Mastering modularity - OSGi in Action - page 30

JFrame

JToolBar

JPanel

JComponent

Figure 2.4 The paint program is
a simple Swing application.

A static main() method on PaintFrame launches the paint program, which creates an
instance of the PaintFrame and each shape implementation, adding each shape
instance to the created PaintFrame instance. For further explanation, figure 2.5
captures the paint program classes and
their interrelationships.
To r u n t h i s n o n m o d u l a r v e r s i o n o f t h e
paint program, go into the chapter02/
paint-nonmodular/ directory of the com-
panion code. Type ant to build the pro-
gram, and then type java -jar main.jar
to run it. Feel free to click around and see
how it works; we won't go into any more
details of the program's implementation,
because GUI programming is beyond the
scope of this topic. The important point is to understand the structure of the program.
Using this understanding, you'll divide the program into bundles so you can enhance
and enforce its modularity.
Currently, the paint program is packaged as a single JAR file, which we'll call ver-
sion 1.0.0 of the program. Because everything is in a single JAR file, this implies that
the program isn't already modularized. Of course, single- JAR -file applications can still
be implemented in a modular way—just because an application is composed of multi-
ple JAR files, that doesn't mean it's modular. The paint program example could have
both its logical and physical modularity improved. First, we'll examine the program's
logical structure and define modules to enhance this structure. Where do you start?
One low-hanging fruit you can look for is public API s. It's good practice in OSG i
(you'll see why later) to separate your public API s into packages so they can be easily
shared without worrying about exposing implementation details. The paint program
has a good example of a public API : its SimpleShape interface. This interface makes it

1

Paint
frame

Simple
shape

1 1

1

Shape
component

Circle Square

Triangle

Figure 2.5 Paint program class relationships