Need assistance getting my Java UDP client application working

Question

I am working on a UDP client to communicate with a server that I have no control over. Part of the requirement of communicating with this server is that I must supply a UDP port number [as part of the message] to the server for its reply communications.

I have several classes in my application, that require a DatagramSocket, representing different requests to this server, so I created a static class:

package mypackage;

import java.net.DatagramSocket;
import java.net.InetSocketAddress;
import java.net.SocketException;

public class DatagramSocketGrabber {

    public DatagramSocketGrabber(){}

    public static DatagramSocket getSocket() throws SocketException {
        DatagramSocket newSocket = new DatagramSocket();
        InetSocketAddress newSocketAddress = new InetSocketAddress(15346);
        newSocket.bind(newSocketAddress);
        return newSocket;
    }
}

In my main() class, I am grabbing this socket connection and using it throughout my application:

package mypackage;

import java.net.DatagramSocket;
import java.net.SocketException;

public class MyApp {
    public static void main(String[] args){
        DatagramSocket mySocket = null;

        try {
            mySocket = DatagramSocketGrabber.getSocket();
        } catch(SocketException se){
            System.out.println(se);
        }

        DSClass01 class01 = new DSClass01(mySocket);
        DSClass02 class02 = new DSClass02(mySocket);
        DSClass03 class03 = new DSClass03(mySocket);

        mySocket.close();
    }
}

However, whenever I run my application, I always get the error:

java.net.SocketException: already bound

NEED TO KNOW:

The UDP server is on my machine (localhost), but it is a compiled application, so I cannot examine its code. Yes, it is from a trustworthy source.

There are no firewall or networking issues because it is a stand-alone computer, with everything wide open.

Before I created the static DatagramSocketGrabber class, I was creating a new DatagramSocket in all of my message classes, and the server was able to successfully receive my messages.

However, the reason I created the static class is because I was not receiving any replies, and I determined that it was because I did not bind my socket to a "fixed" receive port. In order to do that, and be able to use that bound socket in all of my message classes, I figured I needed to create a DatagramSocket creation class to have a single, bound socket that I could pass around.

It seems like you're trying to bind port 15346 multiple times — mr.celo

JFPicard JFPicard · Accepted Answer · 2015-11-25T16:19:18

My guess, since you don't share the code that is throwing the SocketException is that one of the following.

1) You call DatagramSocketGrabber.getSocket(); a second time in your program, hence the SocketException.

2) You left some code that is binding to the same port (like new InetSocketAddress(15346);)

Try to search your code for the socket binding (15346) and try to find if there is any other DatagramSocketGrabber.getSocket(); call in your program.

In the mean time, you can change your DatagramSocketGrabber to a singleton and keeping the DatagramSocket as an attribute, so you can always call your singleton instead of passing the socket to all your methods.

Need assistance getting my Java UDP client application working

2 Answers