Java and ME: November 2010

Sunday, November 28, 2010

Servlet-GSON vs JSONME-JavaME

In this post I’m going to show you a simple example of data transmition between a mobile client using Java Me and a Server using Servlets. The data format to use in the transmition is JSON.

Requirements:

Servlets: On the server side, servlets will wait for requests. More info:
http://www.oracle.com/technetwork/java/index-jsp-135475.html

Java Me: Used to create mobile applications using Java. More info:
http://www.oracle.com/technetwork/java/javame/overview/index.html

JSON: Data interchange format. More info:
http://www.java-n-me.com/2010/10/json-without-fear-of-friday-13th.html

GSON: Google's library to create JSON Strings from and to objetos. More info:
http://www.java-n-me.com/2010/11/gson-json-in-just-two-lines-of-code.html

JSON Me: Library to work with JSON in Java ME. More info:
http://www.java-n-me.com/2010/11/java-me-and-json.html

Following is the Client class that is used on the server side which we will use to create the Objects to pass through the network in JSON format:

package model;

public class Client {

   /** Name of the client*/
   private String name;

   /** Last name of the client*/
   private String lastName;

   /** Identification of the client*/
   private String id;

   //Getters and Setters...
}

As shown, there is nothing strange. Just a simple class with some attributes. This is because on the server side we have GSON. On the mobile client side, this class has a little more lines of code as we shall see later.

The servlet will respond to POST requests, will create 3 Client objects, convert them to JSON Strings (using GSON) and send the response to the client:

package servlets;

import com.google.gson.Gson;
import java.io.*;
import java.util.Vector;
import javax.servlet.ServletException;
import javax.servlet.http.*;
import model.Client;

public class ClientsServlet extends HttpServlet {

   protected void doPost(HttpServletRequest request, HttpServletResponse response)
   throws ServletException, IOException {

   //sets the type of response and the charset used
   //Be careful, in your mobile client, read the response using the same charset
   response.setContentType("application/json; charset=UTF-8");

   //create some clients and add them to the vector
   Vector vClients = new Vector();

   Client clientOne = new Client();
   clientOne.setName("Alexis");
   clientOne.setLastName("Lopez");
   clientOne.setId("123456");

   vClients.add(clientOne);

   Client clientTwo = new Client();
   clientTwo.setName("Second");
   clientTwo.setLastName("Client");
   clientTwo.setId("987534");

   vClients.add(clientTwo);

   Client clientThree = new Client();
   clientThree.setName("Colombia");
   clientThree.setLastName("JUG");
   clientThree.setId("555555");

   vClients.add(clientThree);

   //convert the clients vector to JSON using GSON, very easy!
   Gson gson = new Gson();
   String jsonOutput = gson.toJson(vClients);

   System.out.println("*****JSON STRING TO RESPONSE*****");
   System.out.println(jsonOutput);
   System.out.println("*********************************");

   //print the response
   PrintWriter out = response.getWriter();
   out.println(jsonOutput);
   out.flush();
   out.close();
   }
}

When the servlet is run and it receives a POST request, the following message is shown on the console:

INFO: *****JSON STRING TO RESPONSE*****

INFO: [{"name":"Alexis","lastName":"Lopez","id":"123456"},{"name":"Second","lastName":"Client","id":"987534"},{"name":"Colombia","lastName":"JUG","id":"555555"}]
INFO: *********************************

As you can see, is the JSON representation of the newly created Vector of Client objects. Note that the servlet defines the type of response as JSON ("application/json") and also uses the "UTF-8" charaset. This is important to remember, because when the mobile client is reading the answer, you must use the same charset or you could get strange characters when reading accents or 'Ñ' letter, etc. You may also notice how easy it is to implement GSON to convert objects to JSON Strings. Only two lines of code.

