J2ME REST Client

Christoph Hartmann — Mon, 29 Dec 2008 16:41:00 +0000

Unfortunately J2ME does not provide the best basis for writing a REST Web Service. As I already mentioned in previous posts the J2ME reference implementation does not support all HTTP methods. To overcome this restriction I wrote a J2ME Rest Client that uses HTTP method emulation like Rails does. Even though I heard that some J2ME platforms support PUT and DELETE I decided to use the standard. Method emulation may become a challenge if you’re not talking to your own services since it require a suitable server implementation like I posted for Grails earlier.

The following code illustrates how the Rest Client works. It uses the BasicAuth class from Sun. I removed the imports, because it should be obvious. In future versions of the class I plan to use one method to read the data from stream an check the status to reduce the file size. But for my first prototype it works that way.

/**
 * This class provided a REST implementation for J2ME.
 * @author Christoph Hartmann
 */
public class RestClient {
	String userid;
	String password;
	// User Agent
	private String ua;
 
	public RestClient() {}
 
	public RestClient(String userid, String password) {
		this.userid = userid;
		this.password = password;
		ua = "Profile/" + System.getProperty("microedition.profiles")
				+ " Configuration/"
				+ System.getProperty("microedition.configuration");
	}
 
	/**
	 * prepare the HTTP connection
	 * @param conn
	 * @throws IOException
	 */
	private void configureConncetion(HttpConnection conn) throws IOException {
		conn.setRequestProperty("User-Agent", ua);
		String locale = System.getProperty("microedition.locale");
		if (locale == null) { locale = "en-US";	}
		conn.setRequestProperty("Accept-Language", locale);
		conn.setRequestProperty("Content-Type", "text/plain");
		conn.setRequestProperty("Accept", "text/plain");
		// set HTTP basic authentification
		if (userid != null &#038;&#038; password != null) {
			conn.setRequestProperty("Authorization", "Basic " + BasicAuth.encode(userid, password));
		}
	}
 
	public HttpConnection getConnection(String url) throws IOException {
		HttpConnection conn = (HttpConnection) Connector.open(url);
		configureConncetion(conn);
		return conn;
	}
 
	public HttpConnection getConnection(String url, int access) throws IOException {
		HttpConnection conn = (HttpConnection) Connector.open(url, access);
		configureConncetion(conn);
		return conn;
	}
 
	/**
	 * READ
	 * @param url
	 * @return
	 * @throws IOException
	 */
	public String get(String url) throws IOException {
		HttpConnection hcon = null;
		DataInputStream dis = null;
		StringBuffer responseMessage = new StringBuffer();
 
		try {
			int redirectTimes = 0;
			boolean redirect;
 
			do {
				redirect = false;
 
				// a standard HttpConnection with READ access
				hcon = getConnection(url);
				// obtain a DataInputStream from the HttpConnection
				dis = new DataInputStream(hcon.openInputStream());
				// retrieve the response from the server
				int ch;
				while ((ch = dis.read()) != -1) {
					responseMessage.append((char) ch);
				}// end while ( ( ch = dis.read() ) != -1 )
				// check status code
				int status = hcon.getResponseCode();
 
				switch (status) {
				case HttpConnection.HTTP_OK: // Success!
					break;
				case HttpConnection.HTTP_TEMP_REDIRECT:
				case HttpConnection.HTTP_MOVED_TEMP:
				case HttpConnection.HTTP_MOVED_PERM:
					// Redirect: get the new location
					url = hcon.getHeaderField("location");
					System.out.println("Redirect: " + url);
 
					if (dis != null) dis.close();
					if (hcon != null) hcon.close();
 
					hcon = null;
					redirectTimes++;
					redirect = true;
					break;
				default:
					// Error: throw exception
					hcon.close();
					throw new IOException("Response status not OK:" + status);
				}
 
				// max 5 redirects
			} while (redirect == true &#038;&#038; redirectTimes < 5);
 
			if (redirectTimes == 5) {
				throw new IOException("Too much redirects");
			}
 
		} catch (Exception e) {
			e.printStackTrace();
			// TODO bad style
			responseMessage.append("ERROR: ");
		} finally {
			try {
				if (hcon != null)
					hcon.close();
				if (dis != null)
					dis.close();
			} catch (IOException ioe) {
				ioe.printStackTrace();
			}// end try/catch
		}// end try/catch/finally
		return responseMessage.toString();
 
	}// end sendGetRequest( String )
 
	/**
	 * UPDATE
	 *
	 * @param url
	 * @param data
	 *            the request body
	 * @throws IOException
	 */
	public String post(String url, String data) throws IOException {
 
		HttpConnection hcon = null;
		DataInputStream dis = null;
		DataOutputStream dos = null;
		StringBuffer responseMessage = new StringBuffer();
 
		try {
			int redirectTimes = 0;
			boolean redirect;
 
			do {
				redirect = false;
				// an HttpConnection with both read and write access
				hcon = getConnection(url, Connector.READ_WRITE);
				// set the request method to POST
				hcon.setRequestMethod(HttpConnection.POST);
				// overwrite content type to be form based
				hcon.setRequestProperty("Content-Type", "application/x-www-form-urlencoded");
				// set message length
				if (data != null) {
					hcon.setRequestProperty("Content-Length", ""
							+ data.length());
				}
 
				if (data != null) {
					// obtain DataOutputStream for sending the request string
					dos = hcon.openDataOutputStream();
					byte[] request_body = data.getBytes();
					// send request string to server
					for (int i = 0; i < request_body.length; i++) {
						dos.writeByte(request_body[i]);
					}// end for( int i = 0; i < request_body.length; i++ )
					dos.flush(); // Including this line may produce
					// undesiredresults on certain devices
				}
 
