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.APPLICATION_JAVA_OBJECT_GWT));                                                                          r.greetServer(new Name(nameField.getText()), new AsyncCallback() {

@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+"
"+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)