Question

EDIT: The code below throws no exception but has no output and hangs. It should output “Test message”. In main(), we start a thread that’s given a server socket listening on a random port. The main thread the tries to connect and communicate with the ServerSocket on that same random port, but is apparently failing. Why?

public class IntraProcSockTest {
private static int port;

private class Listener extends Thread {
    public Listener() {
    }

    public void run() {
        try {
            ServerSocket serverSocket = new ServerSocket(0);
            port = serverSocket.getLocalPort();
            Socket socket = serverSocket.accept();

            BufferedReader in;
            String fromClient;

            in = new BufferedReader(new InputStreamReader(
                    socket.getInputStream()));

            while ((fromClient = in.readLine()) != null) {
                System.out.println("From client: " + fromClient);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

public IntraProcSockTest() {
    new Listener().start();
}

public static void main(String[] args) {
    new IntraProcSockTest();

    try {
        Thread.sleep(5000);
        Socket socket = new Socket("localhost", port);
        PrintWriter socketOut = new PrintWriter(socket.getOutputStream());

        socketOut.println("Test message");
    } catch (UnknownHostException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
}

Answer 1

A process can connect to a socket created by itself, there is no problem. Show us the code that throws an exception and/or more details about the exception.

First of all, be careful not to specify a local port for the client socket (the one connecting to the other which is listening). Let the OS choose a random port. Remember that any socket is identified by four elements (remote host, local host, remote port, local port), if you bind both the server socket and the client socket on the same local port, let it be 4498, both sockets are defined as follows: (localhost, localhost, 4498, 4498) and this doesn’t work. I suspect this might be your problem.
To avoid such problems, client sockets are often bound to a random port, chosen by the OS. Show us your code, expecially the part in which the client sockets gets created and connects to the server socket.

And about IPC, it is not always bad to use sockets as an inter-process or even intra-process communication technique. The performance is worse, obviously, and you might loose some code readability, but your software will be easily portable to a network (distributed) application. It’s up to your plans, it’s not like IPC sockets == bad.