Using ProcessBuilder to read/write to telnet process

Question

I am trying to read/write values from/to telnet process by means of ProcessBuilder.

 public static void main(String[] args) {
        try {
            telnetProcess = new ProcessBuilder("C:\\Windows\\System32\\telnet.exe","x.x.x.x").start();
            telnetInputReader = new BufferedReader(new InputStreamReader(telnetProcess.getInputStream()));
            telnetOuputWriter = new BufferedWriter(new OutputStreamWriter(telnetProcess.getOutputStream()));
            expectPattern("login:");
            sendCmd("user");
            expectPattern("password:");
            sendCmd("pwd");
            expectPattern("switch>#");
            sendCmd("exit");
            expectPattern("Connection to host lost");
        } catch (IOException ex) {
            System.out.println("Exception : " + ex);
        }
    }

I got the following error

java.io.IOException: Cannot run program "C:\Windows\System32\telnet.exe": CreateProcess error=2, The system cannot find the file specified

I tried to change the file path to unix formatted style like C:/Windows/System32/telnet.exe and no luck. (Though I expected it to not to work). Then copied the telnet.exe from it's location to some other user's home directory and I was not getting any errors. (???)

But, I didn't see the output as expected. I didn't get any response from the process and the code exited.

    public static void sendCmd(String cmd) {
        System.out.println(cmd);
        try {
            telnetOuputWriter.write(cmd + "\n", 0, cmd.length());
        } catch (IOException ex) {
            Logger.getLogger(TelnetProcessHandler.class.getName()).log(Level.SEVERE, null, ex);
        }

    }

    public static String expectPattern(String pattern) {
        String cmdResponse = "";
        try {
            String line = "";
            // Always getting 'null' here
            while ((line = telnetInputReader.readLine()) != null) {
                System.out.println(line);
                cmdResponse += line;
                if (line.contains(pattern)) {
                    break;
                }
            }
        } catch (IOException ex) {
            System.out.println("ex : " + ex);
        }
        return cmdResponse;
    }

What is wrong in this ? Then, one other query. I have tried using PrintWriter for writing to process which in turn has BufferedWriter in it, like,

telnetOuputWriter = new PrintWriter(new BufferedWriter(new OutputStreamWriter(telnetProcess.getOutputStream())));

Is this fine to use PrintWriter in this context ?

Note : Due to some reasons, I would like to stick with using telnet by means of process, not with Socket or TelnetClient or expect4j.

Answer 1

The telnet program does not use the standard input and output streams to communicate with the user, it needs to use the console device directly. You'll have to find an alternative way of doing what you're trying to do.

For example you could use a Java library that implements the telnet protocol. See this question for example: Open source Telnet Java API