That's it on the server side. Now we will see the Client class on the mobile side and will notice that it has more lines of code than the Client class on the server side. As mentioned before, this is because in Java ME there is NO Java Reflection API and therefore we can not use GSON but we use JSON ME and we have to manually map the attributes of the classes. For more information about the next class, check this previous post: http://www.java-n-me.com/2010/11/java-me-and-json.html

import java.util.Vector;
import org.json.me.JSONArray;
import org.json.me.JSONException;
import org.json.me.JSONObject;
public class Client {

   /** Name of the client*/
   private String name;

   /** Constant name of the attribute name*/
   private static final String ATT_NAME = "name";

   /** Last name of the client*/
   private String lastName;

   /** Constant name of the attribute last name*/
   private static final String ATT_LAST_NAME = "lastName";

   /** Identification of the client*/
   private String id;

   /** Constant name of the attribute id*/
   private static final String ATT_ID = "id";

   /**
   * Allows to get the JSON String from the Client passed as parameter
   * @param client Client object to convert to JSON
   * @return JSON String of the Client passed as parameter
   */
   public static String toJSON(Client client)
   {
   return toJSONObject(client).toString();
   }

   /**
   * This method should be used by this class only, that's why it is private.
   * Allows to get a JSONObject from the Client passed as parameter
   * @param client Client Object to convert to JSONObject
   * @return JSONObject representation of the Client passed as parameter
   */
   private static JSONObject toJSONObject(Client client) {
   JSONObject json = new JSONObject();
   try {
   json.put(ATT_NAME, client.getName());
   json.put(ATT_LAST_NAME, client.getLastName());
   json.put(ATT_ID, client.getId());
   } catch (JSONException ex) {
   ex.printStackTrace();
   }
   return json;
   }

   /**
   * Allows to get JSON String from a Vector of Clients
   * @param clients Vector of clients to convert to JSON
   * @return JSON String of the Vector of clients
   */
   public static String toJSONs(Vector clients) {
   JSONArray clientsArray = new JSONArray();
   for (int i = 0; i < clients.size(); i++) {
   Client client = (Client)clients.elementAt(i);

   JSONObject jsonObject = toJSONObject(client);
   clientsArray.put(jsonObject);
   }
   return clientsArray.toString();
   }

   /**
   * Allows to get a Client Object from a JSON String
   * @param jsonText JSON String to convert to a Client Object
   * @return Client Object created from the String passed as parameter
   */
   public static Client fromJSON(String jsonText) {
   Client client = new Client();
   try {
   JSONObject json = new JSONObject(jsonText);

   //check if the JSON text comes with the attribue Name
   if (json.has(ATT_NAME)) {
   //asign the String value of the attribute name to the client
   client.setName(json.getString(ATT_NAME));
   }

   //check if the JSON text comes with the attribue last name
   if (json.has(ATT_LAST_NAME)) {
   client.setLastName(json.getString(ATT_LAST_NAME));
   }

   //check if the JSON text comes with the attribue id
   if (json.has(ATT_ID)) {
   client.setId(json.getString(ATT_ID));
   }
   } catch (JSONException ex) {
   ex.printStackTrace();
   }
   return client;
   }

   /**
   * Allows to get a Vector of Clients from a JSON String
   * @param jsonArray String that contains Clients JSON Text
   * @return Vector of Clients from the String passed as parameter
   */
   public static Vector fromJSONs(String jsonArray) {
   Vector clients = new Vector();
   try {
   JSONArray jsonArr = new JSONArray(jsonArray);
   for (int i = 0; i < jsonArr.length(); i++) {
   //get a single client JSON Object
   JSONObject json = jsonArr.getJSONObject(i);

   //pass the JSON String to the method in order
   //to get the Client Object
   Client client = fromJSON(json.toString());

   clients.addElement(client);
   }
   } catch (JSONException ex) {
   ex.printStackTrace();
   }
   return clients;
   }

