Re: Please help me kill this java thread..... code example

Eric Sosman <esosman@comcast-dot-net.invalid>
Thu, 15 Nov 2012 11:49:05 -0500
On 11/15/2012 10:06 AM, wrote:

I am using a JVM for a mobile device based on java 1.4, ~old I know, but it is what I must use.

I am creating a simple swing application with a gui and a barcode laser scanner api. I place the swing gui thread in the AWT-EventQueue, and then I place barcode scanner code in a worker thread to feed the gui scanned item codes. Problem is, that when I click exit on the gui, ONLY the gui shuts down. I can't seem to kill the worker thread. Have a look:

     You've not given us much to look at, but I'll have a try.

public class Main extends javax.swing.JFrame implements ActionListener, ScannerListener, ItemListener, KeyListener {
    Scanner scanner;

public static void main(String args[]) {
         java.awt.EventQueue.invokeLater(new Runnable() {

     Aside: Why not SwingUtilities.invokeLater()? Probably makes
little or no difference, but ...

             public void run() {
                 new Main().setVisible(true);

  public Main() {
          //display GUI - a simple field to display the item number when the
          // barcode scanner trigger is pressed, and a exit button to close the app
         // launch the scanner code to feed the gui scanned items
         worker = new Runnable()
                               public void run()
                                   //a while loop to scan and wait
                                  catch (Exception e) { }

     Ugh, barf, bleahh! This is a bit like unplugging that noisy
smoke alarm so you can get back to sleep. It is *very* occasionally
all right to catch and ignore an exception, but even then you should
write a comment explaining *why* you believe *this* exception is safe
to ignore. If you can come up with an explanation of why *every*
kind of Exception is safe to ignore, you're bolder than I will ever be.

                             scannerThread = new Thread(worker);

     Aside: It's not a good idea to start threads inside constructors.
The problem is that the constructed object isn't "truly constructed"
until the entire chain of constructors has returned, so you shouldn't
allow the outside world to see `this' yet. Starting a thread exposes
`this' (via the nested Runnable), thus letting the rest of the program
observe the Main object before it's complete. You'll probably get
away with it in this instance, but cure yourself of this bad habit.


   private void shutdown(java.awt.event.ActionEvent evt) {
           //this will shutdown the gui when the exit button is pressed. BUT how do I kill the scanner thread?

     This should shut down the entire JVM: GUI, worker threads,
garbage collector, daemons, the whole shebang. I have only three
guesses about why you observe the worker still running:

     1) The shutdown() method is never called. You haven't shown
        us any of the arrangements you've made to have it called,
        so we can't tell whether they're correct (or even whether
        they exist). Try `System.out.println("Kilroy was here");'
        at the start of shutdown(), just to find out whether you
        do or do not ever get there.

     2) A security manager forbids exit() from doing anything.
        If this happens a SecurityException will be thrown, and
        that might (or might not) be what stops the GUI. Perhaps
        your habit of catching and ignoring exceptions has blinded
        you to what's going on here.

     3) You're wrong: The worker thread does in fact stop. You've
        not explained why you think it's still running, so we can't
        tell whether your conclusion is correct.

     I'm not a betting man, but if I were my money would be on (1).


Eric Sosman

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