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Socket in J2ME

Andrew Byrne

Joined: Jan 20, 2004
Posts: 3
I have a J2ME Client connecting to a C server using sockets.
I'm able make the socket connection and send messages from the J2ME client to the server but I don't know how to send messages back.
I want to be able to read messages coming in on the InputStream when I send them from the server but I don't seem to be able to access my InputStream,SocketConnection etc from within "public void commandAction" and therefore can't call ""
Does any one know how to do this?
Here is a sample of my code...
import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;
public class Login_MIDlet implements Runnable, CommandListener {
private static Display display;
private TextBox password;
private Project_MIDlet parent;
private boolean stop;

private MainMenu_MIDlet startMenu;

private Command exitCommand = new Command("Exit", Command.EXIT, 1);
private Command startCommand = new Command("Login", Command.ITEM, 1);

InputStream is;
OutputStream os;
SocketConnection sc;
Sender sender;

public Login_MIDlet(Project_MIDlet m) {

parent = m;
display = Display.getDisplay(parent);
password = new TextBox("Enter System Password", "", 20, TextField.PASSWORD);

public void pauseApp(){

public void start() {
Thread t = new Thread(this);

public void destroyApp(boolean unconditional) {
password = null;
exitCommand = null;
startCommand = null;
display = null;

public void run() {
try {
sc = (SocketConnection)"socket://");

is = sc.openInputStream();
os = sc.openOutputStream();
// Start the thread for sending messages - see Sender's main
// comment for explanation
sender = new Sender(os);

// Loop forever, receiving data
while (true) {
StringBuffer sb = new StringBuffer();
int c = 0;
while (((c = != '\n') && (c != -1)) {
sb.append((char) c);

} catch (ConnectionNotFoundException cnfe) {
Alert a = new Alert("Client", "Please run Server MIDlet first",
null, AlertType.ERROR);
} catch (IOException ioe) {
if (!stop) {
} catch (Exception e) {
public void commandAction(Command c, Displayable d) {
if (c == exitCommand) {
* Close all open streams
try {
stop = true;
if (is != null) {
if (os != null) {
if (sc != null) {
catch (IOException ioe) {


else if (c == startCommand) {
String t = password.getString();
String getPassword = "1";
StringBuffer PASSWORD = new StringBuffer();
int i = 0;
while (((i = != '\n') && (i != -1)) {
PASSWORD.append((char) i);
if (t.equals(PASSWORD)) {
startMenu = new MainMenu_MIDlet(parent, sender, is, os, sc);
else {
Alert error = new Alert ("Incorrect Password");
display.setCurrent(error, password);
serge masse
Ranch Hand

Joined: Jul 23, 2003
Posts: 102
I did not read all the code yet but based on your question, I can ask: Did you try creating a new Thread in the commandAction() method and in the new thread then you use your InputStream (is) and do If you did not try this, then I recommend that you try, it should work :-)
for an example of an API that does this, you can look at

serge - - my android apps
Andrew Byrne

Joined: Jan 20, 2004
Posts: 3
Thanks for the reply,
I did not create a new Thread in the commandAction() method to use the InputStream. How would I do that? I'm new to J2ME and I don't fully understand how to use sockets in MIDlets.
Ko Ko Naing
Ranch Hand

Joined: Jun 08, 2002
Posts: 3178
Originally posted by Andrew Byrne:
I'm new to J2ME and I don't fully understand how to use sockets in MIDlets.

If u've ever used socket programming in J2SE, they are a lot similar to J2ME's..... If u r still new to the sockets, then I would suggest u to learn socket programmin in J2SE first and go to J2ME later...
U might want to have a look at the following article on
J2ME Network Programming

Co-author of SCMAD Exam Guide, Author of JMADPlus
Andrew Byrne

Joined: Jan 20, 2004
Posts: 3
Hi thanks for the link,
It's not that I don't know how to use sockets. Sockets aren't all that complicated. What I don't know how to do is send/recieve messages over the socket connection from within the commandAction() method. Well, I'm using this other class Sender(from example in WTK sample folder) to send messages and I can see the server recieving them. Is there perhaps a similar class for recieving messages from the server?
I agree. Here's the link:
subject: Socket in J2ME
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