   //Getters and Setters...
}

Finally we'll see the MIDlet that runs and creates an HTTP connection to access the servlet. The connection is established using POST method and we read the data using the same charset that was used when creating the http response to avoid getting strange characters. Then the console displays the values of the attributes of the clients received:

import java.io.*;
import java.util.Vector;
import javax.microedition.io.*;
import javax.microedition.midlet.*;

public class JSONMIDlet extends MIDlet {

   public void pauseApp() {
   }

   public void destroyApp(boolean unconditional) {
   notifyDestroyed();
   }

   public void startApp() {
   try {

   //ask for the clients
   Vector clients = getClients();

   //show information of the clients
   System.out.println("*****CLIENT INFORMATION*****");
   for (int i = 0; i < clients.size(); i++) {
   Client cl = (Client) clients.elementAt(i);

   System.out.println("Client " + (i + 1) + " name = " + cl.getName());
   System.out.println("Client " + (i + 1) + " last name = " + cl.getLastName());
   System.out.println("Client " + (i + 1) + " ID = " + cl.getId());
   System.out.println("");
   }
   } catch (IOException ex) {
   //manage the exception
   }
   //close the application
   destroyApp(true);
   }

   /**
   * Connects to server in order to obtain information of the clients
   * @return Vector of <code>Client</code> objects
   * @throws IOException if errors with the connections
   */
   public Vector getClients() throws IOException {
   //This is my servlet’s URL
   String URL = "http://localhost:8080/GSON_Servlet/ClientsServlet";
   Vector vClients = new Vector();

   HttpConnection http = null;
   DataInputStream din = null;
   try {
   //Open HttpConnection using POST
   http = (HttpConnection) Connector.open(URL);
   http.setRequestMethod(HttpConnection.POST);
   //Content-Type is must to pass parameters in POST Request
   http.setRequestProperty("Content-Type", "application/x-www-form-urlencoded");

   //Open InputStream to read the response
   din = new DataInputStream(http.openInputStream());
   int i = 0;
   StringBuffer str = new StringBuffer();
   while ((i = din.read()) != -1) {
   str.append((char) i);
   }

   //IMPORTANT: Read the response in the same charset that it was written
   String jSonString = new String(str.toString().getBytes(), "UTF-8");

   System.out.println("*****JSON STRING RECEIVED*****");
   System.out.println(jSonString);
   System.out.println("******************************");

   //Parse the JSON String to obtain a Vector of clients
   vClients = Client.fromJSONs(jSonString);

   } //whatever happens, close the streams
   finally {
   if (din != null) {
   din.close();
   }

   if (http != null) {
   http.close();
   }
   }

   return vClients;
   }
}

When we run the MIDlet the console prints the following lines:

*****JSON STRING RECEIVED*****
[{"name":"Alexis","lastName":"Lopez","id":"123456"},{"name":"Second","lastName":"Client","id":"987534"},{"name":"Colombia","lastName":"JUG","id":"555555"}]
****************************** *****CLIENT INFORMATION***** Client 1 name = Alexis Client 1 last name = Lopez Client 1 ID = 123456 Client 2 name = Second Client 2 last name = Client Client 2 ID = 987534 Client 3 name = Colombia Client 3 last name = JUG Client 3 ID = 555555

Obviously, in a business application the servlet would connect to a database in order to get the clients information, then the servlet would send that information to the mobile client using JSON. The mobile client would have a graphical interface that would allow the user to view the information received from the server... and so many other improvements. But this simple example, shows you the possibilities offered by JSON for communication between server and mobile client in a standard way without having to invent new formats for data exchange.

This concludes the communication Servlets/GSON vs JSON Me/Java Me. I know there have been a lot of code, it may be difficult to read. For the following posts I will emphasize on the really important parts of the code and let the entire source code to be downloaded.

see you soon!

References:

Using JavaScript Object Notation (JSON) in Java ME for Data Interchange. Agosto 2008. Oracle [online].
Available on Internet: http://java.sun.com/developer/technicalArticles/javame/json-me/
[accessed on October the 26th 2010].

meapplicationdevelopers: Subversion. 2007. Oracle [online].
Available on Internet: https://meapplicationdevelopers.dev.java.net/source/browse/meapplicationdevelopers/demobox/mobileajax/lib/json/
[accessed on October the 27th 2010].

