Java Cookbook

Handling Network Errors

Problem

You want more detailed reporting than just IOException if something goes wrong.

Solution

Catch a greater variety of exception classes. There are several subclasses of SocketException; the most notable of these are ConnectException and NoRouteToHostException. The names are self-explanatory: the first means that the connection was refused by the machine at the other end (the server machine), and the second completely explains the failure. Example 15-3 is an excerpt from the Connect program, enhanced to handle these conditions.

Example 15-3. ConnectFriendly.java

/* Client with error handling */
public class ConnectFriendly {
    public static void main(String[] argv) {
        String server_name = argv.length == 1 ? argv[0] : "localhost";
        int tcp_port = 80;
        try {
            Socket sock = new Socket(server_name, tcp_port);

            /* Finally, we can read and write on the socket. */
            System.out.println(" *** Connected to " + server_name  + " ***");
            /* ... */

            sock.close(  );

        } catch (UnknownHostException e) {
            System.err.println(server_name + " Unknown host");
            return;
        } catch (NoRouteToHostException e) {
            System.err.println(server_name + " Unreachable" );
            return;
        } catch (ConnectException e) {
            System.err.println(server_name + " connect refused");
            return;
        } catch (java.io.IOException e) {
            System.err.println(server_name + ' ' + e.getMessage(  ));
            return;
        }
    }
}