How to write "enter key pressed" to a stream?

Question

Sorry for this odd-sounding title...

I have the following situation: I want my Java program to interact with an external console. In order to "send" the individual commands to that console, I need to simulate what would be an "enter key pressed" on a normal console. To clarify what I want, imagine mysql had no other API and I would need to interact via console. Although this is not my actual problem, it is close enough.

I have the following code:

        String command = "/usr/local/mysql/bin/mysql";
        Process child = Runtime.getRuntime().exec(command);

        StreamGobbler gobbler = new StreamGobbler(child.getInputStream());
        gobbler.start();

        BufferedWriter out = new BufferedWriter(new OutputStreamWriter(child.getOutputStream()));
        out.write("help");
        // here enter key needs to be pressed
        out.flush();
        // out.close();

If the call to out.close() is executed, everything is fine. But of course, this way I can only send a single command, which is not what I want. But if out.close() is omitted, the other program never executes the command. My guess is that it still waits for the command to "finish", which on a normal console would be done by pressing enter. out.write(System.getProperty("line.separator")); and out.newLine(); (which are the same) do not solve the problem, neither does out.write("\r\n"); and out.write((char) 26); (EOF).

Of course, it might be, that I am doing it completely wrong (i.e., wrong approach). Then I would appreciate a pointer into the right direction...

Any help on this highly appreciated.

Answer 1

The following code works fine on both Windows 7 using Java 1.6.0_23 and on Ubuntu 8.04 using Java 1.6.0_22:

public class Laj {

  private static class ReadingThread extends Thread {
    private final InputStream inputStream;
    private final String name;

    public ReadingThread(InputStream inputStream, String name) {
      this.inputStream = inputStream;
      this.name = name;
    }

    public void run() {
      try {
        BufferedReader in = new BufferedReader(
            new InputStreamReader(inputStream));
        for (String s = in.readLine(); s != null; s = in.readLine()) {
          System.console().writer().println(name + ": " + s);
        }
      } catch (Exception e) {
        e.printStackTrace();
      }
    }
  }

  public static void main(String[] args) throws Exception {
    String command = "psql -U archadm arch";
    final Process child = Runtime.getRuntime().exec(command);
    new ReadingThread(child.getInputStream(), "out").start();
    new ReadingThread(child.getErrorStream(), "err").start();
    BufferedWriter out = new BufferedWriter(
        new OutputStreamWriter(child.getOutputStream()));
    out.write("\\h");
    out.newLine();
    out.flush();
    out.write("\\q");
    out.newLine();
    out.flush();
  }

}

newLine() is the same as writing the platform line separator. As one would expect, it prints help preceded with "out: ", then exits. If I don't send "\q", it doesn't exit (obviously) but still prints help. Using "\r\n" or "\r" instead of the platform line separator doesn't look like a good idea to me, because such command-line utilities will usually detect that they don't get input from the terminal and assume it is in the native text format (think "psql < script.sql"). Good software should properly detect and accept all reasonable line endings though.