FAQ

Table of contents

  1. How to use Restlet in an Applet
  2. Solve 405 status code responses
  3. How to trace the internal client and server connectors?
  4. How do I implement the traditional MVC pattern?

How to use Restlet in an Applet

In version 2.0, the Restlet engine creates its own classloader, instance of the EngineClassLoader class. This is fine most of the time, except when a security manager is used, such as for Applets running inside a sandbox.

The solution is to use a custom Restlet engine as below, that won’t create this new classloader:

public class AppletEngine extends Engine {

    @Override
    protected ClassLoader createClassLoader() {
        return getClass().getClassLoader();
    }

}

Now you just need to call this line before using the Restlet API:

Engine.setInstance(new AppletEngine());

Solve 405 status code responses

Having set up your server resource with annotated methods, you’re ready to send it requests and eager to get JSON, XML representations of its state. But unfortunately, it fails. In some cases, you’re returned an error response with a HTTP 405 status code.By definition, it means that the sent method is not supported which keep you quite disappointed as the resource does declare a “@Get” method!

The solution is all about content negotiation. Let’s say your client requires a JSON representation. Before the resource is sollicitated to generate the desired representation, the content negotiation algorithm will detect if the resource is really able to generate a JSON representation, that is to say there is a Java method in the code of the server resource which is (A/) properly annotated, and (B/) having a compatible return type. the case (A/) is easy to understand, let’s focus on case (B/). This may happen when you server resource use annotation with media type parameters:

public class MyResource extends ServerResource {
   @Get("html")
   public String toHtml() {
      return "<html><body>hello, world</body></html>";
   }
}

In this case, the client requires a JSON representation but the server resource is not able to generate it.

This may also happen when you rely on the converter service, and that you don’t have properly configured the classpath of your project. Did you reference the archive of the right Restlet extension (such as the Jackson extension) and the archives of its library dependencies?

How to trace the nio client and server connectors?

These connectors are configured via the parameter called “tracing” of their context (see the javadocs).

Here is a sample code that illustrates how to configure the HTTP  server connector of a Component:

Component c = new Component();
Server s = new Server(Protocol.HTTP, 8182);
c.getServers().add(s);
s.getContext().getParameters().add("tracing", "true");

Here is a sample code that illustrates how to configure the HTTP  client connector of a resource:

Client client = new Client(new Context(), Protocol.HTTP);
client.getContext().getParameters().add("tracing", "true");
ClientResource resource = new ClientResource("http://localhost:8182/<resource>");
resource.setNext(client);

How do I implement the traditional MVC pattern?

There is only a rough correspondence between the MVC pattern and the Restlet framework; some debate exists as to whether it should be employed at all. For those who wish to follow the MVC pattern with Restlet, here is the basic proposition:

  • Controller == Restlets (mainly Filters, Routers, Finders). You can visualize the controller as a sort of processing chain, where the last node should be a Finder with all the information necessary to locate the target Resource for the call. Note that Finders are generally implicitely created when attaching Resource classes to a Router.
  • Model == Resource + Domain Objects. Just start from the org.restlet.resource.Resource class and load the related Domain Objects in the constructor based on the request attributes (ex: identifier extracted from the URI). Then you can declare the available variants with getVariants() and override methods like represent(Variant) for GET, acceptRepresentation(Representation) for POST, removeRepresentations() for DELETE or storeRepresentation(Representation) for PUT.
  • View == Representation. To expose views of your model, you create new Representations for your Resources. You can leverage on one of the numerous Representation subclasses (InputRepresentation for example) available in the org.restlet.resource package or in extension packages like for JSON documents, FreeMarker and Velocity templates.