Friday, November 12, 2010

Java ME and JSON

In previous posts, we saw how JSON has been gaining ground as data transmission format. We also introduced GSON, Google’s API to create JSON text from an Object and Objects from JSON text in a fairly simple way, but that can’t be used in Java ME because of the Java Reflection API.

In this post we are going to stand on the mobile client side using JSON ME as application library to create JSON text. The library has disappeared from the JSON’s official web site (http://www.json.org), I'm not sure why, but the project is in the repository of java.net. The steps needed to obtain the source code from java.net SVN to NetBeans can be found in the following link:

http://www.java-n-me.com/2010/11/how-to-import-source-code-from-svn-to.html

If you don’t want to build the source code, contact me so I can send you the library. Once you have the JSON ME .jar file, you are able to implement JSON in your mobile application. Although you have to write more lines of code than when you are using GSON, it is still easy to use and the benefit in terms of ease of data transmission between mobile client and server is worth it!

As a example, we will develop a simple MIDlet that does not have UI, but only the necessary classes from Java ME to be able to generate JSON text from Objects and Objects from JSON text.

Next is the source code of the MIDlet:

import java.util.Vector;
import javax.microedition.midlet.*;
public class JSONMIDlet extends MIDlet {
   public void startApp() {
   Vector clients = new Vector();
   Client client = new Client();
   client.setName("Mr Developer");
   client.setLastName("level 1");
   client.setId("123456");

   //shows the client JSON String
   String jsonOne = Client.toJSON(client);
   System.out.println("***** FIRST OBJECT AS JSON ******");
   System.out.println(jsonOne);
   //add the client to the vector
   clients.addElement(client);

   client = new Client();
   client.setName("Mrs Developer");
   client.setLastName("level 2");
   client.setId("987654");

   //shows the client JSON String
   String jsonTwo = Client.toJSON(client);
   System.out.println("***** SECOND OBJECT AS JSON ******");
   System.out.println(jsonTwo);
   //add the client to the vector
   clients.addElement(client);

   //Now get the Clients Vector JSON String
   String jsons = Client.toJSONs(clients);
   System.out.println("***** ALL OBJECTS AS JSON ******");
   System.out.println(jsons);

   System.out.println("");

   //now map objectos from JSON Strings
   Client newClient = Client.fromJSON(jsonOne);
   System.out.print("***** NAME OF THE NEW FIRST CLIENT = ");
   System.out.println(newClient.getName());

   newClient = Client.fromJSON(jsonTwo);
   System.out.print("***** NAME OF THE NEW SECOND CLIENT = ");
   System.out.println(newClient.getName());

   Vector newClients = Client.fromJSONs(jsons);
   System.out.print("***** SIZE OF THE NEW CLIENT VECTOR = ");
   System.out.println(newClients.size());

   destroyApp(true);
   }

   public void pauseApp() {
   }

   public void destroyApp(boolean unconditional) {
   notifyDestroyed();
   }
}

As you can see, the MIDlet is creating Client Objects with defaults values. Surely in a real application, these data are coming from text fields in a UI. The MIDlet output to the console JSON Strings from these Objects and some attributes of Objects created from JSON strings.

Most of the work is done in the classes you wish to convert to JSON text. The following classes are the key to work with JSON ME:

org.json.me.JSONObject: Contains key / value pairs with which the JSON text is built. It also provides the method for obtaining the JSON text.
org.json.me.JSONArray: Ordered sequence of values that are separated by commas. These values can be JSONObjects. Practically, it can be understood as an array of JsonObject Objects.

Below is the Client class which implements the methods that convert to and from JSON and makes use of the previously named classes:

The Client class contains methods that allow us to get a Client Object from JSON text and get the JSON text from a Client Object. As good practice, you can create constants with the attribute names so they can be used in the methods avoiding hard coding. So you can see the big difference with GSON, given that Java ME can’t use the Reflection API, it is necessary to assign values to attributes manually.

