Bundle Plugin for Maven

This plugin for Maven 2 is based on the BND tool from Peter Kriens. The way BND works is by treating your project as a big collection of classes (e.g., project code, dependencies, and the class path). The way you create a bundle with BND is to tell it the content of the bundle's JAR file as a subset of the available classes. This plugin wraps BND to make it work specifically with the Maven 2 project structure and to provide it with reasonable default behavior for Maven 2 projects.

Simple Example

Rather than going straight to a detailed list of plugin features, we will first look at a simple example of how to use the plugin to give an immediate flavor. A detailed "how to" will follow.

Assume that we have a simple bundle project that has a pubic API package an several implementation packages, such as:

org.foo.myproject.api
org.foo.myproject.impl1
org.foo.myproject.impl2
...

If we also assume that we have a bundle activator in one of the implementation packages, then the <plugins> section of the POM file for this bundle project would look like this:

...
<plugins>
  <plugin>
    <groupId>org.apache.felix</groupId>
    <artifactId>maven-bundle-plugin</artifactId>
    <extensions>true</extensions>
    <configuration>
      <instructions>
        <Export-Package>org.foo.myproject.api</Export-Package>
        <Private-Package>org.foo.myproject.*</Private-Package>
        <Bundle-Activator>org.foo.myproject.impl1.Activator</Bundle-Activator>
      </instructions>
    </configuration>
  </plugin>
</plugins>
...

The <Export-Package> and <Private-Package> instructions tell the plugin about the contents of the resulting bundle JAR file. The <Export-Package> instruction tells the plugin which of the available packages to copy into the bundle and export, while the <Private-Package> instruction indicates which of the available packages to copy into the bundle but not export. If the two sets overlap, as they do in the case, then the export takes precedence. Since we did not specify any values for any other bundle manifest headers, they will assume default values which are described below. One specific behavior to highlight is that the plugin generates the Import-Package bundle manifest header based on the contents of the bundle, which means that you generally do not ever need to explicitly specify it yourself. That's it.

Features

The BND library underlying the plugin defines instructions to direct its behavior. For this Maven plugin, these instructions are issued in the plugin configuration section of the POM file, as was illustrated above. BND recognizes three types of instructions:

Manifest headers - Any instruction that starts with a capital letter will appear in the resulting bundle's manifest file; the value for the header will either be copied, augmented, or generated by BND depending on the instruction.
Variables - Any instruction starting with a lowercase letter is assumed to be a variable in the form of a name-value pair, such as version=3.0, that can be used for property substitution, but is not copied to the manifest.
Directives - Any instruction starting with a '-' character is considered to be a directive that informs BND to perform some special processing and is not copied to the manifest.

The remainder of this section covers the most important aspects of BND's instructions; for complete details refer to the BND documentation.

Instructions

`<Export-Package>`

The <Export-Package> instruction is a list of packages for the bundle to export. These packages are copied into the resulting bundle JAR file from the available classes (i.e., project classes, dependencies, and class path); thus, it is possible to include classes into your bundle that are not associated with source files in your project. <Export-Package> can be specified with package patterns using the '*' wildcard. Also, it is possible to exclude packages using negation by starting the package pattern with '!'. Thus, non-negated patterns indicate which of the available packages to include in the bundle, whereas negated patterns indicate which should not be included in the bundle.

The list of package patterns is ordered and earlier patterns are applied before later patterns. For example, if you specify "org.foo.*,!org.foo.impl" the second pattern has no effect since all org.foo packages have already been selected by the first pattern. Instead, you should specify "!org.foo.impl,org.foo.*", which will export all org.foo packages except org.foo.impl.

Following standard OSGi R4 syntax, package patterns can include both directives and attributes, which will be copied appropriately into the generated Export-Package manifest header. Besides explicitly listing package version attributes, BND will also determine package versions by examining the source JAR file or from packageinfo files in the package directory.

`<Private-Package>`

The <Private-Package> instruction is similar in every way to the <Export-Package> instruction, except for the fact that these packages will not be exported by the bundle. If a package is selected by both the export and private package headers, then the export takes precedence.

`<Include-Resource>`

The <Include-Resource> instruction is a list of arbitrary resources that should be copied into the bundle JAR file. The specified resources are declared as clauses that can have the following forms:

clause ::= assignment | inline | simple
assignment ::= PATH '=' PATH
simple ::= PATH
inline ::= '@' PATH

For the <Include-Resource> instruction, actual file paths are relative to the pom.xml, while file copy destinations are relative to the root of the resulting bundle JAR file. In the case of assignment or simple forms, the PATH parameter can point to a file or directory. The simple form will place the resource in the bundle JAR with only the file name, i.e., without any path component. For example, including src/main/resources/a/b.c will result in a resource b.c in the root of the bundle JAR. If the PATH points to a directory, the entire directory hierarchy is copied into the resulting bundle JAR file relative to the specified directory. If a specific resource must be placed into a subdirectory of the bundle jar, then use the assignment form, where the first path is the the destination path (including file name if the resource is a file) and the second path is the resource to copy. The inline form requires a ZIP or JAR file, which will be completely expanded in the bundle JAR.

