Question: I have a question about declarative OSGi Services. I have the following interface : 

```
  public interface PrintService {
    public void print();
  }

```

and its implementation: 

```
    public class PrintServiceImpl implements PrintService {

      @Override
      public void print() {
        System.out.println("Hello from PrintServiceImpl!");
      }
    }

```

OSGI-INF/component.xml :

```
<scr:component xmlns:scr="http://www.osgi.org/xmlns/scr/v1.1.0" name="service">
   <implementation class="service.PrintServiceImpl"/>
   <service>
      <provide interface="print.PrintService"/>
   </service>
</scr:component>

```

MANIFEST.MF :

```
Service-Component: OSGI-INF/component.xml

```

After I install the service and start it nothing happend. How can I activate it and print "*Hello from PrintServiceImpl!*" to console.

Answer: 
Why do you expect your `print` method to be invoked? It is part of the interface of the service, so it will not be invoked until you can a client that binds to it and calls it.

If you type the `services` command in the console you should see that your bundle is publishing the `print.PrintService` service; this means that your component is working. If you don't see this then you may be missing something like the SCR bundle as suggested by Tom Seidel in the comments above.
You should make explicit that you want you service to start when the bundle is started. Otherwise, it will be instantiated lazily, and give that no other service requires the PrintService, it will not be instantiated at all.

You can control your service lifecycle (with regard to the bundle lifecycle) using the attributes `enabled` and `immediate`. `enabled` is trivial: indicates whether your service is enabled or not when the bundle is started. `immediate` indicates whether this service is lazy ot not. `immediate="true"` will start your service *immediatly*, `immediate="false"` will apply lazy instantiation (i.e. wait until some other service requests this service as a dependency)

e.g.

```
<scr:component xmlns:scr="http://www.osgi.org/xmlns/scr/v1.1.0" name="PrintService"
activate="activate" 
deactivate="deactivate"
modified="modified"
enabled="true" 
immediate="true">

```

As Chris mentioned, if you want your service to do something when it's activated, you should add some code to the `activate` lifecycle method (note that the name of the method can be specified in the component descriptor like it was done explicitly above)

```
protected final void activate() {
    print();
}

```