Concurrent/Non-blocking console keyboard input in Java

Question

I'm working on a MUD in java. I read player input every tick, but I'm using Scanner which uses blocking operations. I want to have non-blocking input. I've looked at the nio package which has a Selector class, but I'm not sure how to use it with regard to System.in. I figure I'll definitely need it once I'm running a server, but for now everything is offline. I've tried extending the main class from Applet and overriding keyDown, but that just meant input was no longer accepted after the first one. Sure, I wasn't blocking anything anymore, but then there was no more input. keyDown never got called again, I guess.

Perhaps threads can be interrupted even when they are executing blocking operations?

Thanks for any insight into this problem.

Answer 1

You can't do that with the system console because by now it can't be done in a multi-platform way.

You can use swing window as console or find a JNI based approach but it might not work on some platforms.

Here's an explanation that mention JCurses. It might work, it's based on JNI and supports Windows and Linux.

Answer 2

JLine might fit your bill as well:

http://jline.sourceforge.net/apidocs/src-html/jline/ConsoleReader.html#line.1447

Answer 3

keyDown() is deprecated so I'd suggest to use processKeyEvent and a keyListener instead.

Perhaps threads can be interrupted even when they are executing blocking operations?

Yes if you have a reference to the thread object you want to interrupt, you can simply call interrupt() method on that instance. And in the run method of the thread you can handle the interrupted exception. However, this seems a little bit hack-ish. I don't see how this is more helpful than using a simple KeyListener.

Answer 4

I had had to solve similar problem with blocking writing/reading from http. In that particular case I used local buffer and Threads.

Idea is simple, one Thread read from stdin and put content in buffer. Second do same with Writing.

And then you use nonblocking quieries into your buffer.
Samle code:

class NonBlockingReader implements Runnable{
  Reader in;
  List buffer;
  public void run(){
    String line=null;
    while((line=in.readLine())!=null){
      storeLine(line);
    }
  }
  private synchronized storeLine(String line){
    buffer.add(line);
  }
  public synchronized String getLine(){
    if(list.size()>0)
       return list.removeFirst();
    return null;
  }
}

// .. same for writer, then you jast pass stdin and stdout ...