The RESTlet Framework and Building Web Services

Overview

All of you should have completed 601 and the infamous web mail project. Given your presumably intimate knowledge of building a web application, you should be able to envision how you would build a RESTful web application from scratch, using your own HTTP server. You could also build a RESTful application using a standard HTTP servere (like Apache) and CGI or something similar. More commonly, developers use some kind of framework such as Ruby on Rails or Django (for Python).

The RESTlet Framework

The RESTlet framework started as an effort to build RESTful applications using Java Servlets. It has evolved into a standalone framework that is focuses purely on building applications that are RESTful. The framework can be downloaded from http://www.restlet.org/ and much of the material below has been taken from the tutorial and other documentation available on the RESTlet web site.

The generic RESTlet API provides a "a neutral API supporting the concepts of REST and facilitating the handling of calls for both client-side and server-side applications". The Noelios Restlet Engine (NRE) is a reference implementation of the API. The API and implementation come bundled together when you download the framework.

Conveniently, the framework provides a standalone web server. Therefore, you can develop your application without having to install Apache or another web server. The critical elements of the framework that you will need to understand are as follows:

• Connectors - "A connector in REST is a software element that enables the communication between components, typically by implementing one side of a network protocol." (from restlet.org). You can create a client connector in order to, for example, issue HTTP requests to another server. You can also create a server connector, for example an HTTP server connector to enable communication for a web server.
• Components - "A component is an abstract unit of software instructions and internal state that provides a transformation of data via its interface."(Roy T. Fielding) To create a basic web application you must create a component, then add an HTTP connector to the component.
• Applications - The Application is the coordination point that can manage the connectors and receive calls. The Application is attached to the VirtualHost that can be retrieved from the Component.
• Application.createRoot - The Application has a createRoot method that returns the Restlet that is responsible for receiving incoming calls.
• Router - The Router Restlet is capable of receiving incoming calls and routing them to the appropriate Resource based on a set of URI templates.
• Resources - The Resource class handles the requests (e.g., GET) coming from the client. Several of its methods (e.g., getRepresentation) can be overridden to return the appropriate Representation when a client request is issued.
• Filters - Filters, such as the Transformer, provide the ability to do pre/post processing of a representation returned by a resource.

Classpath required to run the example app:

org.restlet.jar:com.noelios.restlet.jar:com.noelios.restlet.ext.net.jar:com.noelios.restlet.ext.simple_3.1.jar:com.db4o.jar:org.restlet.example.jar:org.simpleframework.jar