If a resource clause is specified inside of "{ ... }" brackets, then variable substitution will be performed on the resource, where variables in the resources are denoted with "${ ... }" syntax.

`<Import-Package>`

The <Import-Package> instruction is a list of packages that are required by the bundle's contained packages. The default for this header is "*", resulting in importing all referred packages. This header rarely has to be explicitly specified. However, in certain cases when there is an unwanted import, such an import can be removed by using a negation package pattern. The package patterns work in the same way as for <Export-Package>, which means they are ordered. For example, if you wanted to import all packages except org.foo.impl you would specify "!org.foo.impl,*"

Default Behavior

To use this plugin, very little information is required by BND. As part of the Maven integration, the plugin tries to set reasonable defaults for various instructions. For example:

<Bundle-SymbolicName> is assumed to be "${groupId}.${artifactId}".
<Export-Package> is assumed to be "${groupId}.${artifactId}.*", unless <Private-Package> is specified, then <Export-Package> is assumed to be empty.
<Private-Package> is assumed to be empty by default.
<Import-Package> is assumed to be "*", which imports everything referred to by the bundle content, but not contained in the bundle.
<Include-Resource> is assumed to be "src/main/resources/", which will copy the specified project directory hierarchy into the resulting bundle JAR file, mirroring standard Maven behavior.
<Bundle-Version> is assumed to be "${pom.version}" with '-' character separator of the qualifier replaced with a '.' character.
<Bundle-Name> is assumed to be "${pom.name}".
<Bundle-Description> is assumed to be "${pom.description}".
<Bundle-License> is assumed to be "${pom.licenses}".
<Bundle-Vendor> is assumed to be "${pom.organization.name}".
<Bundle-DocURL> is assumed to be "${pom.organization.url}".

Since the plugin creates bundles for OSGi R4, it hard codes Bundle-ManifestVersion to be '2'. Additionally, it generates imports for every export to ensure package substitutability, which is very important when working with collaborating services. It is possible to override any of these values (except Bundle-ManifestVersion) just by specifying the desired value in the plugin configuration section of the POM file.

Detailed "How To"

Get Maven2

The first step in the process of using the plugin is downloading and installing the latest version of the Maven2 runtime. The latest Maven2 release and instuctions for getting started with Maven2 can be found at the Maven website.

Using the Plugin

To use the maven-bundle-plugin, you first need to add the plugin and some appropriate plugin configuration to your bundle project's POM. Below is an example of a simple OSGi bundle POM for Maven2:

<project>
  <modelVersion>4.0.0</modelVersion>
  <groupId>my-osgi-bundles</groupId>
  <artifactId>examplebundle</artifactId>
  <packaging>bundle</packaging>    <!-- (1) -->
  <version>1.0</version>
  <name>Example Bundle</name>
  <dependencies>
    <dependency>
      <groupId>org.apache.felix</groupId>
      <artifactId>org.osgi.core</artifactId>
      <version>0.8.0-incubator</version>
    </dependency>
  </dependencies>
  <build>
    <plugins>
      <plugin>    <!-- (2) START -->
        <groupId>org.apache.felix</groupId>
        <artifactId>maven-bundle-plugin</artifactId>
        <extensions>true</extensions>
        <configuration>
          <instructions>
            <Export-Package>com.my.company.api</Export-Package>
            <Private-Package>com.my.company.*</Private-Package>
            <Bundle-Activator>com.my.company.Activator</Bundle-Activator>
          </instructions>
        </configuration>
      </plugin>    <!-- (2) END -->
    </plugins>
  </build>
  <repositories>
    <repository>    <!-- (3) START -->
      <id>apache.m2.incubator</id>
      <name>Apache M2 Incubator Repository</name>
      <url>http://people.apache.org/repo/m2-incubating-repository/</url>
    </repository>   <!-- (3) END -->
  </repositories>
  <pluginRepositories>
    <pluginRepository>    <!-- (4) START -->
      <id>apache.m2.incubator</id>
      <name>Apache M2 Incubator Repository</name>
      <url>http://people.apache.org/repo/m2-incubating-repository/</url>
    </pluginRepository>   <!-- (4) END -->
  </pluginRepositories>
</project>

There are four main things to note: (1) the <packaging> specifier must be "bundle", (2) the plugin and configuration must be specified (the configuration section is where you will issue instructions to the plugin), and the snapshot repository to resolve dependencies (3) and plugins (4).

