I have a issue with java threads and KeyEvents?

Question

I need a bit of help with my program here. I have a blocking function waiting in my main thread for the user to click "Enter". Then, when the user hits enter, the keypressed event should fire, which will unblock the blocking function. However, when the program hits the blocking function, it simply freezes up and doesn't register the key pressed event.

So, my question is, is a event a runnable, which is added to a thread whenever the user clicks enter? If so, my code should have worked, right? If this is not the case, and each event is not a separate thread, could anyone enlighten me on how I should fix my problem here? my blocking function:

public String getInput() {
    synchronized(waitObject) {
        try {
            System.out.println("waiting");
            waitObject.wait(); // throws exception, cba to add it here
        } catch (Exception ex) {
            ex.printStackTrace();
        }
    }
    return(myString);
}

my keylistener code:

public void keyPressed(KeyEvent e) {
    System.out.println("key pressed");
    char c = e.getKeyChar();
    if (c == e.VK_ENTER) {
        System.out.println("Enter pressed");
        synchronized(waitObject) {
            waitObject.notifyAll();
        }
    }
}

and the function getting the input:

private String getCommand() {
    System.out.println("getting command");
    CommandField command = new CommandField((JFrame)(this));
    command.setPreferredSize(new Dimension(getWidth(), 30));
    m_panel.add(command, BorderLayout.NORTH);
    validate();
    command.requestFocus();
    System.out.println(command.getInput());

    return null;
}

And this function is called from another keylistener:

public class Listener implements KeyListener {
    public void keyPressed(KeyEvent e) {
        if (e.getKeyCode() == KeyEvent.VK_F2) {
            System.out.println(getCommand());
        }

    }
    public void keyReleased(KeyEvent e) {

    }
    public void keyTyped(KeyEvent e) {

    }
}

AWT uses an [*event dispatch thread*](http://en.wikipedia.org/wiki/Event_dispatching_thread). If you occupy this thread, it will not be able to do work -- and you will not receive your `KeyEvent`. — obataku, Aug 08 '12 at 13:41
The second keylistener is added to the same jframe which the CommandField(a jpanel with the other keylistener) is added to. Does that clarify anything? — AlphaUserGuru, Aug 08 '12 at 14:23
see edited answer. 1+ to your question for helping us help you by providing more code. — Hovercraft Full Of Eels, Aug 08 '12 at 14:25

Hovercraft Full Of Eels · Answer 1 · 2012-08-08T14:37:27.693

2

OK, getCommand() is called on the event thread which then calls getInput() on the same thread, which then calls Object#wait() on the event thread, so yes you are tying up the event thread and effectively freezing your program.

None of this is needed, and a much easier fix is possible if you code using the concepts of event-driven programming. You don't want to call wait() in a Swing GUI and in your situation don't need to call it, but rather you want to change how your program responds to input based on its state. If you tell us more on the exact behavior you're trying to elicit, we can probably help you find a much better solution.

Edit
Consider using a JOptionPane or a modal JDialog for displaying a "blocking" window that stops the main program until the dialog has been dealt with.

edited Aug 08 '12 at 14:37

answered Aug 08 '12 at 14:12

Hovercraft Full Of Eels

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Well, I don't know what I should expand on - the behavior I want to have: the CommandField(my version of a textfield which extends JPanel) is supposed to do this: the user presses a key in the JFrame, the commandfield pops up. The user can type in a string, and then click enter. The JFrame, which has been waiting on the blocking function commandfield.getInput(), will then unwait and get the input. – AlphaUserGuru Aug 08 '12 at 14:27
@user1493275: Why not simply use a JOptionPane for this? Or a modal JDialog? A caveat: you should avoid using KeyListeners as they are very touchy with regards to focus. If the component being listened to does not have focus, the KeyListener won't work. Much better to use Key Bindings. – Hovercraft Full Of Eels Aug 08 '12 at 14:31

score 2 · Answer 2 · edited May 23 '17 at 11:56

Starting with ImageApp, I added the following key binding in the constructor. It will show() the popup menu when pressing the Enter key. You can change the arbitrary location to suit your usage.

this.getInputMap().put(KeyStroke.getKeyStroke(KeyEvent.VK_ENTER, 0), "showPopup");
this.getActionMap().put("showPopup", new AbstractAction("showPopup") {

    @Override
    public void actionPerformed(ActionEvent e) {
        popup.show(ImageApp.this, 42, 42);
    }
});

Addendum: To bring up a modal input dialog, so something like this:

this.getInputMap().put(KeyStroke.getKeyStroke(KeyEvent.VK_ENTER, 0), "dialog");
this.getActionMap().put("dialog", new AbstractAction("dialog") {

    @Override
    public void actionPerformed(ActionEvent e) {
        String value = JOptionPane.showInputDialog("What?");
        System.out.println(value);
    }
});

I have a issue with java threads and KeyEvents?

2 Answers2