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2.6. API/Provider Combination

Overview
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This section explains how to set up a Maven project for an API/provider bundle.
The hello-boston bundle exemplifies an API/provider combination bundle. The API/provider bundle exports only its API packages, while the implementation packages are kept private. The API/provider bundle also instantiates and exports an OSGi service, which is accessed through the HelloBoston interface.

Directory structure
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The hello-boston bundle has the following directory structure: 
hello-boston/
  |
  \--src/
     |
     \--main/
     |  |
     |  \--java/
     |  |  |
     |  |  \--org/fusesource/example/hello/boston/
     |  |  |  |
     |  |  |  \--HelloBoston.java
     |  |  |
     |  |  \--org/fusesource/example/hello/boston/impl/
     |  |     |
     |  |     \--HelloBostonImpl.java
     |  |
     |  \--resources/
     |     |
     |     \--OSGI-INF/blueprint/
     |        |
     |        \--boston-svc.xml
     |   
     \--test/
The org.fusesource.example.hello.boston package is public and all of its classes and interfaces are exported from the bundle.
The org.fusesource.example.hello.boston.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, boston-svc.xml. Any file matching the pattern, *.xml, in this directory is assumed to be a blueprint configuration file.

Sample API
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The hello-boston bundle contains an API, which consists of the interface, HelloBoston. The HelloBoston interface returns a localized greeting and a Clock object that tells the local time. It is defined as follows: 
// Java
package org.fusesource.example.hello.boston;

import org.fusesource.example.time.Clock;

public interface HelloBoston {
    public String getGreeting();
    
    public Clock getLocalTime();
}

Sample implementation
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The hello-boston bundle also implements the API. In this example, a single HelloBostonImpl class implements the HelloBoston interface, as follows: 
// Java
package org.fusesource.example.hello.boston.impl;

import org.fusesource.example.hello.boston.HelloBoston;

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

public class HelloBostonImpl implements HelloBoston {
    protected Clock localTime = null;

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

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

}

Publish OSGi service
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The natural way to bootstrap the HelloBoston implementation in OSGi is to publish the class, HelloBostonImpl, as an OSGi service. Use the bean element to create a HelloBostonImpl instance and use the service element to publish the bean, advertising it as a service of HelloBoston type.
For example, the blueprint file, OSGI-INF/blueprint/boston-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-boston-impl"
        class="org.fusesource.example.hello.boston.impl.HelloBostonImpl"/>

    <service ref="hello-boston-impl"
        interface="org.fusesource.example.hello.boston.HelloBoston"/>

</blueprint>

Maven dependencies
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In the Maven POM file, the hello-boston bundle defines dependencies on the following Maven artifact:
  • time-util

Import and export rules
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The following import and export rules apply to the hello-boston bundle:
  • Exporting own packages—export the public API package, org.fusesource.example.hello.boston. Do not export the private package, org.fusesource.example.hello.boston.impl.
  • Importing own packages—import the public API package, org.fusesource.example.hello.boston.
  • Importing dependent packages—any external package dependencies must be imported.

Maven bundle plug-in settings
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You must export the API package, org.fusesource.example.hello.boston, by including it in the Export-Package instruction, and add the provide:=true clause to it. This signals that this bundle is acting as the provider of the hello.boston 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:

Example 2.2. Plug-In Settings for API/Provider Combination

<instructions>
  <Bundle-SymbolicName>${project.groupId}.${project.artifactId}</Bundle-SymbolicName>
  <Import-Package>*</Import-Package>
  <Export-Package>
    !${project.groupId}*.impl*,
    !${project.groupId}*.internal*,
    ${project.groupId}.hello.boston*;provide:=true;version=${project.version}
  </Export-Package>
</instructions>

Generated MANIFEST.MF file
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When you build the bundle using Maven, the Maven bundle plug-in automatically generates the following MANIFEST.MF file: 
Manifest-Version: 1.0
Built-By: JBLOGGS
Created-By: Apache Maven Bundle Plugin
Import-Package: org.fusesource.example.hello.boston;version="[1.0,1.1)
 ",org.fusesource.example.time;version="[1.0,2)",org.osgi.service.blue
 print;version="[1.0.0,2.0.0)"
Bnd-LastModified: 1297357441566
Export-Package: org.fusesource.example.hello.boston;uses:="org.fusesou
 rce.example.time";version="1.0"
Bundle-Version: 1.0.0
Bundle-Name: hello-boston
Build-Jdk: 1.5.0_08
Bundle-ManifestVersion: 2
Export-Service: org.fusesource.example.hello.boston.HelloBoston
Bundle-SymbolicName: org.fusesource.example.hello-boston
Tool: Bnd-1.15.0
The Import-Package header imports both the public API package, org.fusesource.example.hello.boston, and the external package dependencies—for example, org.fusesource.example.time.
The Export-Package header exports the public API package, org.fusesource.example.hello.boston, while the uses clause declares a transitive dependency on the org.fusesource.example.time package.
The Export-Service header advertises the OSGi service as a HelloBoston instance. This enables clients to find the HelloBostonImpl instance by searching for a service of HelloBoston type.
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