OSGi Service Ranking, Filter, and Target are important concepts in AEM when it comes to managing and using OSGi services. Here is a detailed explanation of each concept:
OSGi Service Ranking
When multiple services are registered for a particular interface, OSGi uses the service ranking to determine which service to use. The service with the highest ranking number is selected.
- The ranking number is specified in the OSGi @Service annotation.
- If no ranking is specified, the default ranking is 0.
The service ranking is used by the OSGi framework to determine which service implementation to use when multiple services are registered for the same interface. The service with the highest ranking number is selected.
For example, let’s say we have two service implementations for the same interface, MyService. One implementation has a service ranking of 100, and the other implementation has a service ranking of 200. In this case, the implementation with the service ranking of 200 will be selected.
To specify the service ranking number in AEM, we can use the @Service annotation as follows:
@Service(ranking = 200)
@Component
public class MyServiceImpl implements MyService {
// implementation
}
In this example, the MyServiceImpl service implementation has a service ranking of 200. If there are other implementations of the MyService interface, the implementation with the highest service ranking will be selected.
It’s important to note that if no service ranking is specified, the default ranking is 0. So, if multiple service implementations have the same ranking, the OSGi framework will choose one based on its own criteria, such as the order in which they were registered. Therefore, it’s always a good practice to specify a service ranking for each service implementation in AEM to ensure the desired implementation is used.
OSGi Service Filter
OSGi Service Filter is another important concept in AEM when it comes to working with OSGi services. A service filter is used to select a specific service implementation based on certain criteria.
In AEM, we can use the @Reference annotation to specify the service filter for a particular service reference. The filter is specified using LDAP syntax and can include attributes such as the service interface, service properties, and service ranking.
For example, let’s say we have multiple service implementations for the MyService interface, but we only want to use the implementation that has a specific property value. We can specify a service filter to select that implementation, as shown in the following code snippet:
@Reference(target="(myProperty=myValue)") private MyService myService;
In this example, the @Reference annotation specifies a service filter that selects the service implementation that has a property named “myProperty” with a value of “myValue”.
It’s important to note that the service filter syntax is based on LDAP syntax, so it can be a bit tricky to get right. There are many resources available online to help with constructing service filters.
OSGi Service Target
In AEM, OSGi Service Target is used to specify the name of a specific OSGi service to be used when multiple services are registered for the same interface. The service target is specified in the @Reference annotation.
For example, let’s say we have two service implementations for the same interface, MyService, and we want to use a specific implementation in our component. We can use the OSGi Service Target to specify the name of the service implementation we want to use.
Here’s an example:
@Component
public class MyComponent {
@Reference(target = "(service.pid=com.mycompany.serviceImpl2)")
private MyService myService;
// implementation
}
In this example, the @Reference annotation is used to inject the MyService interface into the MyComponent class. The target attribute is used to specify the name of the OSGi service implementation we want to use. In this case, we are specifying the service implementation with the PID (Persistent Identifier) “com.mycompany.serviceImpl2”.
It’s important to note that the service target must be specified using the service name or the service PID. Also, if the specified service is not available, the component will not activate, and an error will be logged.
