java.net.ConnectException: Connection refused when SocketChannel.open is invoked

Question

Hi, I am trying to write a simple program to open a socket channel to a local address. I get a connection refused exception whenever I run this program

import java.net.InetAddress;
import java.net.InetSocketAddress;
import java.nio.channels.SocketChannel;

public class testSocket {

        public static void main(String [] args) {
                try {
                        InetAddress addr = InetAddress.getByName("localhost");
                        InetSocketAddress remoteAddress = new InetSocketAddress(addr, 19015);

                        // Open a new Socket channel and set it to non-blocking
                        SocketChannel socketChannel = SocketChannel.open();
                        socketChannel.configureBlocking(false);

                        // Issue the Connect call on the remote address.
                        socketChannel.connect(remoteAddress);
                } catch (Exception e) {
                        e.printStackTrace();
                }
        }
}

The exception that I get is

java.net.ConnectException: Connection refused
        at sun.nio.ch.Net.connect(Native Method)
        at sun.nio.ch.SocketChannelImpl.connect(SocketChannelImpl.java:464)
        at testSocket.main(testSocket.java:17)

I encounter this issue with Sun Solaris and HP - UX. It seems to work fine on a Linux machine. Can anyone let me know why the connection is being refused? I did a netstat -a and confirmed that the port is not in use.

Thanks in advance!

Answer 1

The "Connection refused" message is what you'll receive when there is no process listening on your specified port (19015). It looks like you're trying to connect to a service that isn't there. netstat is even telling you that the port isn't in use!

Answer 2

From the Javadoc for SocketChannel.connect()

If this channel is in non-blocking mode then an invocation of this method initiates a non-blocking connection operation. If the connection is established immediately, as can happen with a local connection, then this method returns true. Otherwise this method returns false and the connection operation must later be completed by invoking the finishConnect method.

When I run your code on Linux, connect() returns false hence there is no exception. If you add a call to socketChannel.finishConnect() you will see the same connection refused exception as you get on Solaris/HP-UX.

I suspect that on Solaris/HP-UX connect() returns true hence the exception is thrown immediately.