking is to usethe pax-construct pax-create-bundle script. In this instance, we will create a sample bean with the necessaryboilerplate files needed for Spring Dynamic Modules. 
    /> pax-create-bundle -p org.example.bean 
       -- -Dspring -Djunit

    Notice that another directory org.example.bean has been created. Furthermore, this maven module has been added to the root (i.e. parent) ./pom.xml file. Also conveniently created, is asample bean and associated unit tests in org.example.bean/src directory.Edit the org.example.bean/src/main/resources/META-INF/spring/bundle-context-osgi.xml file to contain the following.

    <?xml version="1.0" encoding="UTF-8"?>
      <beans xmlns="http://www.springframework.org/schema/beans"			  
        ...
        xmlns:osgi="http://www.springframework.org/schema/osgi"
        ...>
    	<osgi:service ref="myExampleBean">			        
    	  <osgi:interfaces>			            
    	    <value>org.example.bean.ExampleBean</value>			        
    	  </osgi:interfaces>			    
    	</osgi:service>
    </beans>
  6. We should be able to build this and fire it up in the Apache Felix OSGi implementation.  But wait!  We have not downloaded, nor installed Felix.  Not to worry, maven-pax-plugin will download Felix and fire it up for you, allpart of the pax:provision goal.
    /> mvn clean install pax:provision
    ... properties (org.springframework.context.service.name=org.example.bean)
    ->

    Let’s check if our org.example.bean bundle is installed and active. From the Felix console, enter the following.Also notice that the spring-osgi-extender bundle and spring 2.5 jars are now available as bundles as well.

    -> ps
    START LEVEL 6			   ID   State         Level  Name
    [   0] [Active     ] [    0] System Bundle (1.0.1)
    [   1] [Active     ] [    1] org.osgi.r4.compendium (1.0.0)
    [   2] [Active     ] [    1] Apache Felix Shell Service (1.0.0)
    [   3] [Active     ] [    1] Apache Felix Shell TUI (1.0.0)
    [   4] [Active     ] [    5] spring-osgi-extender (1.0.0.rc1)
    [   5] [Active     ] [    5] jcl104-over-slf4j (1.4.3)
    [   6] [Active     ] [    5] slf4j-api (1.4.3)
    [   7] [Active     ] [    5] spring-osgi-core (1.0.0.rc1)
    [   8] [Active     ] [    5] spring-osgi-io (1.0.0.rc1)
    [   9] [Active     ] [    5] spring-aop (2.5.0)
    [  10] [Active     ] [    5] spring-beans (2.5.0)
    [  11] [Active     ] [    5] spring-context (2.5.0)
    [  12] [Active     ] [    5] spring-core (2.5.0)
    [  13] [Active     ] [    5] spring-web (2.5.0)
    [  14] [Active     ] [    5] spring-test (2.5.0)
    [  15] [Active     ] [    5] aopalliance.osgi (1.0.0.SNAPSHOT)
    [  16] [Active     ] [    5] backport-util-concurrent.osgi (3.0.0.SNAPSHOT)
    [  17] [Active     ] [    5] slf4j-simple (1.4.3)			
    [  18] [Active     ] [    5] org.example.bean (1.0.0.SNAPSHOT)
    ->

    Now, check that the service is available.

    -> services
    ...
    org.example.bean (18) provides:
    -------------------------------			
    org.example.bean.ExampleBean
    org.springframework.osgi.context.support.OsgiBundleXmlApplicationContext, 
    ...

That’s it for this recipe.  Overall, pax makes it really painless to get your OSGi implementation up and running in your developmentenvironment.  Combined with the convenience of spring-dm, we have the start of something really productive.

3 thoughts on “OSGi Cookbook: #1. A Simple Bean

  1. Hi Gavin,
    The syntax is actually “dash dash” “dash”Dsnapshots. Hope that makes sense.
    If you just cut ‘n pasted, then the double dash is presented as a single “long” dash which is a WordPress weirdo when I saved the post.
    In pax, the double dash is used to preceed any optional maven arguments, i.e. all the -Dxxx bits that follow on.
    Aslam

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