How to interrupt java.util.Scanner nextLine call

Question

I am using a multi threaded environment were one Thread is constantly listening for user input by repeatedly calling scanner.nextLine(). To end the application, this runloop is stopped by another thread, but the listening thread won't stop until a last user input was made (due to the blocking nature of nextLine()).

Closing the stream seems not to be an option since I am reading from System.in, which returns an InputStream that is not closable.

Is there a way to interrupt the blocking of scanner, so that it will return?

thanks

Answer 1

This article describes an approach to avoiding blocking when reading. It gives the code snippet, which you could amend as I indicate in a comment.

import java.io.*;
import java.util.concurrent.Callable;

public class ConsoleInputReadTask implements Callable<String> {
  public String call() throws IOException {
    BufferedReader br = new BufferedReader(
        new InputStreamReader(System.in));
    System.out.println("ConsoleInputReadTask run() called.");
    String input;
    do {
      System.out.println("Please type something: ");
      try {
        // wait until we have data to complete a readLine()
        while (!br.ready()  /*  ADD SHUTDOWN CHECK HERE */) {
          Thread.sleep(200);
        }
        input = br.readLine();
      } catch (InterruptedException e) {
        System.out.println("ConsoleInputReadTask() cancelled");
        return null;
      }
    } while ("".equals(input));
    System.out.println("Thank You for providing input!");
    return input;
  }
}

You could either use this code directly, or write a new closable InputStream class, wrapping up the logic described in this article.

Answer 2

To start off with: this will not solve the issue that, to close the whole program, requires a System.exit() call if there has been an unfulfilled input request (even if canceled). You could potentially circumvent this by spoofing a keystroke into console, but that's a whole other ball park.

If you want to do it in console, is impossible to do without polling, as it's impossible to actually un-block a thread waiting for input from System.in, as System.in itself does not have interruptible get() methods. Because of this, without using polling to only request input if you know it will not be blocking.

If you truely want something that will act as an interruptible nextLine() for a console, you should probably look into making a Swing window, or the like, and making a simple input interface for it. This isn't really difficult, and would have all the functionality you're asking for, outside of some edge cases.

However, I was working on this myself, as I wanted a way for a thread to stop waiting for input from System.in, without closing the program (and while avoiding polling), and this is what I came up with, before switching to my own window.

I can't say with any confidence it's best-practice, but it should be thread-safe, seems to be working fine, and I can't think of any immediate issues. I would like to switch failures from alternate (albeit, otherwise unobtainable) outputs, to actual errors though. You can cancel active requests for input either by interrupting the thread, or by calling cancel(), which canceles the currently waiting request.

It uses Semaphores and threads to create a blocking nextLine() method that can be interrupted/canceled elsewhere. Canceling is not perfect - you can only cancel the currently waiting thread's request, for instance, but interrupting threads should work fine.

package testapp;

/**
 *
 * @author Devlin Grasley
 */
import java.util.concurrent.Semaphore;
import java.util.Scanner;

public class InterruptableSysIn {
    protected static Scanner input = new Scanner (System.in);
    protected static final Semaphore waitingForInput = new Semaphore(0,true); //If InterruptableSysIn is waiting on input.nextLine(); Can also be cleared by cancel();
    protected static String currentLine = ""; //What the last scanned-in line is
    private static final Input inputObject = new Input();
    private static final Semaphore waitingOnOutput = new Semaphore (1); // If there's someone waiting for output. Used for thread safety
    private static boolean canceled = false; //If the last input request was cancled.
    private static boolean ignoreNextLine = false; //If the last cancel() call indicated input should skip the next line.
    private static final String INTERRUPTED_ERROR = "\nInterrupted";
    private static final String INUSE_ERROR = "\nInUse";
    private static boolean lasLineInterrupted = false;

    /**
     * This method will block if someone else is already waiting on a next line.
     * Gaurentees on fifo order - threads are paused, and enter a queue if the
     * input is in use at the time of request, and will return in the order the
     * requests were made
     * @return The next line from System.in, or "\nInterrupted" if it's interrupted for any reason
     */
    public static String nextLineBlocking(){
        //Blocking portion
        try{
            waitingOnOutput.acquire(1);
        }catch(InterruptedException iE){
            return INTERRUPTED_ERROR;
        }
        String toReturn = getNextLine();
        waitingOnOutput.release(1);
        return toReturn;
    }

    /**
     * This method will immediately return if someone else is already waiting on a next line.
     * @return The next line from System.in, or 
     * "\nInterrupted" if it's interrupted for any reason
     * "\nInUse" if the scanner is already in use
     */
    public static String nextLineNonBlocking(){
        //Failing-out portion
        if(!waitingOnOutput.tryAcquire(1)){
            return INUSE_ERROR;
        }
        String toReturn = getNextLine();
        waitingOnOutput.release(1);
        return toReturn;
    }

