Important concepts and terms analysis of the OSGI service condition framework
OSGI is a service -oriented modular framework that allows developers to use loose coupling components to build and manage modular services in the application.In OSGI, the service condition framework is a mechanism for managing and filtering services. It allows developers to choose services according to specific conditions.This article will analyze the important concepts and terms in the OSGI service condition framework, and provide the corresponding Java code example.
1. Service Condition: Service condition is an object to describe the service selection condition.It can choose specific services based on service attributes or other related conditions.The service condition is one of the core concepts of the ServiceTracker class.
Here are a simple service condition example. Select the corresponding service based on service attributes:
import org.osgi.framework.BundleContext;
import org.osgi.framework.ServiceReference;
import org.osgi.util.tracker.ServiceTracker;
public class MyServiceCondition extends ServiceTracker<SomeService, SomeService> {
public MyServiceCondition(BundleContext context) {
super(context, SomeService.class, null);
}
@Override
public SomeService addingService(ServiceReference<SomeService> reference) {
String version = (String) reference.getProperty("version");
// Select only services with specific version attributes
if ("1.0".equals(version)) {
SomeService service = super.addingService(reference);
// Processing service ...
return service;
}
return null;
}
}
In the above example, the MyServiceCondition class has extended the ServicesRacker class and rewritten the AddingService method.In this method, we can choose according to the attributes of the service, and only choose services with specific versions attributes.
2. Service Tracker: The service tracker is a class used to track and manage services, which plays an important role in the service condition framework.It can automatically track and manage services to register and cancel, and provide corresponding callback methods.
The following is an example of a simple service tracker:
import org.osgi.framework.BundleContext;
import org.osgi.framework.ServiceReference;
import org.osgi.util.tracker.ServiceTracker;
public class MyServiceTracker extends ServiceTracker<SomeService, SomeService> {
public MyServiceTracker(BundleContext context) {
super(context, SomeService.class, null);
}
@Override
public SomeService addingService(ServiceReference<SomeService> reference) {
SomeService service = super.addingService(reference);
// Treatment the newly added service ...
return service;
}
@Override
public void removedService(ServiceReference<SomeService> reference, SomeService service) {
// Treat the removal service ...
super.removedService(reference, service);
}
}
In the above example, the MyServiceTracker class inherits the ServiceTracker class and rewrite the AddingService and RemoveDservice methods.In these methods, we can handle newly added services and removed services.
3. Service Selector: The service selector is an object used to select the service according to a set of conditions.It uses service conditions to screen and select qualified services, and return a service selection result.
The following is a simple service selection example. It is used to select a service list with specific attributes:
import org.osgi.framework.BundleContext;
import org.osgi.framework.ServiceReference;
import org.osgi.util.tracker.ServiceTracker;
import org.osgi.util.tracker.ServiceTrackerCustomizer;
import java.util.ArrayList;
import java.util.List;
public class MyServiceSelector implements ServiceTrackerCustomizer<SomeService, SomeService> {
private List<SomeService> selectedServices = new ArrayList<>();
public MyServiceSelector(BundleContext context) {
ServiceTracker<SomeService, SomeService> tracker = new ServiceTracker<>(context, SomeService.class, this);
tracker.open();
}
@Override
public SomeService addingService(ServiceReference<SomeService> reference) {
SomeService service = reference.getBundle().getBundleContext().getService(reference);
String property = (String) reference.getProperty("property");
// Select only services with specific attributes
if ("value".equals(property)) {
selectedServices.add(service);
}
return service;
}
@Override
public void modifiedService(ServiceReference<SomeService> reference, SomeService service) {
// Processing service modification ...
}
@Override
public void removedService(ServiceReference<SomeService> reference, SomeService service) {
selectedServices.remove(service);
service.getBundle().getBundleContext().ungetService(reference);
}
public List<SomeService> getSelectedServices() {
return selectedServices;
}
}
In the above example, the MyServiceSelector class implements the ServiceTrackerCustomizer interface and rewrite the callback method.In the AddingService method, we can choose the service according to the attributes of the service, and add the qualified services to the SELECTEDSERVICES list.
These are some important concepts and terms in the OSGI service condition framework, and provide corresponding Java code examples.By understanding these concepts and examples, developers can better use and manage service conditions in OSGI.
