“BindException: Address already in use” exception when transferring file using java socket in a same pc

Question

I am very new to java network programming. I am trying to write a program that can transfer file over the network. I have write two separate program for sender and receiver. And run that two programs in two separate Intellij window in my pc. I have put the same port number in server socket as well as receiver socket. And the IP Address is the localhost. When I run that two program either of that two throws exception.

After some googling I found Java sockets: multiple client threads on same port on same machine? question where people said that it is legitimate to run two program in same port. I haven't to test this code from two different from two PC.

So my question what am I missing ? Can I run this two program in my PC simultaneously?

here is my written code -

for Sender--

public class Sender {
public static void main(String[] args) throws IOException
{
    String fileLocation;
    int portNo;
    portNo = 6000;
    fileLocation = "/files/A.cpp";
    Sender.send(portNo,fileLocation);
}
public static  void send(int portNo,String fileLocation) throws IOException
{

    FileInputStream fileInputStream = null;
    BufferedInputStream bufferedInputStream = null;

    OutputStream outputStream = null;
    ServerSocket serverSocket = null;
    Socket socket = null;
    try {
        serverSocket = new ServerSocket(portNo);
        System.out.println("Waiting for receiver...");
        try {
            socket = serverSocket.accept();
            System.out.println("Accepted connection : " + socket);

            File file = new File (fileLocation);
            byte [] byteArray  = new byte [(int)file.length()];
            fileInputStream = new FileInputStream(file);
            bufferedInputStream = new BufferedInputStream(fileInputStream);
            bufferedInputStream.read(byteArray,0,byteArray.length); // copied file into byteArray

            //sending file through socket
            outputStream = socket.getOutputStream();
            System.out.println("Sending " + fileLocation + "( size: " + byteArray.length + " bytes)");
            outputStream.write(byteArray,0,byteArray.length);
            outputStream.flush();
            System.out.println("Done.");
        }
        finally {
            if (bufferedInputStream != null) bufferedInputStream.close();
            if (outputStream != null) bufferedInputStream.close();
            if (socket!=null) socket.close();
        }
    } catch (IOException e) {
        e.printStackTrace();
    }
    finally {
        if (serverSocket != null) serverSocket.close();
    }
}
}

for Receiver --

public class Receiver {

public static void main (String [] args ) throws IOException {


    String fileLocation,ipAddress;
    int portNo;
    ipAddress = "localhost";
    portNo = 6000;
    fileLocation = "/files/A.cpp";
    Receiver.receiveFile(ipAddress, portNo, fileLocation);


}
public static void receiveFile(String ipAddress,int portNo,String fileLocation) throws IOException
{
    int bytesRead=0;
    int current = 0;
    FileOutputStream fileOutputStream = null;
    BufferedOutputStream bufferedOutputStream = null;
    Socket socket = null;
    try {
        socket = new Socket(ipAddress,portNo);
        System.out.println("connected.");

        byte [] byteArray  = new byte [6022386];                    
        System.out.println("Please wait downloading file");

        InputStream inputStream = socket.getInputStream();
        fileOutputStream = new FileOutputStream(fileLocation);
        bufferedOutputStream = new BufferedOutputStream(fileOutputStream);
        bytesRead = inputStream.read(byteArray,0,byteArray.length);         

        current = bytesRead;
        do {
            bytesRead =inputStream.read(byteArray, current, (byteArray.length-current));
            if(bytesRead >= 0) current += bytesRead;
        } while(bytesRead > -1);
        bufferedOutputStream.write(byteArray, 0 , current);             

        System.out.println("File " + fileLocation  + " downloaded ( size: " + current + " bytes read)");
    } catch (IOException e) {
        e.printStackTrace();
    }
    finally {
        if (fileOutputStream != null) fileOutputStream.close();
        if (bufferedOutputStream != null) bufferedOutputStream.close();
        if (socket != null) socket.close();
    }
}
}

Thank you.

Answer 1

Your receiver does not need to specify a port to use, leave that to the OS (it will automatically assign a free ephemeral port number; the port number is not important).

Instead, use the Socket.connect(SocketAddress) method to connect the client socket to the server. Specifiy the socket address using the IP/Name of the server plus the port number used, eg:

InetAddress iadr = InetAddress.getByName("localhost");
SocketAddress sadr = new InetSocketAddress(iadr, portNo);
Socket socket = new Socket();
socket.connect(sadr);

This is how you normally connect to a server at a specfied port. The local (client) endpoint is not explicitly specified.

Of course, the server must already be running for this to succeed.

Edit: Since the client endpoint is automatically assigned, the OS will assign a free endpoint for every client, thus you can run multiple clients (receivers) on the same machine without any conflicts.

Answer 2

First it's impossible to use the same port for two serverSockets so your code should work if you don't run the Sender Class twice , you need to correct the name of your class Receiver_ to be Receiver then the steps for your code to be working are :

• First you run Receiver Class (always start by the server)
• Then you run Sender Class (which is the Client here)
• if it doesn't work it might be because another program is using the port 6000, try to change it to another one like 9010 in both of your classses