				// obtain DataInputStream for receiving server response
				dis = new DataInputStream(hcon.openInputStream());
				// retrieve the response from server
				int ch;
				while ((ch = dis.read()) != -1) {
					responseMessage.append((char) ch);
				}// end while( ( ch = dis.read() ) != -1 ) {
				// check status code
				int status = hcon.getResponseCode();
				switch (status) {
				case HttpConnection.HTTP_OK: // Success!
					break;
				case HttpConnection.HTTP_TEMP_REDIRECT:
				case HttpConnection.HTTP_MOVED_TEMP:
				case HttpConnection.HTTP_MOVED_PERM:
					// Redirect: get the new location
					url = hcon.getHeaderField("location");
					System.out.println("Redirect: " + url);
 
					if (dis != null) dis.close();
					if (hcon != null) hcon.close();
					hcon = null;
					redirectTimes++;
					redirect = true;
					break;
				default:
					// Error: throw exception
					hcon.close();
					throw new IOException("Response status not OK:" + status);
				}
 
				// max 5 redirects
			} while (redirect == true &#038;&#038; redirectTimes < 5);
 
			if (redirectTimes == 5) {
				throw new IOException("Too much redirects");
			}
		} catch (Exception e) {
			e.printStackTrace();
			responseMessage.append("ERROR");
		} finally {
			// free up i/o streams and http connection
			try {
				if (hcon != null) hcon.close();
				if (dis != null) dis.close();
				if (dos != null) dos.close();
			} catch (IOException ioe) {
				ioe.printStackTrace();
			}// end try/catch
		}// end try/catch/finally
		return responseMessage.toString();
	}
 
	/**
	 * DELETE
	 * not possible on J2ME therefore we use Rails emulation on PUT and
	 * DELETE like
	 * http://localhost:8080/CandyStreamServer/airport/1?_method=delete
	 * @param url
	 */
	public String delete(String url) throws IOException {
		url += "?_method=DELETE";
		return post(url, null);
	}
 
	/**
	 * CREATE not possible on J2ME therefore we use Rails emulation on PUT and
	 * DELETE
	 * @param url
	 * @param data
	 */
	public String put(String url, String data) throws IOException {
		data += "&_method=PUT";
		url += "?" + data;
		return post(url, null);
	}
}

J2ME, REST and Grails

Christoph Hartmann — Sat, 15 Nov 2008 12:56:00 +0000

I set up an mobile application that sends data via a REST based interface to a Grails-based server. Unfortunately J2ME does not support all HTTP methods. To overcome this weakness we have to emulate the methods PUT and DELETE. Like Rails just did this by adding a “_method=PUT” parameter to the query URL, we will do the same. Rails does the HTTP method emulation handling automatically, whilst in Grails we have do this by ourselves. By using the UrlMapping.groovy it will become quite easy to overcome this weakness. A standard REST mapping should look like:

 
class UrlMappings {
   static mappings = {
          // identifies a rest object like /rest/airport/1
	  "/rest/$domain/$id"{
		  controller = "rest"
		  action = [GET:"show", PUT:"create", POST:"update",DELETE:"delete"]
         }
   }
}

The action attribute may be a simple string, a hash map or a closure. Grails calls the following method evaluateNameForValue(Object value, GrailsWebRequest webRequest) to retrieve the value. The method is located within the org.codehaus.groovy.grails.web.mapping.DefaultUrlMappingInfo.

private String evaluateNameForValue(Object value, GrailsWebRequest webRequest) {
        if (value == null) {
            return null;
        }
        String name;
        if (value instanceof Closure) {
            Closure callable = (Closure) value;
            Object result = ((Closure) callable.clone()).call();
            name = result != null ? result.toString() : null;
        } else if (value instanceof Map) {
            Map httpMethods = (Map) value;
            name = (String) httpMethods.get(webRequest.getCurrentRequest().getMethod());
        } else {
            name = value.toString();
        }
        return name;
    }

Due to our knowledge of the action retrieval process, we are able to implement a “_method” emulation. This could look like:

 
class UrlMappings {
   static mappings = {
          // identifies a rest object like /rest/airport/1
	  "/rest/$domain/$id"{
		  controller = "rest"
		  // action = [GET:"show", PUT:"create", POST:"update",DELETE:"delete"]
 
	         action = {
		   // closure will not be invoked with request, log ... therefore we have to do this manually
		  GrailsWebRequest webRequest = (GrailsWebRequest) RequestContextHolder.getRequestAttributes();
		  String method = webRequest.getCurrentRequest().getMethod();
		  String methodParam = webRequest.getCurrentRequest().getParameter("_method");
 
		  // check if we can use _method
		  if (methodParam != null) {
		    methodParam = methodParam.toUpperCase()
 
		    switch (methodParam) {
			case "PUT" :
			case "DELETE" :
			case "POST" :
			case "GET" :
			  method = methodParam;
			break
		  }
		} 
 
		switch (method.toUpperCase()) {
		  case "PUT" : return "create"
			break
		  case "DELETE" : return "delete"
			break
		  case "POST" : return "update"
			break
		  case "GET" : return "show"
			break
		  default:  return "show"
		        break
		}
		return method;
		}
         }
   }
}

I hope that helps all Grails REST developers to enable their services for J2ME.

acidum.de » J2ME

J2ME REST Client

J2ME, REST and Grails