Learning lifecycle - OSGi in Action - page 103

The constants indicate the name of the shape, the bundle resource file for the shape's
icon, and the bundle class name for the shape's class. The draw() method draws the
shape on the canvas,
From the constants, it's fairly straightforward to see how you'll describe a specific
shape implementation. You only need to know the name, an icon, and the class imple-
menting the shape. As an example, for the circle implementation you add the follow-
ing entries to its bundle manifest:

Extension-Name: Circle
Extension-Icon: org/foo/shape/circle/circle.png
Extension-Class: org.foo.shape.circle.Circle
The name is just a string, and the icon
and class refer to a resource file and a
class inside the bundle JAR file, respec-
tively. You add similar metadata to the
manifests of all shape implementation
bundles, which converts them all to
extensions. Next, you need to tweak the
architecture of the paint program to
make it cope with dynamic addition and
removal of shapes. Figure 3.18 captures
the updated design.
Comparing the new design to the
original, you add two new classes: Shape-
Tracker and DefaultShape . They help
you dynamically adapt the paint frame to
deal with SimpleShape implementations
dynamically appearing and disappearing. In a nutshell, the ShapeTracker is used to
track when extension bundles start or stop, in which case it adds or removes Default-
Shape s to/from the PaintFrame , respectively.
The concrete implementation of the ShapeTracker is a subclass of another class,
called BundleTracker . The latter class is a generic class for tracking when bundles are
started or stopped. Because BundleTracker is somewhat long, we'll divide it across
multiple listings; the first part is shown next.

Shape
tracker

Default
shape

1
1

1
1

1

Paint
frame

Simple
shape

1
1

1

Shape
component

Circle Square

Triangle

Figure 3.18 Dynamic paint program class
relationships

Listing 3.14 BundleTracker class declaration and constructor

package org.foo.paint;
import java.util.*;
import org.osgi.framework.*;

public abstract class BundleTracker {
final Set m_bundleSet = new HashSet();
final BundleContext m_context;
final SynchronousBundleListener m_listener;
boolean m_open;