Here is the output text that is displayed by the console, once the MIDlet has been run:

***** FIRST OBJECT AS JSON ****** {"lastName":"level 1","name":"Mr Developer","id":"123456"}
***** SECOND OBJECT AS JSON ****** {"lastName":"level 2","name":"Mrs Developer","id":"987654"}
***** ALL OBJECTS AS JSON ****** [{"lastName":"level 1","name":"Mr Developer","id":"123456"},{"lastName":"level 2","name":"Mrs Developer","id":"987654"}] ***** NAME OF THE NEW FIRST CLIENT = Mr Developer ***** NAME OF THE NEW SECOND CLIENT = Mrs Developer ***** SIZE OF THE NEW CLIENT VECTOR = 2

If many classes of your application need to be converted to JSON text, you may create an Interface that defines methods for all classes wishing to implement JSON. A good example of this practice can be found on the following link:

http://jimmod.com/blog/2010/03/java-me-j2me-json-implementation-tutorialsample/

OK that's it for this post, I hope I was clear enough. Any further explanation do not hesitate to contact me.

In the next post we will integrate the pairs Servlet / GSON vs JSON ME / Java ME, so we can communicate mobile clients with servers and vice versa.

See you soon!

References:

Java ME (J2ME) JSON Implementation Tutorial/Sample. March 2010. Jimmy’s Blog [online].
Available on Internet: http://jimmod.com/blog/2010/03/java-me-j2me-json-implementation-tutorialsample/
[accessed on November 09 2010].

Sunday, November 7, 2010

GSON: JSON in just two lines of code, second part

As I mentioned in my last post, in this post we'll make a little more complicated example than the previous one. For those who have not yet revised the previous post, you can do it now using this link:

http://www.java-n-me.com/2010/11/gson-json-in-just-two-lines-of-code.html

Well, now that we have reviewed the above example, we will modify the classes. Find the changes in bold:

import java.util.*;

public class Book

{

/** Name of the book*/

private String name;

/** number of pages*/

private Integer numPages;

/** How much the book costs*/

private Double cost;

/**authors's names*/

private String[] authors;

/** Publication Date*/

private Date published;

/** Pages of the book*/

private List<page> pages;

//Constructor, Getters and Setters...

}

public class Page

{

/** Number of the page*/

private int numPage;

/** Text of the page*/

private String text;

//Constructor, Getters and Setters...

}

import com.google.gson.Gson;
import java.util.*;
public class Control
{
public static void main(String[] args)
{
//create a book
   Book myBook = new Book();
   myBook.setName("Java and JSON");
   myBook.setNumPages(2);
   myBook.setCost(1.5);
   myBook.setAuthors(new String[]{"Alexis López", "Duke"});
   myBook.setPublished(new Date());

     //Create the pages
   Vector<Page> pages = new Vector();
   Page pOne = new Page();
   pOne.setNumPage(1);
   pOne.setText("This text is from page one");

   Page pTwo = new Page();
   pTwo.setNumPage(2);
   pTwo.setText("This text is from page two");

   pages.add(pOne);
   pages.add(pTwo);

   //Add the pages to the book
   myBook.setPages(pages);

//Convert myBook to JSON using GSON
   Gson gson = new Gson();
   String json = gson.toJson(myBook);

//print the result
System.out.println("*** MyBook as JSON ***");
   System.out.println(json);

   //Map JSON text to a new book
   Book newBook = gson.fromJson(json, Book.class);

   //print some attributes of the new book
System.out.println("*** NewBook from JSON ***");
   System.out.println("Name = " + newBook.getName());
   System.out.println("Authors = " + Arrays.toString(newBook.getAuthors()));
   System.out.println("Number of pages = " + newBook.getNumPages());
   System.out.println("Publication date = " + newBook.getPublished());
   System.out.println("Page One = " +
   ((Page)(newBook.getPages().get(0))).getText());
   System.out.println("Page two = " +
   ((Page)(newBook.getPages().get(1))).getText());
   //...
}
}

