Java console application: accepting commands from infinite loop

Question

So I have an infinite loop running in the main thread of my console application like so.

public static void main(String[] args) {
    while(true){
        doSomething();
        doSomethingElse();
        System.out.println("some debugging info");
        doAnotherThing();
    }
}

I want this code to run over and over and over.

Once in a while, I would like to input a command into the console, such as the string "give me more info plox", and then if that command equals something, I want my code to do something.

Normally I would just use a scanner, but I can't do that here - since scanner.next(); pauses my code... I want my code to keep running whether or not I enter a command. The only workaround I can see is by using a file. But is there any other option?

Hint: you don't want to repeat this things infinitely, you just want it to WAIT until something happens. — sashkello
– sashkello, Commented Jun 26, 2013 at 23:37

morgano · Accepted Answer · 2013-06-26 23:38:47Z

3

Use threads, a main thread to read from console and another one to do the loop, the first thread updates a list of strings (producer) and the loop thread reads the list to see if there is something new for it (consumer)

answered Jun 26, 2013 at 23:38

morgano

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orak · Accepted Answer · 2013-06-26 23:55:09Z

You may do something like below

public class MainApp implements Runnable
{

    static String command;
    static boolean newCommand = false;

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args)
    {
        MainApp reader = new MainApp();
        Thread t = new Thread(reader);
        t.start();

        while (true)
        {
            doSomething();

            if (newCommand)
            {
                System.out.println("command: " + command);
                newCommand = false;
                //compare command here and do something
            }
        }
    }

    private static void doSomething()
    {
        try
        {
            System.out.println("going to do some work");
            Thread.sleep(2000);
        } catch (InterruptedException ex)
        {
            Logger.getLogger(MainApp.class.getName()).log(Level.SEVERE, null, ex);
        }
    }

    @Override
    public void run()
    {
        Scanner scanner = new Scanner(System.in);

        while(true)
        {
            command = scanner.nextLine();
            System.out.println("Input: " + command);
            newCommand= true;
        }
    }



}

tofi9 · Accepted Answer · 2013-06-26 23:52:09Z

0

You could try System.in.available(), it is non-blocking. However, the method is known as not well spec'ed. On some systems (Unix based OpenJDK) it only returns > 0 after the user has confirmed the input with the enter key.

Otherwise, morgano's suggestion to continuously block for System.in.read() on a separate thread.

answered Jun 26, 2013 at 23:52

tofi9

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