Why I am getting the bad length exception when I am running this application? - java-me

Question

I want to communicate to a server from J2me app using UDP.However, when I am running the app, I am getting a bad length exception.My codes and output are given below.
client code
import javax.microedition.midlet.MIDlet;
import javax.microedition.lcdui.Display;
import javax.microedition.lcdui.Command;
import javax.microedition.lcdui.CommandListener;
import javax.microedition.lcdui.Displayable;
import javax.microedition.lcdui.TextBox;
import javax.microedition.lcdui.TextField;
import javax.microedition.io.Connector;
import javax.microedition.io.Datagram;
import javax.microedition.io.DatagramConnection;
import java.io.IOException;
public class DatagramTest extends MIDlet
implements CommandListener, Runnable
{
private static final int BUF_SIZE = 1024;
private static Command exit = new Command("Exit", Command.EXIT, 1);
private static DatagramTest instance;
private Display display;
private TextBox dgramText;
private DatagramConnection conn;
private Datagram dgram;
private String address = "datagram://myip:9876";
public DatagramTest()
{
super();
instance = this;
}
public DatagramTest(String service)
{
this();
address = service;
}
/**
Returns the single instance of this class. Calling
this method before constructing an object will return
a null pointer.
#return an instance of this class.
*/
public static DatagramTest getInstance()
{
return instance;
}
public void startApp()
{
display = Display.getDisplay(this);
dgramText = new TextBox("Datagram contents",
null,
2048,
TextField.ANY);
dgramText.setCommandListener(this);
display.setCurrent(dgramText);
System.out.println("Starting run....");
run();
System.out.println("Stopping run....");
}
public void run()
{
System.out.println("In run....");
try
{
int maxLength;
conn = (DatagramConnection)Connector.open(address);
maxLength = conn.getMaximumLength();
dgram = conn.newDatagram(1024);
dgram.reset();
conn.send(dgram);
conn.receive(dgram);
byte[] data = dgram.getData();
// Extract the response string.
String str = new String(data);
System.out.println(str);
dgram.reset();
System.out.println("Exit run....");
}
catch (IOException ioe)
{
System.out.println(ioe.getMessage());
ioe.printStackTrace();
quit();
}
return;
}
public void pauseApp()
{
}
void quit()
{
destroyApp(true);
notifyDestroyed();
}
public void destroyApp(boolean destroy)
{
try
{
conn.close();
}
catch (IOException ioe)
{
ioe.printStackTrace();
}
}
public void display()
{
Display.getDisplay(this).setCurrent(dgramText);
}
public void commandAction(Command c, Displayable d)
{
if (c == exit)
{
quit();
}
}
}
Server code
import java.io.*;
import java.net.*;
class UDPServer
{
public static void main(String args[]) throws Exception
{
DatagramSocket serverSocket = new DatagramSocket(9876);
byte[] receiveData = new byte[1024];
byte[] sendData = new byte[1024];
while(true)
{
DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
serverSocket.receive(receivePacket);
String sentence = new String( receivePacket.getData());
System.out.println("RECEIVED: " + sentence);
InetAddress IPAddress = receivePacket.getAddress();
int port = receivePacket.getPort();
String capitalizedSentence = sentence.toUpperCase();
sendData = capitalizedSentence.getBytes();
DatagramPacket sendPacket =
new DatagramPacket(sendData, sendData.length, IPAddress, port);
serverSocket.send(sendPacket);
}
}
}
Output at clientside
Starting run.... In run.... Bad
datagram length java.io.IOException:
Bad datagram length
at com.sun.midp.io.j2me.datagram.Protocol.receive(Protocol.java:367)
at hello.DatagramTest.run(DatagramTest.java:89)
at hello.DatagramTest.startApp(DatagramTest.java:69)
at javax.microedition.midlet.MIDletProxy.startApp(MIDletProxy.java:43)
at com.sun.midp.midlet.Scheduler.schedule(Scheduler.java:374)
at com.sun.midp.main.Main.runLocalClass(Main.java:466)
at com.sun.midp.main.Main.main(Main.java:120)
Stopping run....
Why I am getting this bad length exception and how do I sort it out?

Answer 1

One thing you need to try is to send and receive datagrams in a separate thread.
The documentation for DatagramConnection.receive() says: "This method blocks until a datagram is received"
You are calling it from inside MIDlet.startApp().
Blocking the application management system thread that calls startApp() is bad practice.