Real-World Example

Consider this more real-world example using Felix' Log Service implementation. The Log Service project is comprised of a single package: org.apache.felix.log.impl. It has a dependency on the core OSGi interfaces as well as a dependency on the compendium OSGi interfaces for the specific log service interfaces. The following is its POM file:

<project>
  <modelVersion>4.0.0</modelVersion>
  <groupId>org.apache.felix</groupId>
  <artifactId>org.apache.felix.log</artifactId>
  <packaging>bundle</packaging>
  <name>Apache Felix Log Service</name>
  <version>0.8.0-SNAPSHOT</version>
  <description>
    This bundle provides an implementation of the OSGi R4 Log service.
  </description>
  <dependencies>
    <dependency>
      <groupId>${pom.groupId}</groupId>
      <artifactId>org.osgi.core</artifactId>
      <version>0.8.0-incubator</version>
    </dependency>
    <dependency>
      <groupId>${pom.groupId}</groupId>
      <artifactId>org.osgi.compendium</artifactId>
      <version>0.9.0-incubator-SNAPSHOT</version>
    </dependency>
  </dependencies>
  <build>
    <plugins>
      <plugin>
        <groupId>org.apache.felix</groupId>
        <artifactId>maven-bundle-plugin</artifactId>
        <extensions>true</extensions>
        <configuration>
          <instructions>
            <Export-Package>org.osgi.service.log</Export-Package>
            <Private-Package>org.apache.felix.log.impl</Private-Package>
            <Bundle-SymbolicName>${pom.artifactId}</Bundle-SymbolicName>
            <Bundle-Activator>${pom.artifactId}.impl.Activator</Bundle-Activator>
            <Export-Service>org.osgi.service.log.LogService,org.osgi.service.log.LogReaderService</Export-Service>
          </instructions>
        </configuration>
      </plugin>
    </plugins>
  </build>
  <repositories>
    <repository>
      <id>apache.m2.incubator</id>
      <name>Apache M2 Incubator Repository</name>
      <url>http://people.apache.org/repo/m2-incubating-repository/</url>
    </repository>
  </repositories>
  <pluginRepositories>
    <pluginRepository>
      <id>apache.m2.incubator</id>
      <name>Apache M2 Incubator Repository</name>
      <url>http://people.apache.org/repo/m2-incubating-repository/</url>
    </pluginRepository>
  </pluginRepositories>
</project>

Notice that the <Export-Package> instruction specifies that the bundle exports the Log Service package, even though this package is not contained in the bundle project. By declaring this, the plugin will copy the Log Service package into the resulting bundle JAR file. This is useful in this case because now the bundle can resolve without having to download the entire compendium bundle. The resulting manifest for the Log Service bundle looks like this (notice how the imports/exports automatically have version information associated with them, which was obtained from packageinfo files in the source packages):

Manifest-Version: 1
Bundle-License: http://www.apache.org/licenses/LICENSE-2.0.txt
Bundle-Activator: org.apache.felix.log.impl.Activator
Import-Package: org.osgi.framework;version=1.3, org.osgi.service.log;v
 ersion=1.3
Include-Resource: src/main/resources
Export-Package: org.osgi.service.log;uses:=org.osgi.framework;version=
 1.3
Bundle-Version: 0.8.0.SNAPSHOT
Bundle-Name: Apache Felix Log Service
Bundle-Description: This bundle provides an implementation of the OSGi
  R4 Log service.
Private-Package: org.apache.felix.log.impl
Bundle-ManifestVersion: 2
Export-Service: org.osgi.service.log.LogService,org.osgi.service.log.L
 ogReaderService
Bundle-SymbolicName: org.apache.felix.log

The resulting bundle JAR file has the following content (notice how the LICENSE and NOTICE files were automatically copied from the src/main/resources/ directory of the project):

META-INF/MANIFEST.MF
LICENSE
META-INF/
META-INF/maven/
META-INF/maven/org.apache.felix/
META-INF/maven/org.apache.felix/org.apache.felix.log/
META-INF/maven/org.apache.felix/org.apache.felix.log/pom.properties
META-INF/maven/org.apache.felix/org.apache.felix.log/pom.xml
NOTICE
org/
org/apache/
org/apache/felix/
org/apache/felix/log/
org/apache/felix/log/impl/
org/apache/felix/log/impl/Activator.class
org/apache/felix/log/impl/Log.class
org/apache/felix/log/impl/LogEntryImpl.class
org/apache/felix/log/impl/LogException.class
org/apache/felix/log/impl/LogListenerThread.class
org/apache/felix/log/impl/LogNode.class
org/apache/felix/log/impl/LogNodeEnumeration.class
org/apache/felix/log/impl/LogReaderServiceFactory.class
org/apache/felix/log/impl/LogReaderServiceImpl.class
org/apache/felix/log/impl/LogServiceFactory.class
org/apache/felix/log/impl/LogServiceImpl.class
org/osgi/
org/osgi/service/
org/osgi/service/log/
org/osgi/service/log/LogEntry.class
org/osgi/service/log/LogListener.class
org/osgi/service/log/LogReaderService.class
org/osgi/service/log/LogService.class
org/osgi/service/log/package.html
org/osgi/service/log/packageinfo

Building the Plugin

The plugin is hosted at the Apache Felix incubator project. The following steps describe how to build and install the plugin into your local Maven2 repository.

Using the SVN client of your choice, checkout the maven-bundle-plugin project.

$ svn co http://svn.apache.org/repos/asf/felix/trunk/tools/maven2/maven-bundle-plugin

Using Maven2, build and install the maven-bundle-plugin by issuing the following Maven2 command in the project directory that was created as a result of the previous step.

$ mvn install