Java Reference
In-Depth Information
Listing 6.4
Declaring special offer service that has an Inventory dependency
<?xml version='1.0' encoding='utf-8'?>
<component name='fancyfoods.cheese.offer' enabled='true'>
class='fancyfoods.department.cheese.offers.DesperateCheeseOffer'/ >
<provide interface='fancyfoods.offers.SpecialOffer'/>
Expose this
as service
<reference name='inventory' interface=''
bind='setInventory' unbind='unsetInventory' />
Inject Inventory instance using
named set and unset methods
Because it's an older technology than Blueprint, Declarative Services has a wider
range of tooling support. Some of the more popular tools, such as bnd and bndtools,
support both of them, which makes it convenient to use whichever dependency injec-
tion technology you prefer. (More on bnd can be found in chapter 8.)
One limitation of both Declarative Services and Blueprint is that, in their original,
strictly specified forms, neither supported annotations. Annotations are a frequent
feature request for both Declarative Services and Blueprint, and unofficial support
has been developed for both platforms. Annotation support for Blueprint has been
prototyped in Apache Aries, but not developed further at the time of writing. Anno-
tation support for Declarative Services is available in Apache Felix's SCR subproject.
The SCR support has now been standardized. As well as being more concise than
XML metadata (which isn't hard!), annotations are less brittle if you refactor your
code frequently.
Felix's annotation support works by using a Maven plug-in to convert the annotations
to normal Declarative Services XML at compile time. To use the support, you'll first need
to add a Maven dependency on the maven-scr-plugin . The annotation syntax is rich,
supporting almost everything that can be specified in DS XML , but in its simplest form,
to identify a class as a component and expose it as a service, you'd write this:
public class DesperateCheeseOffer implements SpecialOffer {
To inject a reference to another service, it's even simpler:
Inventory inventory;
Handy as it is, the Felix SCR annotation support does feel slightly tacked-on. Although
Declarative Services is part of the existing OSG i specifications, annotations aren't
(although there are plans to add them soon). Other dependency injection frame-
works support both XML and annotations as first-class programming models.
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