What's New in Declarative Services 1.3?
I’d like to tell you about two really cool new features in OSGi Declarative Services. They’re so cool that I feel kind of guilty for blogging about them, because some of the kudos may accrue to me even though I have done absolutely none of the work to either specify or implement them. So bear in mind that I’m only your humble messenger.
Configuration Property Types
As a bit of background, in Declarative Services (DS) it has always been possible to write configurable components by passing a Map into the activate method:
There are some great things about the way DS handles configuration. First, the component itself has no idea about the origin of the config data… it could come from a properties file, or an XML file, or a database record, or transmitted over the air from a cellular provider, etc. Because the component is not coupled to the physical mechanism, we can change that mechanism easily without updating the code of all our components.
Second, the configuration is passed all-at-once and can be dynamically changed.
Some other DI frameworks inject configuration data one field at a time
setPort…). Where they support dynamic reconfiguration at all, they
tend to need methods to let the component know that a series of config changes
is starting and ending, so that it knows when it’s safe to reconfigure. Otherwise
the component could end up using a mixture of old and new config.
There is a problem, however. Did you notice that my code sample above had no
error handling at all? What if the
host property was missing from the map?
What if the port was given as a String rather than an integer? What if it
contained invalid numeric characters? Some fields may be optional, where do we
specify the defaults? It turns out that most components have to do quite a bit
of work to convert, parse and validate their configuration. There are libraries
that encapsulate this – such as the Configurable API from bnd – but
they add a runtime dependency, which is inconvenient (though manageable in OSGi
with static linking).
Declarative Services 1.3 – part of the OSGi Release 6 compendium spec, which was released to the public last week – improves this enormously. It’s now possible to define a custom type for the configuration, rather than a generic Map. For example:
Now the DS runtime is doing all the work of pulling out fields, converting them to the correct type and so on. If there are any conversion errors, or missing fields that don’t have defaults, then DS will refuse to load our component and will write an appropriate message into the OSGi Log.
This feature is known as “Configuration Property Types”. You may wonder why an annotation is used rather than an interface, given that we don’t actually use it like an annotation. There are a few reasons: in annotations it’s trivial to provide defaults; the methods cannot have parameters; and the return types allowed in an annotation are restricted to types that mesh well with configuration data.
Of course this version of DS interoperates with the OSGi Metatype specifcation, so that the administrators of our application can know what fields and types are actually expected. The metadata is generated at build time if we just add a couple more annotations:
… and, from this definition, tools like the Apache Felix Web Console can automatically generate an admin GUI for our component:
Prototype Scope Services
The second really cool feature is support for prototype scope in services. The prototype scope has been available in the OSGi Core R6 specification for about a year now, but there was a significant gap between the release of the Core and Compendium documents. So until now we had to drop down to the low-level OSGi API in order to use prototype scope. In DS 1.3 we can use annotations and a more convenient runtime API instead.
Prototype scope is where a service has the ability to create multiple instances on demand. Traditionally, most OSGi services were conceptually singletons: each registry entry corresponded to a single back-end service instance that was shared by all consumers. (Aside: there were additionally bundle-scoped services, where each consumer bundle got its own instance of the service object, but if you used a service multiple times from within the same bundle you would still share the same instance).
This model had to be enhanced, since sometimes each consumer really needs to have its own instance of the service. Also some consumers need to have programmatic control over the creation and destruction of instances, in order to fit in with an external lifecycle. An example of this would be in web requests – some web standards (for example JAX-RS) require services to be instantiated and then destroyed in every request. Hence the prototype scope was added. This is an opt-in feature: both the provider and the consumer of the service need to be aware that it is being used.
In DS 1.3, a provider can opt in to the prototype scope very easily with an attribute on the @Component annotation:
As a consumer there are a few more choices. When we consume a service with the @Reference annotation, we can use an attribute to request a new instance per component:
Using this annotation, each component that uses the Foo service will get its own instance. However within each component, the same Foo instance will be used repeatedly. Alternatively, the consumer can create and destroy instances programmatically whenever it wants – this requires the consumer to use a bit of API from DS as follows:
The use of the ComponentServiceObjects interface means we are no longer building strict POJOs, but still this interface is easily mockable for testing purposes.
The specification for DS 1.3 is available now from http://www.osgi.org/Download/Release6. Felix SCR 2.0 implements the specification and can be downloaded from http://felix.apache.org/downloads.cgi. For full metatype support you will need Felix Metatype 1.1 from the same page.
To build components using the annotations you will need a development build of bnd/Bndtools 3.0. To install this, go to http://bndtools.org/installation.html and follow the instructions for installing a developer snapshot build.