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2.3. Library Bundle

Overview
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This section explains how to set up a Maven project for a typical library bundle.
The time-util bundle exemplifies a library bundle, where the main purpose of a library is to make interfaces and classes available to other bundles. Hence, the library should export all of its own packages and associate a version number with the exported packages.

Directory structure
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Assuming it is built as a Maven project, the time-util bundle has the following directory structure: 
time-util/
  |
  \--src/
     |
     \--main/
     |  |
     |  \--java/
     |     |
     |     \--org/fusesource/example/time/
     |        |
     |        \-TimeUtil.java
     |        |
     |        \-Clock.java
     |   
     \--test/
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The Java source code is located under the src/main/java sub-directory. The org.fusesource.example.time 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 implementation
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The time-util bundle is essentially a wrapper around some of the standard time utilities in Java. It provides a Clock class, which returns the local time in a particular time zone when you invoke the Clock.getLocalTime() method. The Clock class is defined as follows: 
// Java
package org.fusesource.example.time;

import java.util.Calendar;
import java.util.Locale;
import java.util.TimeZone;

public class Clock {
    ...    
    public Clock(TimeUtil.TimeZone tz) {
        ...
    }
    
    public String getLocalTime() {
        return jcal.getTime().toString();
    }
}
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The TimeUtil class is a factory that is used to create Clock instances for particular time zones. Two time zones are supported: TimeZone.BOSTON and TimeZone.PARIS. The TimeUtil class is defined as follows: 
// Java
package org.fusesource.example.time;

public class TimeUtil {
  public enum TimeZone {
    BOSTON,
    PARIS
  }

  public static Clock createClock(TimeZone tz) {
      return new Clock(tz);
  }
}
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Import and export rules
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The following import and export rules apply to the time-util bundle:
  • Exporting own packages—the org.fusesource.example.time package is public, and must be exported.
  • Importing own packages—none of the bundle's own packages should be imported, which is the usual case for a library bundle.
  • Importing dependent packages—any external package dependencies must be imported. In this particular example, however, there are none (the time-util bundle depends only on classes from the JVM).

Maven bundle plug-in settings
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The Maven bundle plug-in is configured to export the library package, org.fusesource.example.time (coded as ${project.groupId}.time). The Export-Package instruction also contains entries to block the export of any packages containing .impl or .internal. In this case, the bundle plug-in instructions are as follows: 
<instructions>
  <Bundle-SymbolicName>${project.groupId}.${project.artifactId}</Bundle-SymbolicName>
  <Import-Package>*</Import-Package>
  <Export-Package>
    !${project.groupId}*.impl*,
    !${project.groupId}*.internal*,
    ${project.groupId}.time*;version=${project.version}
  </Export-Package>
</instructions>
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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
Bundle-Name: time-util
Built-By: JBLOGGS
Build-Jdk: 1.5.0_08
Created-By: Apache Maven Bundle Plugin
Bundle-ManifestVersion: 2
Bundle-SymbolicName: org.fusesource.example.time-util
Tool: Bnd-1.15.0
Bnd-LastModified: 1297357076457
Export-Package: org.fusesource.example.time;version="1.0"
Bundle-Version: 1.0.0
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