The key OSGi configuration is highlighted in this snippet from the core extension's pom.xml.

<artifactId>ext-core</artifactId>
<packaging>bundle</packaging>
 ...
<build>
   <plugins>
      <plugin>
          <groupId>org.apache.felix</groupId>
          <artifactId>maven-bundle-plugin</artifactId>
          <extensions>true</extensions>
          <configuration>
             <instructions>
                <!-- Fragment-Host gives the symbolic name of the core wrapper bundle to attach to. -->
                <Fragment-Host>ext-commerce-engine-wrapper</Fragment-Host>
                <Import-Package>
                   !*
                </Import-Package>
                ...
             </instructions>
          </configuration>
      </plugin>
      ...

The first OSGi configuration data in the POM is the project's packaging: <packaging>bundle</packaging>. This tells Maven to build the project as an OSGi bundle. The project still builds into a JAR file, so the project can be used outside of an OSGi setting, which means you can leave this as is even if you are not using the core extension in an OSGi setting.

The maven-bundle-plugin configuration turns the core extension into a OSGi fragment and tells the fragment to attach to Cortex's ext-commerce-engine-wrapper bundle. The extension commerce engine wrapper bundle wraps the core extension in an OSGi bundle. Fragments are treated as part of the host bundles. Relevant definitions of the fragment are merged with the host bundle's definitions before the host is resolved, as long as the information does not conflict. Fragment dependencies are resolved if possible. If the fragment dependencies cannot be resolved, the fragment does not attach to the host bundle. A fragment cannot have its own class loader or bundle activator. It cannot override the information present in the host bundles. Fragments extend bundles with resources, classes, and permitted headers enabling you to customize your bundles.

The <Import-Package> element contributes any additional imports to the core bundle. The core bundle already has everything we need, so we are telling it to not import anything by specifying !* If your core extension relies on additional third party libraries (other than those already used by core), you must list the required packages in the <Import-Package> instruction. If you do this, include the manifest header directive value identifying a mandatory resolution type to enable attachment problem logging. For example:

<Import-Package>
  org.thirdparty.library.package; resolution:=mandatory,
  !*
</Import-Package>

If you don't specify the mandatory resolution, OSGI won't find your third party library package at run time and the fragment will fail to attach, but no error message will appear.

This file tells OSGi where to find the spring configuration for the extension project.

<beans xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns="http://www.springframework.org/schema/beans"
    xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd" >

    <!-- Let OSGI know the spring configuration of this project. -->
    <import resource="../../META-INF/conf/ep-core-plugin.xml" />
</beans>

• Core extension classes replacing the out-of-the-box core classes (meaning classes that have the exact same package structure and class name) will not work with this approach.
• If the extension uses third-party libraries, additional configuration is required:
  • Add the third-party packages to the <Import-Package> element (as described above in Extension pom.xml)
  • If the third-party libraries are OSGi compliant, add them as a dependency and artifact item in the Cortex Webapp.
  • If the libraries are not OSGi compliant, they need to be embedded in the fragment.