Question

I’m having trouble transitioning to Java from C/C++ for my “Telnet” interface to some modules we work with here. I want to be able to establish a connection with a card that, after starting it’s command line interface, waits for a connection and serves up a prompt (“OK>”) to the clients. This works fine for both C and C# clients I’ve written, but the Java has given me some issues. I’ve attached some code that I grabbed from some examples online, but so far, all I can ascertain for sure is that the socket is being created.

Code:

   private boolean CreateTelnetSession()
   {
        try
        {
            _socket = new Socket();
            _socket.connect(new InetSocketAddress(_ipAddr, _ipPort));
            _socket.setSoTimeout(10000);
            _socket.setKeepAlive(true);

            _out = new PrintWriter(_socket.getOutputStream(), true);
            _in = new BufferedReader(new InputStreamReader(_socket.getInputStream()));

            _out.println("\r\n");

            System.out.println(_in.readLine());

            return true;
        }
        catch(Exception e)
        {
            System.out.println("Exception!");
        }
        return false;
    }

The socket SEEMS to be created correctly, and when the program shuts down, I can see the session close on the card(s) I’m trying to talk to, but I don’t see the carriage return/line feed echoed on the card as I would expect, or a prompt returned via the InputStream. Is it possible that it’s a character encoding issue? Am I doing something incorrectly with the streams (crossing them!?!)? Any insight at all? When I get over this initial learning curve, I would like to acknowledge how easy Java makes these socket reads and writes, but until then…

I read this post:
java simple telnet client using sockets

It seems similar to what I’m running up against, but it’s not the same. I’m willing to take the rep hit if someone has seen something on here that resolves my issue, so feel free to let me know, bluntly, what I missed.

Edit:

private boolean CreateTelnetSession()
{
    try
    {
        _socket = new Socket();
        _socket.connect(new InetSocketAddress(_ipAddr, _ipPort));
        _socket.setSoTimeout(10000);
        _socket.setKeepAlive(true);

        _out = new DataOutputStream(_socket.getOutputStream());
        _in = new DataInputStream(_socket.getInputStream());

        _outBuffer = ByteBuffer.allocate(2048);
        _outBuffer.order(ByteOrder.LITTLE_ENDIAN);
        _inBuffer = ByteBuffer.allocate(2048);
        _inBuffer.order(ByteOrder.LITTLE_ENDIAN);

        System.out.println("Connection Response: " + _in.read(_inBuffer.array()));
        System.out.println("Response: " + WriteCommand("DRS\r\n"));

        return true;
    }
    catch(Exception e)
    {
        System.out.println("Exception!");
    }
    return false;
}

private String WriteCommand(String command)
{
    try
    {
        _outBuffer = encoder.encode(CharBuffer.wrap(command));
        _out.write(_outBuffer.array());
        _out.flush();

        _in.read(_inBuffer.array());
        String retString = decoder.decode(_inBuffer).toString();

        return retString.substring(0, retString.indexOf('>') + 1);
    }
    catch(Exception e)
    {
        System.out.println("Exception!");
    }

    return "E1>";
}   

There are many things to clean up and I’m going to experiment with whether I need to do it in quite this way, but this is the gist of the “solution”. The big killer was the endian-ness. It should be mentioned, once again, that this is ugly and non-production code, but any other input would still be appreciated.

Answer 1

I have a couple things you can try. You are using a PrintWriter for your output, this is a fairly high-level Writer (i.e. it encapsulates a lot of things from you). My concern is that the println() method in PrintWriter adds a line terminating character(s) at the end automatically (as appropriate for your OS). So what you are really sending is “/r/n(line terminator)” so on a unix box you would be sending “/r/n/n”.

I would recommend switching to a DataOutputStream which gives you much more control over the raw bytes that are sent: http://docs.oracle.com/javase/6/docs/api/java/io/DataOutputStream.html

Remember if you switch to DataOutputStream you need to call flush on the output stream.

My other thought is it might be an endianess problem. Java is strictly Big Endian (network byte order). Is it possible your “card” is reading things in little-endian? If you need to write over the network in little endian (if so your card is a bad netizen!) you will need to use a ByteBuffer, set its order to little-endian. Write your bytes to it, then write the bytes from your ByteBuffer to the DataOutputStream.

I would probably switch to a DataInputStream for your input stream too. readline() will only return once the newline character is seen. Is your card returning a newline in its response?

My last thought is that your println methods might have an error and you don’t know it because PrintWriter doesn’t throw exceptions. The PrintWriter JavaDocs says:

“Methods in this class never throw I/O exceptions, although some of its constructors may. The client may inquire as to whether any errors have occurred by invoking checkError().”

Hopefully something in my long rambling response will help you.