What is the difference between Component Factory and Service Factory in OSGi?

Difference Between Component Factory and Service Factory in OSGi

In OSGi, both Component Factory and Service Factory are mechanisms used to create multiple instances of a service or component. However, they serve different purposes and are used in different contexts.

1. Component Factory (OSGi Declarative Services - DS)

A Component Factory is used when you need to dynamically create and manage multiple instances of a Declarative Services (DS) component.

Key Characteristics :

* Used in Declarative Services (DS).
* Allows on-demand instantiation of a component.
* Explicitly managed by the application using ComponentFactory.
* Lifecycle is controlled manually by calling newInstance() and dispose().

Example of Component Factory

1. Declare a Factory Component

import org.osgi.service.component.annotations.Component;
import org.osgi.service.component.ComponentFactory;

@Component(
    service = ComponentFactory.class,
    factory = "com.example.MyComponentFactory"
)
public class MyComponent {
    public void activate() {
        System.out.println("Component instance created!");
    }
}

2. Create an Instance of the Component

import org.osgi.service.component.annotations.Reference;
import org.osgi.service.component.ComponentFactory;
import org.osgi.service.component.ComponentInstance;
import java.util.Dictionary;
import java.util.Hashtable;

public class MyComponentManager {

    @Reference(target = "(component.factory=com.example.MyComponentFactory)")
    private ComponentFactory<MyComponent> factory;

    public void createComponent() {
        Dictionary<String, Object> config = new Hashtable<>();
        ComponentInstance<MyComponent> instance = factory.newInstance(config);
        System.out.println("New component instance created dynamically!");
    }
}

* Use Case: When you need to create multiple independent instances of a component dynamically.

2. Service Factory (OSGi Service Registry)

A Service Factory is used when a service needs to provide different instances for different consuming bundles.

Key Characteristics :

* Used in the OSGi Service Registry.
* Allows per-bundle service instantiation.
* getService() provides a unique instance per consuming bundle.
* ungetService() allows cleanup when the service is no longer needed.

Example of a Service Factory

import org.osgi.framework.Bundle;
import org.osgi.framework.ServiceFactory;
import org.osgi.framework.ServiceRegistration;

public class MyServiceFactory implements ServiceFactory<MyService> {

    @Override
    public MyService getService(Bundle bundle, ServiceRegistration<MyService> registration) {
        System.out.println("Creating new service instance for bundle: " + bundle.getSymbolicName());
        return new MyServiceImpl();
    }

    @Override
    public void ungetService(Bundle bundle, ServiceRegistration<MyService> registration, MyService service) {
        System.out.println("Releasing service instance for bundle: " + bundle.getSymbolicName());
    }
}

* Use Case: When each bundle should receive its own instance of a service.

Key Differences at a Glance

Feature	Component Factory (DS)	Service Factory (Service Registry)
Context	OSGi Declarative Services (DS)	OSGi Service Registry
Instance Creation	Created explicitly using `ComponentFactory.newInstance()`	Created implicitly when a bundle requests the service
Instance Scope	Independent instances	One instance per consuming bundle
Lifecycle Management	Manually controlled	Managed by OSGi
Use Case	When multiple independent components need to be created dynamically	When each bundle should receive its own instance of a service