OSGI - two objects of a bundle service - service

Question

I have a bundle that provides a service.
My bundle implementation looks like this:
class ServiceImpl implements Service
{
Object value;
#Override
public void setValue(Object value)
{
this.value = value;
}
#Override
public Object getValue()
{
return value;
}
}
In my java application, I load this bundle to OSGI framework, and create TWO references to the service, in an attempt to have two objects with different values for "value".
Unfortunately, this does not seem to work. The service always returns the last value set by either objects. How can I overcome this issue?
Here's an example for the problem:
Service object1 = context.getService(reference1);
Service object2 = context.getService(reference2);
Integer one= 1;
Integer two =2;
object1.setValue(1);
object2.setValue(2);
System.out.println(object1.getValue() ); //returns 2 !!!!!!!!!!!!!!!!!!
System.out.println(object2.getValue() ); //returns 2
I used ServiceFactory but it seems not useful for my case. What should I do? Thanks.

Answer 1

Both BJ and Balazs offer valuable information, but no solution that works with current versions of the OSGi specification.
What you can do is register your service with a second "Factory" interface. This factory then allows you to create instances of the service. Because you probably don't want to do that manually, you can hide this logic in a ServiceTracker.
There are a few "downsides" to this approach. First of all, you need to register the service and have the instance implement both Factory and Service. Secondly, you always have to use this custom ServiceTracker to access it. If you use a dependency manager that allows you to extend its dependencies (such as Apache Felix Dependency Manager) you can easily hide all of this in a custom ServiceDependency.
Anyway, to show you that this actually works, here is a simple example:
public class Activator implements BundleActivator {
#Override
public void start(final BundleContext context) throws Exception {
context.registerService(Service.class.getName(), new FactoryImpl(), null);
ServiceTrackerCustomizer customizer = new ServiceTrackerCustomizer() {
#Override
public Object addingService(ServiceReference reference) {
Object service = context.getService(reference);
if (service instanceof Factory) {
return ((Factory) service).createInstance();
}
return service;
}
#Override
public void modifiedService(ServiceReference reference,
Object service) {
// TODO Auto-generated method stub
}
#Override
public void removedService(ServiceReference reference,
Object service) {
// TODO Auto-generated method stub
}
};
ServiceTracker st1 = new ServiceTracker(context, Service.class.getName(), customizer);
ServiceTracker st2 = new ServiceTracker(context, Service.class.getName(), customizer);
st1.open();
st2.open();
Service s1 = (Service) st1.getService();
Service s2 = (Service) st2.getService();
s1.setValue("test1");
s2.setValue("test2");
System.out.println(s1.getValue());
System.out.println(s2.getValue());
}
#Override
public void stop(BundleContext context) throws Exception {
}
static interface Factory {
public Object createInstance();
}
static class FactoryImpl extends ServiceImpl implements Factory, Service {
#Override
public Object createInstance() {
return new ServiceImpl();
}
}
static interface Service {
public void setValue(Object value);
public Object getValue();
}
static class ServiceImpl implements Service {
private Object m_value;
#Override
public void setValue(Object value) {
m_value = value;
}
#Override
public Object getValue() {
return m_value;
}
}
}

Answer 2

You need to wait for R6. Pre-R6, each bundle can be exposed to at most one instance of a service. Even registering a ServiceFactory will not change that since the framework will cache the service object from the ServiceFactory to return to the bundle on subsequent calls to getService.
In R6, we introduce service scopes which allows a service implementation to return multiple service objects to a bundle. Using this requires both the service provider and the service consumer to use new API added in R6.
You can play with this now as it is implemented in Eclipse Equinox Luna.

Answer 3

Even if you use ServiceFactory, for the same bundle the same service object will be returned.
There might be a PrototypeServiceFactory in the future as there is an RFP about it: https://github.com/osgi/design/tree/master/rfcs/rfc0195
That would fit to your needs.
Although there might be a PrototypeServiceFactory in the future, I think it is better to solve this use-case programmatically by yourself. E.g.:
Instead of creating a mutuable OSGi service (I do not think creating mutuable services is a good idea) create a factory.
On the client side you would use:
BusinessLogicFactory factory = context.getService(reference);
BusinessLogic object1 = factory.createInstance();
BusinessLogic object2 = factory.createInstance();
...