Java socket server client to client file transfer

Question

I'm not sure if what I'm trying to do is possible. I want a socket server to run, multiple clients can connect to it. Client A drags and drops a file onto his GUI and client B gets a popup asking if he wants to receive the file. If agreed client A starts uploading the file and client B starts downloading the file directly. Is this possible? should the file first be transferred to the server? If I send a file to the server it works, but when I try to send a file to client BI get the following:

java.net.ConnectException: Connection timed out: connect
    at java.net.DualStackPlainSocketImpl.connect0(Native Method)
    at java.net.DualStackPlainSocketImpl.socketConnect(DualStackPlainSocketImpl.java:79)
    at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:339)
    at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:200)
    at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:182)
    at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:172)
    at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392)
    at java.net.Socket.connect(Socket.java:579)
    at java.net.Socket.connect(Socket.java:528)
    at java.net.Socket.<init>(Socket.java:425)
    at java.net.Socket.<init>(Socket.java:241)

here is the upload class:

public Upload(String addr, int port, File filepath, OpenChatFrame frame) {
        super();
        try {
            file = filepath;
            ui = frame;
            socket = new Socket(InetAddress.getByName(addr), port);
            Out = socket.getOutputStream();
            In = new FileInputStream(filepath);
            length = file.length();
        } catch (Exception ex) {
            ex.printStackTrace();
        }
    }

    @Override
    public void run() {
        try {
            byte[] buffer = new byte[1024];

            int count;

            ui.jProgressBar1.setMaximum((int) length);

            while ((count = In.read(buffer)) >= 0) {
                Out.write(buffer, 0, count);
                bytes += count;
                ui.jProgressBar1.setValue(bytes);

            }
            Out.flush();


            if (In != null) {
                In.close();
            }
            if (Out != null) {
                Out.close();
            }
            if (socket != null) {
                socket.close();
            }
        } catch (Exception ex) {
            ex.printStackTrace();
        }
    }

and here is the download class:

public Download(String saveTo, OpenChatFrame ui) {
    try {
        server = new ServerSocket(0);
        port = server.getLocalPort();
        this.saveTo = saveTo;
        this.ui = ui;
    } catch (IOException ex) {
        //TODO: catch
    }
}

@Override
public void run() {
    try {
        socket = server.accept();
        System.out.println("Download : " + socket.getRemoteSocketAddress());

        In = socket.getInputStream();
        Out = new FileOutputStream(saveTo);

        byte[] buffer = new byte[1024];
        int count;

        while ((count = In.read(buffer)) >= 0) {
            Out.write(buffer, 0, count);
        }

        Out.flush();

        File physicalFile = new File(saveTo);
        System.out.println("FILE FILENAME: " + physicalFile.getName());
        System.out.println("saveTo: " + saveTo);


        if (Out != null) {
            Out.close();
        }
        if (In != null) {
            In.close();
        }
        if (socket != null) {
            socket.close();
        }
    } catch (Exception ex) {
        //TODO: catch
    }
}

Answer 1

I want a socket server to run, multiple clients can connect to it. Client A drags and drops a file onto his GUI and client B gets a popup asking if he wants to receive the file. If agreed client A starts uploading the file and client B starts downloading the file directly. Is this possible? should the file first be transferred to the server?

Yes it is possible, if the circumstances allow it.

Assuming that the client A initiates the Socket connection, then client B's IP address must be addressable by client A. If client B has a public IP address, or a private IP address that client A's network has a route to, then you are OK. However, client B has a private IP address and client A cannot route to it, then it cannot work, and you would need a public server (accessible to both) to relay the data.

The other thing that is required is that there are no firewalls blocking connections from client A to client B on the port that they are trying to use.

---

Your partial stacktrace seems to be from the initiating client (client A), and indicates that the connection attempt is not getting through. Given the nature of the exception, I think that most likely explanation is that there is a firewall blocking the connection.

(If this was a routing problem, or if the server wasn't listening on the designated port, you would see different exceptions ... )

---

Asides:

1. It is bad style to start variable names with an uppercase letter; eg In and Out . The style rules say that In and Out ought to be class names.

2. The terms "upload" and "download" are not normally used to describe a "peer-to-peer" transfer; eg between two clients.