Note that we used java.util.List and not a java.util.Vector for the book's pages Collection. This is because GSON, when we are mapping the new Object, gson will try to use a java.util.LinkedList, and therefore it will not be compatible with java.util.Vector nor with java.util.ArrayList, producing the following exception:

Exception in thread "main" java.lang.IllegalArgumentException: Can not set java.util.Vector field Book.pages to java.util.LinkedList
   at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(UnsafeFieldAccessorImpl.java:146)
   at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(UnsafeFieldAccessorImpl.java:150)
   at sun.reflect.UnsafeObjectFieldAccessorImpl.set(UnsafeObjectFieldAccessorImpl.java:63)
   at java.lang.reflect.Field.set(Field.java:657)
   at com.google.gson.FieldAttributes.set(FieldAttributes.java:182)
   at com.google.gson.JsonObjectDeserializationVisitor.visitFieldUsingCustomHandler
(JsonObjectDeserializationVisitor.java:118)
   at com.google.gson.ObjectNavigator.navigateClassFields(ObjectNavigator.java:158)

//...

For this reason we use java.util.List. The following console output is shown when you run the Control class:

*** MyBook as JSON ***

{"name":"Java and JSON","numPages":2,"cost":1.5,"authors":["Alexis López","Duke"],"published":"7/11/2010 10:10:19 AM","pages":[{"numPage":1,"text":"This text is from page one"},{"numPage":2,"text":"This text is from page two"}]}

*** NewBook from JSON ***

Name = Java and JSON

Authors = [Alexis López, Duke]

Number of pages = 2

Publication date = Sun Nov 07 10:10:19 COT 2010

Page One = This text is from page one

Page two = This text is from page two

Thus, we can have Objects with relationships to another Objects and convert them to and from JSON.

In a future post, we will review examples with Java ME/JSON ME.

See you soon!

References:

google-gson. August 19 de 2010. Google Code [online]. Available from Internet:http://code.google.com/p/google-gson/ [accessed on October the 26th 2010].

Wednesday, November 3, 2010

GSON: JSON in just two lines of code

The increased use of JSON for data exchange has resulted in the emergence of several libraries to handle it. GSON, google's API, allows the production of JSON strings from Objects and Objects from JSON strings quite easily. However, additional steps must be performed when the Object implements Collections of items that are not primitive.

In this post we will have a simple example with no Collections. We'll convert Object to JSON and then JSON to Object.

GSON can be downloaded from:

http://code.google.com/p/google-gson/

It's a .jar file wich you can add to your project. GSON uses the Java Refelction API in order to generate the JSON Strings in just two lines of code! two lines!

Note: Due to the use of the Java Reflection API, GSON can't be used in Java ME, but it is a very good choice if you want to produce JSON Strings of an Object at server side and then send it to a mobile client.

The Java Classes we will use during the example are:

import java.util.Date;

public class Book

{

/** Name of the book*/

private String name;

/** number of pages*/

private Integer numPages;

/** How much the book costs*/

private Double cost;

/**authors's names*/

private String[] authors;

/** Publication Date*/

private Date published;

//Constructor, Getters and Setters...

}

import com.google.gson.Gson;
import java.util.Date;
public class Control
{
public static void main(String[] args)
{
//create a book
   Book myBook = new Book();
   myBook.setName("Java and JSON");
   myBook.setNumPages(2);
   myBook.setCost(1.5);
   myBook.setAuthors(new String[]{"Alexis López", "Duke"});
   myBook.setPublished(new Date());

//Convert myBook to JSON using GSON
   Gson gson = new Gson();
   String json = gson.toJson(myBook);

//print the result
System.out.println("*** MyBook as JSON ***");
   System.out.println(json);

   //Map JSON text to a new book
   Book newBook = gson.fromJson(json, Book.class);

