Learning lifecycle - OSGi in Action - page 102

Install
bundle

Bundle JAR

Installed
bundles

Register
bundle listener

Bundle start
event

Tracker
Inject

Interrogate for metadata resources, classes, and so on

Shape
Impl

Starting shape bundle resolves it

Frame

For the reverse, if the shape bundle is stopped,
tracker removes its associated shape

Figure 3.16 Extender pattern overview

Paint Export

org.foo.shape

Shape

Extension-Name:
Circle
Extension-Class:
org.foo.shape.circle.Circle
Extension-Icon:
org/foo/shape/circle/circle.png

Circle

Square

Figure 3.7 Paint program
as an implementation of the
extender pattern

Triangle

Let's dive in and start converting the application. The first thing you need to do is
define the extension metadata for shape bundles to describe their shape implementa-
tion. In the following snippet, you add a couple of constants to the SimpleShape inter-
face for extension metadata property names; it's not strictly necessary to add these,
but it's good programming practice to use constants:

package org.foo.shape;

import java.awt.Graphics2D;
import java.awt.Point;

public interface SimpleShape {
public static final String NAME_PROPERTY = "Extension-Name";
public static final String ICON_PROPERTY = "Extension-Icon";
public static final String CLASS_PROPERTY = "Extension-Class";

public void draw(Graphics2D g2, Point p);
}