2.5. Provider Bundle

Overview

This section explains how to set up a Maven project for a typical provider bundle.
The hello-paris-impl bundle exemplifies a provider bundle, which provides the implementation of an API. The provider does not export any of its own packages, because the implementation classes are private. But the provider does instantiate and export an OSGi service, which is accessed through the HelloParis interface.

Directory structure

The hello-paris-impl bundle has the following directory structure: 
hello-paris-impl/
  |
  \--src/
     |
     \--main/
     |  |
     |  \--java/
     |  |  |
     |  |  \--org/fusesource/example/hello/paris/impl/
     |  |     |
     |  |     \--HelloParisImpl.java
     |  |
     |  \--resources/
     |     |
     |     \--OSGI-INF/blueprint/
     |        |
     |        \--paris-svc.xml
     |   
     \--test/
The org.fusesource.example.hello.paris.impl package is private. By default, the Maven bundle plug-in treats any packages containing the segments .impl or .internal, as private packages.
The src/main/resources/OSGI-INF/blueprint directory contains a single blueprint file, paris-svc.xml. Any file matching the pattern, *.xml, in this directory is assumed to be a blueprint configuration file.

Sample implementation

The hello-paris-impl bundle is intended to implement all of the interfaces appearing in the hello-paris API bundle. In this example, a single HelloParisImpl class implements the HelloParis interface, as follows: 
// Java
package org.fusesource.example.hello.paris.impl;

import org.fusesource.example.hello.paris.HelloParis;

import org.fusesource.example.time.Clock;
import org.fusesource.example.time.TimeUtil;

public class HelloParisImpl implements HelloParis {
    protected Clock localTime = null;

    public String getGreeting() {
        return new String("Bonjour!");
    }

    public Clock getLocalTime() {
        if (localTime==null) {
            localTime = TimeUtil.createClock(TimeUtil.TimeZone.PARIS);
        }
        return localTime;
    }

}

Publish OSGi service

The natural way to bootstrap the HelloParis implementation in OSGi is to publish the class, HelloParisImpl, as an OSGi service. Use the bean element to create a HelloParisImpl instance and use the service element to publish the bean, advertising it as a service of HelloParis type.
For example, the blueprint file, OSGI-INF/blueprint/paris-svc.xml, has the following contents: 
<?xml version="1.0" encoding="UTF-8"?>
<blueprint xmlns="http://www.osgi.org/xmlns/blueprint/v1.0.0">

    <bean id="hello-paris-impl"
        class="org.fusesource.example.hello.paris.impl.HelloParisImpl"/>

    <service ref="hello-paris-impl"
        interface="org.fusesource.example.hello.paris.HelloParis"/>

</blueprint>

Maven dependencies

In the Maven POM file, the hello-paris-impl bundle defines dependencies on the following Maven artifacts:
  • time-util
  • hello-paris

Import and export rules

The following import and export rules apply to the hello-paris-impl bundle:
  • Exporting own packages—none of the bundle's own packages should be exported; they are all private.
  • Importing own packages—none of the bundle's own packages should be imported.
  • Importing dependent packages—any external package dependencies must be imported.

Maven bundle plug-in settings

You can use the default export rules (that is, omitting the Export-Package instruction), as long as you take care to put all of your code into packages containing .impl or .internal (which are not exported by default). You should explicitly list the implemented API, org.fusesource.example.hello.paris, in the Import-Package instruction and add the provide:=true clause to it. This signals that this bundle is acting as the provider of the hello.paris package (and ensures that the API is imported with the correct version range—see Section 3.3, “Automatic Import Versioning”). In this case, the bundle plug-in instructions are as follows: 
<instructions>
  <Bundle-SymbolicName>${project.groupId}.${project.artifactId}</Bundle-SymbolicName>
  <Import-Package>
    ${project.groupId}.hello.paris*;provide:=true,
    *
  </Import-Package>
</instructions>

Generated MANIFEST.MF file

When you build the bundle using Maven, the Maven bundle plug-in automatically generates the following MANIFEST.MF file: 
Manifest-Version: 1.0
Bundle-Name: hello-paris-impl
Built-By: JBLOGGS
Build-Jdk: 1.5.0_08
Created-By: Apache Maven Bundle Plugin
Import-Package: org.fusesource.example.hello.paris;version="[1.0,1.1)"
 ,org.fusesource.example.time;version="[1.0,2)",org.osgi.service.blu
 eprint;version="[1.0.0,2.0.0)"
Export-Service: org.fusesource.example.hello.paris.HelloParis
Bundle-ManifestVersion: 2
Bundle-SymbolicName: org.fusesource.example.hello-paris-impl
Tool: Bnd-1.15.0
Bnd-LastModified: 1297357383645
Bundle-Version: 1.0.0
The Import-Package header imports external package dependencies only—for example, org.fusesource.example.hello.paris and org.fusesource.example.time.
The Export-Service header advertises the OSGi service as a HelloParis instance. This enables clients to find the HelloParisImpl instance by searching for a service of HelloParis type.