AEM OSGi Components & Services Sample code

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In this answer, we'll go through the code-level implementation of creating OSGi components, services, and connecting them with REST API services. We'll be using Java and the Apache Felix framework for this purpose.

First, let's start by creating an OSGi component. OSGi components are Java classes that implement a specific interface and have metadata that specifies their dependencies and capabilities. To create an OSGi component, we'll create a Java class that implements the org.osgi.service.component.Component interface.

Here's an example of an OSGi component that implements the Component interface:



import org.osgi.service.component.ComponentContext;
import org.osgi.service.component.annotations.*;

@Component(service = MyService.class)
public class MyComponent implements MyService {

@Activate
public void activate(ComponentContext context) {
// component activation code here
}

@Deactivate
public void deactivate(ComponentContext context) {
// component deactivation code here
}

@Override
public void doSomething() {
// service implementation code here
}
}
In this example, we've created an OSGi component that implements the MyService interface and provides it as an OSGi service. The @Component annotation is used to specify that this class is an OSGi component and should be registered as a service. The @Activate and @Deactivate annotations are used to specify methods that should be called when the component is activated and deactivated, respectively.

Now let's create an OSGi service that uses the MyComponent component. OSGi services are Java interfaces that define a set of methods that can be called by other components. To create an OSGi service, we'll create a Java interface that defines the service's methods.

Here's an example of an OSGi service interface:


public interface MyService {
void doSomething();
}
In this example, we've created an OSGi service interface called MyService that defines a single method called doSomething().

Finally, let's create a REST API service that uses the MyService OSGi service. We'll be using the Apache Felix HTTP Service for this purpose, which provides a simple way to expose OSGi services as REST API endpoints.

Here's an example of a REST API service that uses the MyService OSGi service:


import org.osgi.service.component.annotations.Component;
import org.osgi.service.component.annotations.Reference;
import org.osgi.service.http.HttpService;
import org.osgi.service.http.NamespaceException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.PrintWriter;

@Component(service = HttpServlet.class, property = {"osgi.http.whiteboard.servlet.pattern=/my-service"})
public class MyServlet extends HttpServlet {

@Reference
private MyService myService;

@Reference
private HttpService httpService;

public void activate() throws ServletException, NamespaceException {
httpService.registerServlet("/my-service", this, null, null);
}

public void deactivate() {
httpService.unregister("/my-service");
}

@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
PrintWriter writer = resp.getWriter();
writer.println("Hello, world!");
myService.doSomething();
}
}
In this example, we've created a REST API service that uses the MyService OSGi service to perform some functionality when a GET request is received. The @Component annotation is used to specify that this class is an OSGi component and should be registered as a service.