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2.4. API Bundle

Overview

This section explains how to set up a Maven project for a typical API bundle.
The hello-paris bundle exemplifies an API bundle, which contains only public Java interfaces. Hence, the API bundle should export all of its own packages and associate a version number with the exported packages.

Directory structure

The hello-paris bundle has the following directory structure: 
hello-paris/
  |
  \--src/
     |
     \--main/
     |  |
     |  \--java/
     |     |
     |     \--org/fusesource/example/hello/paris/
     |        |
     |        \-HelloParis.java
     |   
     \--test/
The Java source code is located under the src/main/java sub-directory. The org.fusesource.example.hello.paris package is public and all of its classes and interfaces can be exported from the bundle.
There are no blueprint resources associated with this bundle.

Sample API

The hello-paris bundle is a pure API, which means it contains only Java interfaces. In this example, there is a single interface, HelloParis, which returns a localized greeting and a Clock object that tells the local time. The HelloParis interface is defined as follows: 
// Java
package org.fusesource.example.hello.paris;

import org.fusesource.example.time.Clock;

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

Maven dependencies

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

Import and export rules

The following import and export rules apply to the hello-paris bundle:
  • Exporting own packages—the org.fusesource.example.hello.paris package is public, and must be exported.
  • 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

The Maven bundle plug-in is configured to export the API package, org.fusesource.example.hello.paris (coded as ${project.groupId}.hello.paris*). The Export-Package instruction also contains entries to block the export of any packages containing .impl or .internal. In this case, the bundle plug-in instructions are as follows: 
<instructions>
  <Bundle-SymbolicName>${project.groupId}.${project.artifactId}</Bundle-SymbolicName>
  <Import-Package>*</Import-Package>
  <Export-Package>
    !${project.groupId}*.impl*,
    !${project.groupId}*.internal*,
	${project.groupId}.hello.paris*;version=${project.version}
  </Export-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
Built-By: JBLOGGS
Build-Jdk: 1.5.0_08
Created-By: Apache Maven Bundle Plugin
Import-Package: org.fusesource.example.time;version="[1.0,2)"
Bundle-ManifestVersion: 2
Bundle-SymbolicName: org.fusesource.example.hello-paris
Tool: Bnd-1.15.0
Bnd-LastModified: 1296826928285
Export-Package: org.fusesource.example.hello.paris;uses:="org.fusesour
 ce.example.time";version="1.0"
Bundle-Version: 1.0.0
The Import-Package header lists one external package dependency, org.fusesource.example.time. None of the bundle's own packages are imported.
The Export-Package header is used to export the API package, org.fusesource.example.hello.paris, while the uses clause declares a transitive dependency on the org.fusesource.example.time package.