Karaf Features and OSGi Services: A Bundle

After finishing a pair of articles last week on Karaf features, I felt that I had done a poor job of explaining the context in which all of this bundling and featurizing was taking place. Instead I pretty much started in the middle, assuming the existence of OSGi bundles with proper manifests, all snug in their Maven repositories.

So I'm going to correct that and at the same time illustrate another cool OSGi technology: declarative services. To do that will take some time and will require help from a detailed example.

OSGi Bundles of Moderate Joy

To start with, we need to know how to make OSGi bundles. And before we bother doing that, it would help to discuss what a bundle is.

OSGi is an approach to building modular applications. The idea is that each module is very specific about its unique name, its version, what it requires from other modules, and what it provides that other modules are allowed to use. In Java, each of these modules is packaged into a JAR file with some additional information in the META-INF/MANIFEST.MF file. This additional information includes imports and exports in the form of Java packages. A properly packaged JAR file with the right manifest content is known as an OSGi bundle. Here is a simple example manifest, as packed inside the JAR at build time:

Manifest-Version: 1.0
Bnd-LastModified: 1477440189494
Build-Jdk: 1.8.0_91
Built-By: ahohn
Bundle-ManifestVersion: 2
Bundle-Name: Greeter Interfaces
Bundle-SymbolicName: org.anvard.karaf.greeter.interfaces
Bundle-Version: 1.0.0.SNAPSHOT
Created-By: Apache Maven Bundle Plugin
Export-Package: org.anvard.karaf.greeter.api;version="1.0.0.SNAPSHOT"
Tool: Bnd-1.50.0

There are a couple things to note here:

• The Bundle-SymbolicName is important and needs to be unique.
• We can specify a Bundle-Version as well as a version for any packages we export. Next time we'll see a bundle that imports this package; the version is important.

We could create this manifest content ourselves, whether we run the jar command directly or use a build tool like the Maven JAR plugin. But this is a lot of manual effort and involves a lot of redundancies, especially in a Maven project where we are already specifying the artifact name, version, and dependencies. Fortunately, OSGi expert Peter Kriens created a tool called bnd. The bnd tool had the ability to scan Java code to identify package imports from outside a module, and to use a much cleaner configuration file as the source to generate the right OSGi content for META-INF/MANIFEST.MF.

Taking this one step further is the maven-bundle-plugin. Instead of using a separate configuration file, this plugin allows specifying the needed information in the Maven POM. It also leverages the Maven dependencies to figure out what versions of package imports are required. To get the manifest shown above, we need this configuration in our POM file:

    <packaging>bundle</packaging>
    ...
    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.felix</groupId>
                <artifactId>maven-bundle-plugin</artifactId>
                <version>2.3.7</version>
                <extensions>true</extensions>
                <configuration>
                    <instructions>
                        <Bundle-Name>${project.name}</Bundle-Name>
                        <Bundle-SymbolicName>${project.groupId}.${project.artifactId}</Bundle-SymbolicName>
                        <Export-Package>
                            org.anvard.karaf.greeter.api
                        </Export-Package>
                    </instructions>
                </configuration>
            </plugin>
        </plugins>
    </build>

See the GitHub repository for the complete pom.xml.

Bundling and Karafing

Now that we have a way to build an OSGi bundle, we can install it to our local Maven repository and then use Karaf's ability to resolve bundles from Maven to load it into Karaf.

First, we compile the bundle using Maven:

$ mvn clean install

[ Lots of output; Internet downloaded ]

[INFO] --- maven-bundle-plugin:2.3.7:install (default-install) @ interfaces ---
[INFO] Installing org/anvard/karaf/greeter/interfaces/1.0-SNAPSHOT/interfaces-1.0-SNAPSHOT.jar
[INFO] Writing OBR metadata
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 1.829 s
[INFO] Finished at: 2016-10-25T20:33:25-04:00
[INFO] Final Memory: 19M/212M
[INFO] ------------------------------------------------------------------------

Now we can load into Karaf. Starting from a standard Karaf distribution:

apache-karaf-4.0.7$ bin/karaf

...

karaf@root()> install mvn:org.anvard.karaf.greeter/interfaces/1.0-SNAPSHOT
Bundle ID: 52
karaf@root()> list
START LEVEL 100 , List Threshold: 50
ID | State     | Lvl | Version        | Name
----------------------------------------------------------
52 | Installed |  80 | 1.0.0.SNAPSHOT | Greeter Interfaces
karaf@root()>

Karaf has some nice features for doing development on OSGi bundles that take advantage of OSGi's modularity. For example, now that we have the bundle installed and we know its ID, we can use update 52 to grab the latest snapshot from Maven. This includes stopping the existing bundle, installing a newer version, and starting the newer version. This works because OSGi is careful to keep bundles separate, even to the extent of separate class loaders, so it's easy to discard a bundle and load in a new one, even if the classes are the same.

Wrapping Up

At this point we have a bundle that has some Java code in it, but it doesn't do anything other than export that code for other bundles to use. Next time I'll talk a little about the structure of the example and why 5 Java files are split across 4 bundles.