   //print some attributes of the new book
System.out.println("*** NewBook from JSON ***");
   System.out.println("Name = " + newBook.getName());
   System.out.println("Authors = " + Arrays.toString(newBook.getAuthors()));
   System.out.println("Number of pages = " + newBook.getNumPages());
   System.out.println("Publication date = " + newBook.getPublished());

   //...
}
}

Once the Control class has been run, you will notice in the console the JSON String of the Object myBook:

{"name":"Java and JSON","numPages":2,"cost":1.5,"authors":["Alexis López","Duke"],"published":"3/11/2010 09:42:06 AM"}

Also, you will notice the information of the new book (newBook) mapped from the MyBook JSON String:

Name = Java and JSON

Authors = [Alexis López, Duke]

Number of pages = 2

Publication date = Wed Nov 03 09:42:06 GMT-05:00 2010

As I mentioned at the beginning of the post, when the object in question contains among its attributes a Collection, you should consider this:

- Producing the JSON string is the same as we saw in the example above.
- Mapping Objects from the JSON String requires additional steps, especially because it is not possible to recognize the type of Generics used for collection from the plain text (JSON).

In a future post, we will extend the example to clarify this.

See you soon!

References:

google-gson. August 19 de 2010. Google Code [online]. Available from Internet:http://code.google.com/p/google-gson/ [accessed on October the 26th 2010].

Monday, November 1, 2010

How to import source code from SVN to Netbeans 6.9

This post will establish the required steps to import the JSON ME source code from java.net, and in general from a svn repository into your NetBeans 6.9.

The code we want to import can be found at:

http://java.net/projects/meapplicationdevelopers/sources/svn/show/demobox/mobileajax/lib/json

Requirements:

Download and install NetBeans 6.9, if you don't have it yet.
Create an account at Java.net, if you don't have one.

Steps:

Start NetBeans 6.9 and go to the menu Team-->Subversion-->Checkout.

To access the source code using Subversion, you must have Subversion client installed on your system. The first time you start NetBeans to access Subversion, it offers you the possibility to download and install the client automatically or manually.

Note: With previous releases of NetBeans, the installation had to be done manually:

http://www.open.collab.net/downloads/netbeans/
Once the Subversion client has been installed, you can specify the SVN repository in where the source code is. For this example:

https://svn.java.net/svn/meapplicationdevelopers~svn

The last URL can be found at:

http://java.net/projects/meapplicationdevelopers/sources

Next, enter your java.net account info and if you have a proxy, you can configure it here as well.
Next, you can choose the directory you want to import.

For our example, choose:

demobox/mobileajax/lib/json

Also, you have to specify which revision version you want to import, press Browse to find the revision you want. For this case, the last revision was 333.

Finally, you must select the local folder in which you want to import the selected directory and leave the box "Scan for NetBeans Projects after Checkout" checked. Thus, when the process is finished, NetBeans will automatically import the project (in case that the source code contains a NetBeans project).
After importing the source code, there may be problems with the MicroEdition SDK version installed on your system and the one the source code was using. To solve the problem, you can right click on the imported project, choose properties and assign the MicroEdition SDK installed on your system to the project. To troubleshoot other resources, you should check if you have them installed in your system, if not, maybe you should get them before you can compile the source code using NetBeans.

References:

meapplicationdevelopers: Subversion Instructions. 2007. Oracle [online].
Available from Internet:
http://java.net/projects/meapplicationdevelopers/sources
[accessed on October the 27th 2010].

meapplicationdevelopers: Subversion. 2007. Oracle [online].
Available from Internet: http://java.net/projects/meapplicationdevelopers/sources/svn/show/demobox/mobileajax/lib/json
[accessed on October the 27th 2010].

CollabNet Subversion Downloads for NetBeans 6.5. CollabNet 2010 [online].
Available from Internet:
http://www.open.collab.net/downloads/netbeans/
[accessed on October the 29th 2010].