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Making a component add-on OSGi-compatible

In order to use a component jar as an OSGi bundle, the manifest file needs to have additional headers. The headers describe the bundle and provide additional information. Some of the headers are as follows.

  • Bundle-SymbolicName is the only mandatory header. It specifies a unique identifier for the bundle, based on the reverse domain name convention. e.g. com.vaadin.flow.component.button

  • Bundle-Name defines a human-readable name. e.g. vaadin-button-flow

  • Bundle-License specifies the license information of bundle. e.g. https://www.apache.org/licenses/LICENSE-2.0

  • Bundle-ManifestVersion indicates the OSGi specification to use for reading this bundle. The value 1 indicates OSGi release 3, and the value 2 indicates OSGi release 4 and later.

  • Bundle-Version specifies the version of this bundle which consists of up to four parts separated with dots.

  • Import-Package declares the imported packages for this bundle.

  • Export-Package contains a declaration of exported packages.

  • Require-Capability specifies that this bundle requires other bundles to provide a capability e.g. osgi.ee;filter:="(&(osgi.ee=JavaSE)(version=1.8))"

After generating the MANIFEST.MF file, manually or using any tool, it should be added to the output jar file. This job can be done by configuring maven-jar-plugin like this:

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-jar-plugin</artifactId>
    <configuration>
        <archive>
            <manifestFile>${project.build.outputDirectory}/META-INF/MANIFEST.MF</manifestFile>
        </archive>
    </configuration>
</plugin>
Note
 Static resources do not work in OSGi environment for now and it is required to unpack them in the application project.

Tools

Although the headers can be added to MANIFEST.MF manually, it is recommended to use an automated tool to create them. Here two tools, both maven plugins, are briefly introduced.

Bnd maven plugin

This plugin generates required manifest entries based on specified instructions which are declared in either a file (with default name of bnd.bnd) or the plugin <configuration> in the pom. The plugin also set default values to some headers derived from pom elements. For example, Bundle-SymbolicName is set to artifactId, and Bundle-Version is deducted from artifact version. The following is an example of the usage of the plugin.

<plugin>
    <groupId>biz.aQute.bnd</groupId>
    <artifactId>bnd-maven-plugin</artifactId>
    <executions>
        <execution>
            <goals>
                <goal>bnd-process</goal>
            </goals>
        </execution>
    </executions>
    <configuration>
        <bnd><![CDATA[
             -exportcontents: org.example.api,org.example.types
             -sources: true
             Private-Package: org.example.internal.*
             Bundle-Activator: org.foo.myproject.impl.Activator
             ]]>
        </bnd>
    </configuration>
</plugin>

In this example, the instructions are provided using <![CDATA[ ]]> section in bnd parameter. In addition to the instructions, that start with a minus sign ('-'), manifest headers (e.g. Private-Package) can also be added here. For more information about instructions see Bnd Instruction Reference.

For more information about this plugin see bnd-maven-plugin documentation on GitHub.

Apache Felix Maven Bundle Plugin

This plugin is based on Bnd tool with this change that you can provide headers and instructions as nested tags in <configuration> section. So, it may be a better choice when we want to have instructions in the pom file. Although the values of the required entries in the manifest file can be set manually, this plugin generates reasonable default values for various headers. Here is an example on how to use the plugin.

<plugin>
    <groupId>org.apache.felix</groupId>
    <artifactId>maven-bundle-plugin</artifactId>
    <extensions>true</extensions>
    <executions>
        <execution>
            <id>bundle-manifest</id>
            <phase>process-classes</phase>
            <goals>
                <goal>manifest</goal>
            </goals>
        </execution>
    </executions>
    <configuration>
        <instructions>
            <_exportcontents>org.example.api,org.example.types</_exportcontents>
            <_sources>true</_sources>
            <Private-Package>org.example.internal.*</Private-Package>
            <Bundle-Activator>org.foo.myproject.impl.Activator</Bundle-Activator>
        </instructions>
    </configuration>
</plugin>

With these instructions, the plugin generates for instance Bundle-Version deducted from ${pom.version}, Bundle-Name set to ${pom.name} and Bundle-SymbolicName computed from groupId and artifactId. For more information see Apache Felix Maven Bundle Plugin documentation.

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