Advanced component frameworks - OSGi in Action - page 392

Byte-code weaving —i POJO instruments component byte code, which enables it to
provide features not possible (or easily possible) with other approaches.

■

Metadata format agnosticism —Whereas other approaches force you into using
the single approach they support to describe your components ( XML , annota-
tions, or API ), i POJO allows you to use any of these approaches.

■

High level of extensibility —The component man-
agement features provided by the i POJO com-
ponent container are implemented by handlers
from which you can pick and choose. You can
also create custom handlers for specific man-
agement tasks; see figure 12.4.

■

Handler

Container

POJO

In most of the remainder of this chapter, we'll explore
the features of i POJO , but we won't cover everything.
For starters, we'll focus on using the annotation
approach for describing components, because you've
already seen enough XML and API . But keep in mind
that everything you do with annotations you can do
with the XML - and API -based description approaches—
it depends on your preference.

Handler

Figure 12.4 iPOJO components
are an aggregation of handlers
attached to the component
container at execution time.

12.2.1 Building iPOJO components

i POJO uses byte-code manipulation to instrument component class files. This instru-
mentation inserts hooks into the component class file so it can be externally man-
aged. Although i POJO also supports execution-time byte-code instrumentation, the
simplest way to get it done is with a build-time step to process your components. To
achieve this, i POJO integrates with Ant, Maven, and Eclipse. As an example, here's the
Ant task for the circle bundle:

<taskdef name="ipojo"
classname="org.apache.felix.ipojo.task.iPojoTask"
classpath="${lib}/felix/org.apache.felix.ipojo.ant-1.6.0.jar" />
<ipojo input="${dist}/${ant.project.name}-${version}.jar"
metadata = "OSGI-INF/circle.xml"/>
Upon completion of this build step, i POJO has instrumented the byte code of any
components contained in the referenced bundle. The details of how i POJO instru-
ments the component byte code aren't as important; but for the curious, i POJO instru-
ments all components in a single, generic way to enable intercepting member field
and method accesses. All functionality provided by the i POJO component framework
(providing services, requiring services, and so on) is provided by handlers using these
interception hooks. The hooks themselves don't change the component behavior; the
modified component classes behave the same as before and can still be use without
i POJO , although they now have some code dependencies on it.