    /**
     * This method will block if someone else is already waiting on a next line.
     * Gaurentees on fifo order - threads are paused, and enter a queue if the
     * input is in use at the time of request, and will return in the order the
     * requests were made
     * @param ignoreLastLineIfUnused If the last line was canceled or Interrupted, throw out that line, and wait for a new one.
     * @return The next line from System.in, or "\nInterrupted" if it's interrupted for any reason
     */
    public static String nextLineBlocking(boolean ignoreLastLineIfUnused){
        ignoreNextLine = ignoreLastLineIfUnused;
        return nextLineBlocking();
    }

    /**
     * This method will fail if someone else is already waiting on a next line.
     * @param ignoreLastLineIfUnused If the last line was canceled or Interrupted, throw out that line, and wait for a new one.
     * @return The next line from System.in, or 
     * "\nInterrupted" if it's interrupted for any reason
     * "\nInUse" if the scanner is already in use
     */
    public static String nextLineNonBlocking(boolean ignoreLastLineIfUnused){
        ignoreNextLine = ignoreLastLineIfUnused;
        return nextLineNonBlocking();
    }

    private static String getNextLine(){
        String toReturn = currentLine; //Cache the current line on the very off chance that some other code will run etween the next few lines

        if(canceled){//If the last one was cancled
            canceled = false;

            //If there has not been a new line since the cancelation
            if (toReturn.equalsIgnoreCase(INTERRUPTED_ERROR)){
                //If the last request was cancled, and has not yet recieved an input

                //wait for that input to finish
                toReturn = waitForLineToFinish();
                //If the request to finish the last line was interrupted
                if(toReturn.equalsIgnoreCase(INTERRUPTED_ERROR)){
                    return INTERRUPTED_ERROR;
                }

                if(ignoreNextLine){
                    //If the last line is supposed to be thrown out, get a new one
                    ignoreNextLine = false;
                    //Request an input
                    toReturn = getLine();
                }else{
                    return toReturn;
                }

            //If there has been a new line since cancelation
            }else{
                //If the last request was cancled, and has since recieved an input
                try{
                    waitingForInput.acquire(1); //Remove the spare semaphore generated by having both cancel() and having input
                }catch(InterruptedException iE){
                    return INTERRUPTED_ERROR;
                }

                if(ignoreNextLine){
                    ignoreNextLine = false;
                    //Request an input
                    toReturn = getLine();
                }
                //return the last input
                return toReturn;
            }
        }else{
            if(lasLineInterrupted){

                //wait for that input to finish
                toReturn = waitForLineToFinish();
                //If the request to finish the last line was interrupted
                if(toReturn.equalsIgnoreCase(INTERRUPTED_ERROR)){
                    return INTERRUPTED_ERROR;
                }

                //Should the read be thrown out?
                if(ignoreNextLine){
                    //Request an input
                    toReturn = getLine();
                }

            }else{
                ignoreNextLine = false; //If it's been set to true, but there's been no cancaleation, reset it.

                //If the last request was not cancled, and has not yet recieved an input
                //Request an input
                toReturn = getLine();
            }
        }
        return toReturn;
    }

    private static String getLine (){
        Thread ct = new Thread(inputObject);
        ct.start();
        //Makes this cancelable
        try{
            waitingForInput.acquire(1); //Wait for the input
        }catch(InterruptedException iE){
            lasLineInterrupted = true;
            return INTERRUPTED_ERROR;
        }
        if(canceled){
            return INTERRUPTED_ERROR;
        }
        return currentLine;
    }

    public static String waitForLineToFinish(){
        //If the last request was interrupted
        //wait for the input to finish
        try{
            waitingForInput.acquire(1);
            lasLineInterrupted = false;
            canceled = false;
            return currentLine;
        }catch(InterruptedException iE){
            lasLineInterrupted = true;
            return INTERRUPTED_ERROR;
        }
    }

    /**
     * Cancels the currently waiting input request
     */
    public static void cancel(){
        if(!waitingOnOutput.tryAcquire(1)){ //If there is someone waiting on user input
            canceled = true;
            currentLine = INTERRUPTED_ERROR;
            waitingForInput.release(1); //Let the blocked scanning threads continue, or restore the lock from tryAquire()    
        }else{
            waitingOnOutput.release(1); //release the lock from tryAquire()    
        }
    }

    public static void cancel(boolean throwOutNextLine){
        if(!waitingOnOutput.tryAcquire(1)){ //If there is someone waiting on user input
            canceled = true;
            currentLine = INTERRUPTED_ERROR;
            ignoreNextLine = throwOutNextLine;
            waitingForInput.release(1); //Let the blocked scanning threads continue
        }else{
            waitingOnOutput.release(1); //release the lock from tryAquire()    
        }
    }

}

class Input implements Runnable{
    @Override
    public void run (){
        InterruptableSysIn.currentLine = InterruptableSysIn.input.nextLine();
        InterruptableSysIn.waitingForInput.release(1); //Let the main thread know input's been read
    }

}