How can a Swing WindowListener veto JFrame closing.

Question

I have a frame, and want to prompt when the user closes it to save the document. But if they cancel, the frame shouldn't close.

frame.addWindowListener(new SaveOnCloseWindowListener(fileState));
...
public class SaveOnCloseWindowListener extends WindowAdapter {
    private final FileState fileState;

    public SaveOnCloseWindowListener(FileState fileState) {
        this.fileState = fileState;
    }

    public void windowClosing(WindowEvent e) {
        if (!fileState.onQuit())
            cancelClose();  
    }
}

FileState looks at whether the document is dirty. If it isn't it does nothing and returns true. If it is dirty, it asks the user if he wants to save (YES/NO/CANCEL). If the user cancels at this point, it should abort the windowClosing.

All the suggestions I've seen on the net involve explicitly exiting in the windowClosing method, thus overriding the use of JFrame.setDefaultCloseOperation(), and duplicating the code in JFrame.processWindowEvent().

I actually have a dirty solution, but would like to see if there are any cleaner ones.

Cheers

Answer 1

The right way is set JFrame.setDefaultCloseOperation to DO_NOTHING_ON_CLOSE when the window is created. And then just calling setVisible(false) or dispose() when your user accepts the close, or doing nothing when the close isn't accepted.

The whole purpose of JFrame.setDefaultCloseOperation is only to prevent the need to implement WindowListeners for the most simple actions. The actions performed by these default close operations are very simple.

EDIT:

I've added the solution I'm describing. This assumes you want the frame to be fully deleted.

frame.setDefaultCloseOperation(setDefaultCloseOperation);
frame.addWindowListener(new SaveOnCloseWindowListener(fileState));
...

public class SaveOnCloseWindowListener extends WindowAdapter {
    private final FileState fileState;

    public SaveOnCloseWindowListener(FileState fileState) {
        this.fileState = fileState;
    }

    public void windowClosing(WindowEvent e) {
        if (fileState.onQuit())
            frame.dispose();
    }
}

Answer 2

I think that this is the logical expression of Thirler's answer, and kind of what I was trying to avoid.

frame.setDefaultCloseOperation(JFrame.DO_NOTHING_ON_CLOSE);
frame.addWindowListener(new SaveOnCloseWindowListener(fileState, JFrame.EXIT_ON_CLOSE));

public class SaveOnCloseWindowListener extends WindowAdapter {

    private final int closeOperation;
    private final Document document;

    public SaveOnCloseWindowListener(int closeOperation, Document document) {
        this.closeOperation = closeOperation;
        this.document = document;
    }

    public void windowClosing(WindowEvent e) {
        if (!document.onClose())
            doClose((Window) e.getSource());
    }

    private void doClose(Window w) {
        switch (closeOperation) {
        case JFrame.HIDE_ON_CLOSE:
            w.setVisible(false);
            break;
        case JFrame.DISPOSE_ON_CLOSE:
            w.dispose();
            break;
        case JFrame.DO_NOTHING_ON_CLOSE:
        default:
            break;
        case JFrame.EXIT_ON_CLOSE:
            System.exit(0);
            break;
        }
    }
}

Answer 3

Closing an Application might make the process a little easier for you.

Answer 4

Thanks to the input from Thirler and camickr. This is my solution, which I'm sure that some will hate, but avoids duplicating logic in JFrame, and allows client code to setDefaultCloseOperation as usual.

class MyFrame {        
    @Override protected void processWindowEvent(WindowEvent e) {
        try {
            super.processWindowEvent(e);
        } catch (SaveOnCloseWindowListener.VetoException x) {}
    }
}

public class SaveOnCloseWindowListener extends WindowAdapter {

    public static class VetoException extends RuntimeException {
    }

    private final DocumentController documentController;

    public SaveOnCloseWindowListener(DocumentController documentController) {
        this.documentController = documentController;
    }

    public void windowClosing(WindowEvent e) {
        if (!documentController.onClose())
            throw new VetoException();
    }
}