Java UDP Applet Problem

bman

Hey,

I'm not very good at programming in java so this problem may be very simple. I'm trying to get basic communications between a UDP server application and a UDP Client running in an applet. The client works fine when I run the code inside an application, but when I run it as an applet I get the JOptionPane showing "Probe1" but never gets to "Probe2". Has this got something to do with java security not allowing UDP/TCP from an applet? I had a similar project work using TCP last year as long as I was communicating with an application on the localhost so I thought it should work the same here. Any help would be very much appriecated.

Edit: I just checked to see if there was any error raised in the applet. When I print it out it shows:
java.security.AccessControlException: access denied (java.net.SocketPermission 127.0.0.1:9876 connect, resolve)
What does this mean??
______________________________________

import java.applet.*;
import java.awt.*;
import java.io.*;
import java.net.*;
import javax.swing.*;

public class UDPApplet extends Applet
{
//public void paint( Graphics g )
public void init()
{
try
{
DatagramSocket clientSocket = new DatagramSocket();

InetAddress IPAddress = InetAddress.getByName("127.0.0.1");

byte[] sendData = new byte[1024];

byte[] receiveData = new byte[1024];

String sentence = "Hello World";

sendData = sentence.getBytes();

DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, 9876);

JOptionPane.showMessageDialog(null, "Probe1");

clientSocket.send(sendPacket);

JOptionPane.showMessageDialog(null, "Probe2");

DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);

clientSocket.receive(receivePacket);

String modifiedSentence = new String(receivePacket.getData());

JOptionPane.showMessageDialog(null, modifiedSentence);

clientSocket.close();

}
catch (Exception e) {}
}
}
______________________________________________

import java.io.*;
import java.net.*;

class UDPS
{
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());

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);
}
}

}

Webmonkey

Um this might sound a bit obvious but you sure you don't have a firewall running that is blocking this port? First thing I would try.

Edit - now i don't know anything about java didn't touch it yet but you may have to grand permission in your app.

This might look interesting: http://forums.devx.com/showthread.php?t=29907

bman

Thanks for the reply Webmonkey. The reason I didn't think it was a firewall problem is because it works in the application format but not in the applet. I'll have a look at that thread you linked to.

sh_o

The issue is with the security model that is run when running an applet as opposed to an application.
The following url has a good explanation from the source on how to do this.
http://java.sun.com/developer/technicalArticles/Security/applets/

For testing purposes, if you are trying the applet using appletviewer, use the command 'policytool' from the jre and add a new permission "all permissions" to your policy file and then you should be able to connect. Note this is just a quick workaround to save you signing the jar file etc which would be necessary for any production applet.

bman

Thanks sh_o. I don't want to have to go through the whole signing jar files and all that if i can help it, but applet viewer will not do. Is there a quick workaround for running this in a browser without the signing of jar files? Its only a small part of a college project so I want to hack through it as quick as possible.

DublinDilbert

From memory the only server an applet can open a TCP/UDP connection to is the server it was downloaded from, into the web-browser.

Is your UDP server running on the same machine that serves the web-page that calls the applet?

bman

Ok, so I went through signing the jar files and it works well. Now I want to have a C server talk to a java applet. It works fine for a C server and client, a java server and client applet, but for some reason I cannot send a datagram from the java applet to the C server. I have copied in the code I think is relevant below. Is there some sort of problems with interoperability between java and c strings/byte[]? Hopefully someone knows where I'm going wrong, thanks.

java client code for sending datagram:

InetAddress IPAddress = InetAddress.getByName("localhost");//("127.0.0.1");
String s = "Hello";
sendData = s.getBytes();
DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress,1099);
clientSocket.send(sendPacket);


c server code to receive datagram:

int sock, length, fromlen, n, i = 0;
struct sockaddr_in server;
struct sockaddr_in from;
char buf[1024];

sock=socket(AF_INET, SOCK_DGRAM, 0);
if (sock < 0) error("Opening socket");

length = sizeof(server);

bzero(&server,length);

server.sin_family=AF_INET;

server.sin_addr.s_addr=INADDR_ANY;

server.sin_port=htons(PORT);//atoi(PORT));

if (bind(sock,(struct sockaddr *)&server,length)<0)
error("binding");

fromlen = sizeof(struct sockaddr_in);

printf("Waiting for datagram from client\n");
n = recvfrom(sock,buf,1024,0,(struct sockaddr *)&from,&fromlen);
if (n < 0) error("recvfrom");
write(1,"Received a datagram: ",21);
write(1,buf,n);


The c program sits at "Waiting for datagram from client".
Once again, thanks for all the help so far.

sh_o

I can only speak from the java end, but I presume the information is buffered so make sure you call clientSocket.flush() on the java client end to indicate you are finished.
Also, should the c code not be in some form of a loop, put a while loop around the checking for the datagram bit

Webmonkey

Yeah i was wondering about that while loop as well

bman

It seems the problem was with this line although I can see no reason why it wouldn't work as it worked for the application?!

InetAddress IPAddress = InetAddress.getByName("localhost");//("127.0.0.1");

I deleted the "localhost" part and uncommented the "127.0.0.1" part. Works fine now. Thanks for the help.