Build System

The buildsystem is based on Maven. A very common tool for Java development. Maven is a convention centric, declarative system that is extensible via additional plugins. That means if you stick 100% to Mavens idea of a java project, your buildsystem instruction file is not longer than 10 lines.

openHAB has a few extra requirements and we use about 10 additional plugins, ranging from OSGi specific ones (bnd) to publish and testing plugins.

This section talks about some common buildsystem related topics and also some quirks that you will encounter.

Adding Dependencies

Generally all dependencies should be OSGi-bundles and available on Maven Central.

External dependency

In most cases they should be referenced in the project POM with scope compile:

  <dependencies>
    <dependency>
      <groupId>foo.bar</groupId>
      <artifactId>baz</artifactId>
      <version>1.0.0</version>
      <scope>compile</scope>
    </dependency>
  </dependencies>

These dependencies are embedded in the resulting bundle automatically.

There are two exceptions:

1. Dependencies to other openHAB bundles (e.g. org.openhab.addons.bundles/org.openhab.binding.bluetooth/3.1.0-SNAPSHOT or org.openhab.addons.bundles/org.openhab.transform.map/3.1.0-SNAPSHOT).
2. Bundles that are used by more than one binding (e.g. Netty-bundles like io.netty/netty-common/4.1.34.Final).

Dependencies on other openHAB bundles should have the scope provided. To ensure that they are available at runtime they also need to be added to the feature.xml:

  <bundle dependency="true">mvn:org.openhab.addons.bundles/org.openhab.binding.bluetooth/3.1.0-SNAPSHOT</bundle>

Internal dependency

In two cases libraries can be added to the /lib directory:

1. The bundle is not available for download
2. The bundle is not an OSGi bundle but needs to be used for integration tests. Unlike Karaf, BND is not able to wrap bundles on its own. In this case the binding works as wrapper. You need add the library and export all needed packages manually.

The build system automatically picks up all JAR files in /lib and embeds them in the new bundle. In this case they must not be added to the feature.

If the bundles manifest is not properly exporting all needed packages, you can import them manually by adding

  <properties>
    <dep.noembedding>netty-common</dep.noembedding>
  </properties>

It is also necessary to add them to the feature.xml(see above). If the version that is used in other openHAB bundles is incompatible with your library, check if you can use a different version of your library. In case this is not possible, please ask if an exception can be made and a different version can be bundled within your binding. Technically you just need to omit the exclusion-statement above. If you only depend on shared bundles you can also omit the exclusion statement and add a noEmbedDependencies.profile file in the root directory of your binding.

If the imported packages need to be exposed to other bundles (e.g. case 2 above), this has to be done manually by adding

  <properties>
    <bnd.exportpackage>foo.bar.*;version="1.0.0"</bnd.exportpackage>
  </properties>

to the POM. If version="1.0.0" is not set, the packages are exported with the same version as the main bundle. Optional parameters available for importing/exporting packages (e.g. foo.bar.*;resolution:="optional") are available, too. Packages can be excluded from import/export by prepending ! in front of the name (e.g. <bnd.importpackage>!foo.bar.*</bnd.importpackage> would prevent all packages starting with foo.bar from being imported).

Karaf Feature

Each bundle needs a feature.xml which is used for the bundle-installation in openHAB. A "fits-most-cases" feature.xml is automatically generated.

In some cases additional entries need to be added.

Shared Dependencies (JNA, Netty, etc.)

Bundles that are shared by different openHAB bundles are excluded from embedding. The need to be added to the feature to make them available at runtime. Two cases need to be treated differently:

1. Bundles that have a core feature are referenced by the feature (e.g. <feature>openhab-runtime-jna</feature> or <feature>openhab-transport-upnp</feature>).
2. Bundles that do not have a core feature are added directly (e.g. <bundle dependency="true">mvn:commons-codec/commons-codec/1.10</bundle>).

Multi-Bundle Features / Sub-Bundles

In some cases a binding consists of several bundles (e.g. mqtt). The feature.xml of the sub-bundle (e.g. mqtt.homie) needs to add all bundles from the parent-bundle to make sure that the feature verification succeeds:

<feature>openhab-transport-mqtt</feature>
<bundle start-level="80">mvn:org.openhab.addons.bundles/org.openhab.binding.mqtt/${project.version}</bundle>

To make sure that all sub-bundles are installed together with the main-bundle, the feature.xml of the bundles needs to be excluded from the aggregation. Therefore an exclusion needs to be added to features/openhab-addons/pom.xml (starting from l. 47):

<exclude name="**/org.openhab.binding.mqtt*/**/feature.xml"/>

An "aggregated" feature then needs to be created in features/openhab-addons/src/main/resources/footer.xml. The feature is named like the base-bundle and lists all sub-bundles.

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