How to obtain OSGi service references from a Servlet that is registered in OSGi HttpService?

Question

It seems natural that a HttpServlet running in OSGi environment (i.e. registered in OSGi HttpService) would want to call some OSGi services to accomplish it's tasks. The question is how to obtain references to these OSGi service inside the servlet.

One way would be to inject dependencies into the HttpServlet instance that is being registered to the OSGi HttpService like this:

MyServlet servlet = new MyServlet();
servlet.setFooService(fooService);

httpService.registerServlet("/myservlet", servlet, initparams, context);

I'm not sure if this is a valid approach since in non-OSGi environment the servlet life-cycle is managed by the Web Container and hence the service reference would not be injected for the servlet instances created later on.

There is another way to solve this when using PAX Web as an implementation of the OSGi HttpService. PAX Web exports the OSGi BundleContext into the ServletContext as a special attribute "osgi-bundlecontext". The BundleContext can then be used to obtain necessary service references:

public void init(ServletConfig servletConfig) throws ServletException {

    ServletContext context = servletConfig.getServletContext()
    BundleContext bundleContext = 
        (BundleContext) context.getAttribute("osgi-bundlecontext");

    ServiceReference serviceRef =
         bundleContext.getServiceReference("com.foo.FooService")
}

However this approach is rather ugly and ties you to a concrete implementation of the OSGi HttpService. Do you know any other (and possibly better) solution to this problem?

Answer 1

If you use a setter for the dependency on the service, as you have shown, it can work outside of OSGi as well. You just need to use some other dependency injection mechanism. If there is none, you could provide a subclass that initializes the servlet using JNDI lookups or from the servlet context.

public class MyServlet_AdapterForMissingDI extends MyServlet{

    public void init(ServletConfig config){
        setFooService(getItFromSomewhere());
    }

}

The point being that if you have DI capabilities that can inject setFooService, you can just use the same servlet in OSGi and elsewhere, if you do not (and still want to support this case), you provide an adapter.

On a related note, check out Felix SCR to configure your object's dependencies, and Pax Web Extender Whiteboard, which takes care of hooking your servlet up with the HttpService.

Specifically, without SCR and Whiteboard, you need to think about the case when the fooService becomes unavailable later, or the HttpService gets started after your servlet. In these cases your servlet would have a reference to a dead service that prevents the bundle from being garbage-collected, or your servlet would not be registered with the HttpService.

Update: Here is the SCR descriptor I use for one of my servlets. SCR handles servlet instantiation, life-cycle, registration (via Whiteboard), and dependencies. There is no OSGi-specific code in the servlet. There is not even the need for a BundleActivator anymore (SCR registers all services):

<component name="oracle.statusServlet" >
<implementation class="mypackage.DataSourceStatusServlet"/>
<property name="service.description" value="Oracle DataSource status servlet" />
<property name="alias" value="/OracleDataSourceStatus" />
<property name="servlet-name" value="Oracle DataSource status servlet" />
<service>
    <provide interface="javax.servlet.Servlet" />
</service>
    <reference name="DATASOURCES" 
            interface="javax.sql.DataSource"
            cardinality="0..n" policy="dynamic" 
            bind="bindDataSource" unbind="unbindDataSource"/>

</component>

The dependencies for the servlet are specified in the reference tag. SCR will do the service lookup and binding.