RESTful Web Services

Representational State Transfer(REST), a software architecture style used in developing stateless web services. While this style may be used to describe any distributed framework that uses a simple protocol for data transmission and no other additional data transfer rules, the most common use of REST is on on the Web with HTTP being the only protocol used here. In REST each service (called "resource" in REST) is viewed as resource identified by a URL, and the only operations allowed are the HTTP - GET (Read), PUT (Update), POST(Create) and DELETE (Delete). You can find this style similar in SQL, thinking of a resource as equivalent to a Table. The main features and constraints of REST architectural style are:

• Client-Server: A clear separation concerns is the reason behind this constraint. Separating concerns between the Client and Server helps improve portability in the Client and Scalability of the server components.
• Stateless: All REST resources are required to be stateless. The idea is that the application would be resilient enough to work between server restarts. However, it is not uncommon to see some RESTful web services save states between requests.
• Caching: Caching is allowed, however it is required that "response to a request be implicitly or explicitly labeled as cacheable or non-cacheable"
• As there is no interface definition (like in SOAP), it becomes mandatory for a Client and Server to have a mutual understanding of the messages being transmitted between them.

Given that every resource in a RESTful service is represented by a URL, it is easy to write a client for such a web service. The following is the code for a simple Web Service client for the flickr web services interface.
There's More

package main;

import java.net.HttpURLConnection;
import java.net.InetSocketAddress;
import java.net.Proxy;
import java.net.SocketAddress;
import java.net.URL;

import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;

import org.w3c.dom.Document;

public class FlickrClient {
  public static void main(String[] args) {
    String flickrURL = "http://api.flickr.com/services/rest/?method=flickr.test.echo&name=value&api_key=[yourflickrkey]";
    try {
      SocketAddress addr = new InetSocketAddress("[proxy]", 9090);
      Proxy proxy = new Proxy(Proxy.Type.HTTP, addr);
      URL u = new URL("http://api.flickr.com/services/rest/?method=flickr.test.echo&name=value&api_key=[yourflickrkey]");
      HttpURLConnection uc = (HttpURLConnection) u.openConnection(proxy);
      uc.setRequestProperty("Accept", "*/*");
      uc.setRequestProperty("Accept-Charset", "ISO-8859-1,utf-8;q=0.7,*;q=0.7");
      uc.setRequestProperty("Accept-Language", "en-us,en;q=0.5");
      uc.setRequestProperty("Keep-Alive", "300");
      uc.setRequestProperty("ucection", "keep-alive");
      String proxyUser = "[netUserId]";
      String proxyPassword = "[netPassword]";
      uc.setRequestProperty("Proxy-Authorization", "NTLM " + new sun.misc.BASE64Encoder().encode((proxyUser + 
":" + proxyPassword).getBytes()));

      DocumentBuilder docBuilder = DocumentBuilderFactory.newInstance().newDocumentBuilder();
      Document doc = docBuilder.parse(uc.getInputStream());

      System.out.println(doc.getDocumentElement().getTagName());

      System.out.println();
    } catch (Exception e) {
      e.printStackTrace();
    }
  }
}

• Note that the request URL has a method parameter.

  http://api.flickr.com/services/rest/?method=flickr.test.echo

  This is is not strictly adhering to the principles of REST, however it is not uncommon. Ideally the URL should be

  http://api.flickr.com/services/rest/flickrtestecho

  You would make a GET request on this resource.

Links

• Developerworks
• Artima Java