Proof of Concept: GWT, Restlet, OSGi and Maven

After some difficulties I got a project working, which consists of a simple GWT application with a Restlet backend. The application consists of four OSGi bundles, which work inside eclipse PDE. Also, all of these modules and their dependencies are declared using Maven 2.

Overview:

The user interface is based on google‘s standard project from the google eclipse plugin.

The button Ping triggers a very simple REST call to the server (based on example from Restlet documentation)

tstBtn.addClickHandler(new ClickHandler() {

                        @Override
                        public void onClick(ClickEvent event) {
                         ClientResource r = new ClientResource(“/ping”);

                         r.setOnResponse(new Uniform() {
                         public void handle(Request request, Response response) {
                         try {
                         tstBtn.setText(response.getEntity().getText());
                         } catch (IOException e) {
                         e.printStackTrace();
                         }
                         }
                         });
                         r.get();
                                
                        }
                        
                });

Clicking this button should result in the caption of the button being filled with a response from the server.

The server resource is as following:

public class PingResource extends ServerResource {

@Get(“txt”)
public String toText() {
StringBuilder sb = new StringBuilder(“Restlet server alive. Method: “);
sb.append(getRequest().getMethod());

ChallengeResponse challengeResponse = getRequest()
.getChallengeResponse();
if (challengeResponse != null) {
sb.append(“/ Auth. scheme: “);
sb.append(challengeResponse.getScheme());
}

return sb.toString();
}
}

The send button should work in the same way as in the google example; only that the remote procedure call is facilitated using Restlet.

GWT Client:

GreetingServiceAsync r = GWT.create(GreetingServiceAsync.class);
                                r.getClientResource().setReference(“/services/greet”);
                                r.getClientResource().getClientInfo().getAcceptedMediaTypes().add(new
                                                Preference<MediaType>(MediaType.APPLICATION_JAVA_OBJECT_GWT));        
                                
                                r.greetServer(new Name(nameField.getText()), new AsyncCallback<String>() {

                                        @Override
                                        public void onFailure(Throwable caught) {
                                                // Show the RPC error message to the user
                                                dialogBox
                                                                .setText(“Remote Procedure Call – Failure”);
                                                serverResponseLabel
                                                                .addStyleName(“serverResponseLabelError”);
                                                serverResponseLabel.setHTML(SERVER_ERROR+“<br/>”+caught.getMessage());
                                                dialogBox.center();
                                                closeButton.setFocus(true);
                                                
                                        }

                                        @Override
                                        public void onSuccess(String result) {
                                                dialogBox.setText(“Remote Procedure Call”);
                                                serverResponseLabel
                                                                .removeStyleName(“serverResponseLabelError”);
                                                serverResponseLabel.setHTML(result);
                                                dialogBox.center();
                                                closeButton.setFocus(true);
                                                
                                        }
                                        
                                });

Restlet Server:

public class GreetingServiceServerResource extends ServerResource implements GreetingService {

        @Override
        public Representation handle() {
                
                
                 CompositeClassLoader customCL = new CompositeClassLoader();
                 customCL.addClassLoader(Thread.currentThread().getContextClassLoader());
                 customCL.addClassLoader(Name.class.getClassLoader());
                
                 ClassLoader oldCL = Thread.currentThread().getContextClassLoader();
                 Thread.currentThread().setContextClassLoader(customCL);
                
                Representation rep = super.handle();
                
                Thread.currentThread().setContextClassLoader(oldCL);
                
                return rep;
        }

        @Override
        @Post
        public String greetServer(Name name) throws IllegalArgumentException {
                System.out.println(“Name submitted: “+name.name);
                return “Welcome “+name.name+” from Restlet!”;
        }
        

}

OSGi Bundles:

Under equinox, the following modules were required:

id        State Bundle
0        ACTIVE org.eclipse.osgi_3.5.2.R35x_v20100126
         Fragments=2, 3
2        RESOLVED org.eclipse.persistence.jpa.equinox.weaving_1.1.3.v20091002-r5404
         Master=0
3        RESOLVED javax.transaction_1.1.1.v201002111330
         Master=0
17        ACTIVE org.hamcrest.core_1.1.0.v20090501071000
18        ACTIVE thrdGWTUser_0.0.2.SNAPSHOT
19        ACTIVE javax.servlet_2.5.0.v200806031605
21        ACTIVE org.junit4_4.5.0.v20090824
41        ACTIVE org.eclipse.osgi.services_3.2.0.v20090520-1800
49        ACTIVE thrdRestletGWT_0.0.2
50        ACTIVE thrdRestletExtSimple_0.0.2
51        ACTIVE zzSampleGWTClient_0.0.2
52        ACTIVE zzSampleRestletServer_0.0.2

Lessons Learned and Helpful Resources

– org.restlet.ext.simpleframework is deprecated and should not be used
– the current version for the Maven GWT plugin is 1.3-SNAPSHOT or 1.3.1.google. My projects worked with 1.3-SNAPSHOTS at the end.
– Restlet might have problems with transporting String objects. Therefore I encapsulated the String with the username in a Name object.
– Classloaders: GWT RPC, OSGi and Restlet
– GWT Module XXX not found in project sources or resources.
– CLAP Protocol in Restlet and OSGi
– GWT + OSGi + Maven
– Restlet Server: OSGi and Maven
– Deployment of GWT Applications (Part 2)
– Maven, GWT and Eclipse Quick start (Part 1)