Component models and frameworks - OSGi in Action - page 363

cardinality="0..1"
bind="bindLog"
unbind="unbindLog"/>
</scr:component>
The WindowListener component has a static, singular, and mandatory dependency on
a Window service B . You specify a target LDAP filter to locate the specific Window service
of interest C ; recall that the filter references the property you associated with the
PaintFrame component's Window service in its component description in listing 11.3.
Additionally, the WindowListener component has a dynamic, singular, and optional
dependency on a log service.
Target reference properties
You saw earlier that component properties can be used to define the service proper-
ties associated with a component's provided service. Component properties can also
be used to configure service-dependency target filters at execution time. To do this,
the property name must be equal to the name associated with the service reference
appended with .target . In this case, you could override the window target using a
property of this form:
<property name="window.target" value="(name=other)" />
This binds the window listener to windows attributed with the name=other identifier.
Doing this directly in the static component description is of relatively low value. But
if you remember the discussion on component properties, these values can also be
set at execution time via the Configuration Admin Service or using component facto-
ries, which opens up a set of interesting use cases.
Filtering services based on attributes is relatively easy and, at first glance, dealing with
optional services appears equally easy. But there are some subtle mechanics of which
you need to be aware when using the dynamic dependency policy. Here are the rele-
vant lines from the WindowListener component.
Listing 11.5 WindowListener with optional LogService dependency
public class WindowListener extends WindowAdapter {
...
private AtomicReference<LogService> logRef =
new AtomicReference<LogService>();
protected void bindLog(LogService log) {
logRef.compareAndSet(null, log);
}
protected void unbindLog(LogService log) {
logRef.compareAndSet(log, null);
}
...
private void log(int level, String msg) {
LogService log = logRef.get();