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# Declarative Services (DS)

Increasingly Declarative Services (DS) is the backbone of OSGi. While other Dependency Injection (DI) frameworks can be used with OSGi, the OSGi Alliance strongly recommend DS.

All of the enRoute examples are DS based and walk you from the simplest of DS components quickstart through a range of DS capabilities.

## Background

DS (a.k.a Service Component Runtime (SCR)) is an Extender Pattern that creates components from an XML resource in your bundle. Code annotations allow the toolchain to write the required XML on the fly so no direct interaction with XML is required. The XML file defines the Bundle’s dependencies, properties, and registered services; allowing DS to automatically instantiate the class, inject the dependencies, activate the component, and register the services.

## DS and Dynamism

The most significant and compelling difference between DS, and other dependency injection frameworks, is that DS has a lifecycle and handles dynamic dependencies: resource can be found and resource can be lost.

The concepts of time & change are utterly lacking in any other DI engines.

DS deals with the complexity of potentially volatile services through the use of strong guarantees.

### Ordering

DS provides a very strict ordering.

• Constructor – DS will always create a new object, it will never reuse an existing object.
• Bind – The bind methods or field injections are called in alphabetical order when using annotations. (Though dynamic methods or field injections can of course be called at any time.)
• Activate – Only if all static reference methods and field injections are called is the activate method called.
• If this method does not throw an exception, it is guaranteed that the deactivate will be called.
• If an exception is thrown the following phases are not executed.
• Active – During the active phase the following methods can be called in any order from any thread and in parallel:
• Any methods of the registered services.
• A modified methods that dynamically takes the modified configuration properties.
• Any of the updated reference methods if defined.
• Deactivate – Clean up
• Unbinds – And unbind methods are called.
• Release of object – DS will release the object so that no longer any references are held.
• Finalize – Java garbage collects the object.

Lazy services are registered before their constructor is called. The initialization of the DS component will take place when the service is used for the first time. However, this should not be observable by the component itself.

## Static References

The default and simplest model of DS is to use static references. If a component only has static references then it never sees any of the OSGi dynamics. This means that with the given ordering there is no need to use volatile or other synchronization constructs for static references.

## Optional References

Sometimes a component can deliver its functionality even when a reference is absent. This is then an optional reference. By far the simplest method to handle this is to make the reference optional by specifying the cardinality:

	@Component(service=ReluctantOptionalReference.class)
public class ReluctantOptionalReference {

@Reference(cardinality=ReferenceCardinality.OPTIONAL)
Foo reluctantOptionalReference;
}


However, this is a static reference. This implies that the component is started regardless of the presence of Foo. If Foo happens to be there then it is injected otherwise the field remains null. This model is called reluctant.

Unfortunately, this means we miss the Foo service when it is registered a few nanoseconds later. Since the static model has so many advantages there is an option to reconstruct the component when this reference finds a candidate. This is the greedy mode:

	@Component(service=GreedyOptionalReference.class)
public class GreedyOptionalReference {

@Reference(
cardinality=ReferenceCardinality.OPTIONAL,
policyOption=ReferencePolicyOption.GREEDY)
Foo greedyOptionalReference;
}


DS will now reconstruct the component when there is a better candidate for foo. Clearly any candidate will beat no candidate but what means better in the case that we already have foo?

When multiple candidates are available DS will sort them by ranking. Services with a higher ranking are deemed better. Service ranking is indicated by a property called service.ranking. It is an integer, higher is better.

One of the advantages of the static model is that in your activate method all the way to your deactivate method your visible world won’t change.

The previous examples were still static because none of the references changed between the activate and deactivate phase. The greedy policy option achieved its replacement by reconstructing the component. This is acceptable in most cases but sometimes the component does not want to die for the sake of an optional reference. In that case we can handle the situation dynamically.

By far the easiest solution is to mark the field as volatile. A volatile field will automatically get marked as policy=DYNAMIC.

	@Component(service=DynamicOptionalReference.class)
public class DynamicOptionalReference {

@Reference(cardinality=ReferenceCardinality.OPTIONAL)
volatile Foo dynamicOptionalReference;
}


This is simple but there is an obvious price. The following bad code shows a common (but horrible) pattern that people use to use foo:

	if ( foo != null ) // BAD!
foo.bar();


This innocuous looking code is actually a Null Pointer Exception in the waiting. A better way is to do:

	Foo foo = this.foo;
if ( foo != null )
foo.bar();


By using a local variable we guarantee that the check (is foo null?) is using the same object as the one we will call bar() on. This is a very cheap form of synchronization.

## What if the Service Disappears?

Your code should always be prepared to accept exceptions when you call other services. This does not mean you should catch them, on the contrary. It is much better to forward the exceptions to the caller so that they do not unnecessarily get wrapped up in wrapping exceptions and lose the original context.

In almost all cases there is a top level function that initiated your request. It is this function that has the responsibility to make sure the overall system keeps working regardless of failures. This kind of robustness code is extremely difficult to get right and should never be mixed with application code.

## Tracking Multiple Services

If you use a whiteboard pattern or other listener like model then in general you want to use dynamics.

The reason is that you have multiple references and building and destroying the component at every change in the set of services we’re interested in (the tracked services) becomes expensive.

By far the easiest method is to use field injection of a list of services. If you make this field volatile then DS will inject a new list whenever the set of tracked services changes.

	@Component(service=SimpleList.class)
public class SimpleList {

@Reference
volatile List<Foo>		dynamicFoos;
}


However, there are scenarios where the component must interact with the bind and unbind of the references. The most common way is then to create a bind and unbind method.

	@Component(service = DynamicBindUnbind.class)
public class DynamicBindUnbind {

final List<Foo> foos = new CopyOnWriteArrayList<>();

@Reference(
cardinality = ReferenceCardinality.MULTIPLE,
policy = ReferencePolicy.DYNAMIC)

In this example we use a CopyOnWriteArrayList. This is a so called non-locking object. Though it is perfectly safe to use in a concurrent environment it will not use locks and any iteration over that list is guaranteed not to fail.