OSGi: service binding without lifecycle management - service

Question

I am building a Java application on the Equinox OSGi framework and I have been using DS (declarative services) to declare referenced and provided services. So far all the service consumers I have implemented happened to be service providers as well, so it was natural for me to make them stateless (so that they can be reused by multiple consumers, rather than being attached to one consumer) and let them be instantiated by the framework (default constructor, invoked nowhere in my code).
Now I have a different situation: I have a class MyClass that references a service MyService but is not itself a service provider. I need to be able to instantiate MyClass myself, rather than letting the OSGi framework instantiate it. I would then want the framework to pass the existing MyService instance to the MyClass instance(s). Something like this:
public class MyClass {
private String myString;
private int myInt;
private MyService myService;
public MyClass(String myString, int myInt) {
this.myString = myString;
this.myInt= myInt;
}
// bind
private void setMyService(MyService myService) {
this.myService = myService;
}
// unbind
private void unsetMyService(MyService myService) {
this.myService = null;
}
public void doStuff() {
if (myService != null) {
myService.doTheStuff();
} else {
// Some fallback mechanism
}
}
}
public class AnotherClass {
public void doSomething(String myString, int myInt) {
MyClass myClass = new MyClass(myString, myInt);
// At this point I would want the OSGi framework to invoke
// the setMyService method of myClass with an instance of
// MyService, if available.
myClass.doStuff();
}
}
My first attempt was to use DS to create a component definition for MyClass and reference MyService from there:
<scr:component xmlns:scr="http://www.osgi.org/xmlns/scr/v1.1.0" name="My Class">
<implementation class="my.package.MyClass"/>
<reference bind="setMyService" cardinality="0..1" interface="my.other.package.MyService" name="MyService" policy="static" unbind="unsetMyService"/>
</scr:component>
However, MyClass is not really a component, since I don't want its lifecycle to be managed -- I want to take care of instantiation myself. As Neil Bartlett points out here:
For example you could say that your component "depends on" a
particular service, in which case the component will only be created
and activated when that service is available -- and also it will be
destroyed when the service becomes unavailable.
This is not what I want. I want the binding without the lifecycle management.
[Note: Even if I set the cardinality to 0..1 (optional and unary), the framework will still try instantiate MyClass (and fail because of the lack of no-args constructor)]
So, my question: is there a way to use DS to have this "binding-only, no lifecycle management" functionality I'm looking for? If this is not possible with DS, what are the alternatives, and what would you recommend?
Update: use ServiceTracker (suggested by Neil Bartlett)
IMPORTANT: I've posted an improved version of this below as an answer. I'm just keeping this here for "historic" purposes.
I'm not sure how to apply ServiceTracker in this case. Would you use a static registry as shown below?
public class Activator implements BundleActivator {
private ServiceTracker<MyService, MyService> tracker;
#Override
public void start(BundleContext bundleContext) throws Exception {
MyServiceTrackerCustomizer customizer = new MyServiceTrackerCustomizer(bundleContext);
tracker = new ServiceTracker<MyService, MyService>(bundleContext, MyService.class, customizer);
tracker.open();
}
#Override
public void stop(BundleContext bundleContext) throws Exception {
tracker.close();
}
}
public class MyServiceTrackerCustomizer implements ServiceTrackerCustomizer<MyService, MyService> {
private BundleContext bundleContext;
public MyServiceTrackerCustomizer(BundleContext bundleContext) {
this.bundleContext = bundleContext;
}
#Override
public MyService addingService(ServiceReference<MyService> reference) {
MyService myService = bundleContext.getService(reference);
MyServiceRegistry.register(myService); // any better suggestion?
return myService;
}
#Override
public void modifiedService(ServiceReference<MyService> reference, MyService service) {
}
#Override
public void removedService(ServiceReference<MyService> reference, MyService service) {
bundleContext.ungetService(reference);
MyServiceRegistry.unregister(service); // any better suggestion?
}
}
public class MyServiceRegistry {
// I'm not sure about using a Set here... What if the MyService instances
// don't have proper equals and hashCode methods? But I need some way to
// compare services in isActive(MyService). Should I just express this
// need to implement equals and hashCode in the javadoc of the MyService
// interface? And if MyService is not defined by me, but is 3rd-party?
private static Set<MyService> myServices = new HashSet<MyService>();
public static void register(MyService service) {
myServices.add(service);
}
public static void unregister(MyService service) {
myServices.remove(service);
}
public static MyService getService() {
// Return whatever service the iterator returns first.
for (MyService service : myServices) {
return service;
}
return null;
}
public static boolean isActive(MyService service) {
return myServices.contains(service);
}
}
public class MyClass {
private String myString;
private int myInt;
private MyService myService;
public MyClass(String myString, int myInt) {
this.myString = myString;
this.myInt= myInt;
}
public void doStuff() {
// There's a race condition here: what if the service becomes
// inactive after I get it?
MyService myService = getMyService();
if (myService != null) {
myService.doTheStuff();
} else {
// Some fallback mechanism
}
}
protected MyService getMyService() {
if (myService != null && !MyServiceRegistry.isActive(myService)) {
myService = null;
}
if (myService == null) {
myService = MyServiceRegistry.getService();
}
return myService;
}
}
Is this how you would do it?
And could you comment on the questions I wrote in the comments above? That is:
Problems with Set if the service implementations don't properly implement equals and hashCode.
Race condition: the service may become inactive after my isActive check.

