This test shows that Java will not allow a socket to connect to itself via TCP's simultaneous connection feature, by setting the source port identical to the destination port.

NoLoopBackSocket.java

import org.junit.Test;

import java.net.InetAddress;
import java.net.InetSocketAddress;
import java.net.Socket;
import java.net.SocketException;

/**
 * This tests make sure Java prevents a socket from connecting to itself
 * via binding to the same port it connects to
 */
public class LoopBackSocket {

    @Test(expected = SocketException.class)
    public void testNoLoopbackSymConnection() throws Exception {
        final InetAddress localhost = InetAddress.getLocalHost();
        Socket socket = new Socket();
        socket.bind(new InetSocketAddress(localhost, 7777));
        socket.connect(new InetSocketAddress(localhost, 7777));
    }
}