Answer 1

No this falls outside the scope of DS. If you want to directly instantiate the class yourself then you will have to use OSGi APIs like ServiceTracker to obtain the service references.
Update:
See the following suggested code. Obviously there are a lot of different ways to do this, depending on what you actually want to achieve.
public interface MyServiceProvider {
MyService getService();
}
...
public class MyClass {
private final MyServiceProvider serviceProvider;
public MyClass(MyServiceProvider serviceProvider) {
this.serviceProvider = serviceProvider;
}
void doStuff() {
MyService service = serviceProvider.getService();
if (service != null) {
// do stuff with service
}
}
}
...
public class ExampleActivator implements BundleActivator {
private MyServiceTracker tracker;
static class MyServiceTracker extends ServiceTracker<MyService,MyService> implements MyServiceProvider {
public MyServiceTracker(BundleContext context) {
super(context, MyService.class, null);
}
};
#Override
public void start(BundleContext context) throws Exception {
tracker = new MyServiceTracker(context);
tracker.open();
MyClass myClass = new MyClass(tracker);
// whatever you wanted to do with myClass
}
#Override
public void stop(BundleContext context) throws Exception {
tracker.close();
}
}

Answer 2

Solution: use ServiceTracker (as suggested by Neil Bartlett)
Note: if you want to see the reason for the downvote please see Neil's answer and our back-and-forth in its comments.
In the end I've solved it using ServiceTracker and a static registry (MyServiceRegistry), as shown below.
public class Activator implements BundleActivator {
private ServiceTracker<MyService, MyService> tracker;
#Override
public void start(BundleContext bundleContext) throws Exception {
MyServiceTrackerCustomizer customizer = new MyServiceTrackerCustomizer(bundleContext);
tracker = new ServiceTracker<MyService, MyService>(bundleContext, MyService.class, customizer);
tracker.open();
}
#Override
public void stop(BundleContext bundleContext) throws Exception {
tracker.close();
}
}
public class MyServiceTrackerCustomizer implements ServiceTrackerCustomizer<MyService, MyService> {
private BundleContext bundleContext;
public MyServiceTrackerCustomizer(BundleContext bundleContext) {
this.bundleContext = bundleContext;
}
#Override
public MyService addingService(ServiceReference<MyService> reference) {
MyService myService = bundleContext.getService(reference);
MyServiceRegistry.getInstance().register(myService);
return myService;
}
#Override
public void modifiedService(ServiceReference<MyService> reference, MyService service) {
}
#Override
public void removedService(ServiceReference<MyService> reference, MyService service) {
bundleContext.ungetService(reference);
MyServiceRegistry.getInstance().unregister(service);
}
}
/**
* A registry for services of type {#code <S>}.
*
* #param <S> Type of the services registered in this {#code ServiceRegistry}.<br>
* <strong>Important:</strong> implementations of {#code <S>} must implement
* {#link #equals(Object)} and {#link #hashCode()}
*/
public interface ServiceRegistry<S> {
/**
* Register service {#code service}.<br>
* If the service is already registered this method has no effect.
*
* #param service the service to register
*/
void register(S service);
/**
* Unregister service {#code service}.<br>
* If the service is not currently registered this method has no effect.
*
* #param service the service to unregister
*/
void unregister(S service);
/**
* Get an arbitrary service registered in the registry, or {#code null} if none are available.
* <p/>
* <strong>Important:</strong> note that a service may become inactive <i>after</i> it has been retrieved
* from the registry. To check whether a service is still active, use {#link #isActive(Object)}. Better
* still, if possible don't store a reference to the service but rather ask for a new one every time you
* need to use the service. Of course, the service may still become inactive between its retrieval from
* the registry and its use, but the likelihood of this is reduced and this way we also avoid holding
* references to inactive services, which would prevent them from being garbage-collected.
*
* #return an arbitrary service registered in the registry, or {#code null} if none are available.
*/
S getService();
/**
* Is {#code service} currently active (i.e., running, available for use)?
* <p/>
* <strong>Important:</strong> it is recommended <em>not</em> to store references to services, but rather
* to get a new one from the registry every time the service is needed -- please read more details in
* {#link #getService()}.
*
* #param service the service to check
* #return {#code true} if {#code service} is currently active; {#code false} otherwise
*/
boolean isActive(S service);
}
/**
* Implementation of {#link ServiceRegistry}.
*/
public class ServiceRegistryImpl<S> implements ServiceRegistry<S> {
/**
* Services that are currently registered.<br>
* <strong>Important:</strong> as noted in {#link ServiceRegistry}, implementations of {#code <S>} must
* implement {#link #equals(Object)} and {#link #hashCode()}; otherwise the {#link Set} will not work
* properly.
*/
private Set<S> myServices = new HashSet<S>();
#Override
public void register(S service) {
myServices.add(service);
}
#Override
public void unregister(S service) {
myServices.remove(service);
}
#Override
public S getService() {
// Return whatever service the iterator returns first.
for (S service : myServices) {
return service;
}
return null;
}
#Override
public boolean isActive(S service) {
return myServices.contains(service);
}
}
public class MyServiceRegistry extends ServiceRegistryImpl<MyService> {
private static final MyServiceRegistry instance = new MyServiceRegistry();
private MyServiceRegistry() {
// Singleton
}
public static MyServiceRegistry getInstance() {
return instance;
}
}
public class MyClass {
private String myString;
private int myInt;
public MyClass(String myString, int myInt) {
this.myString = myString;
this.myInt= myInt;
}
public void doStuff() {
MyService myService = MyServiceRegistry.getInstance().getService();
if (myService != null) {
myService.doTheStuff();
} else {
// Some fallback mechanism
}
}
}
If anyone wants to use this code for whatever purpose, go ahead.