JFrame's getFocusOwner() not being helpful

Question

I am working with a Swing program and having a little trouble. The program has two windows (both are JFrames). The main window is just fine and should not be relevant to this issue.

The window I am having issues with contains a JScrollPane with a JPanel in it, and has a JMenuBar. The JPanel has a bunch of JTextComponents (some JTextFields, some JTextAreas) on it.

What I want to do is have an ActionListener attached to a JMenuItem find the JTextComponent that has focus.

I have seen the previous posts at focused component reference and How to find out which object currently has focus. My issue is that calling the particular window's getFocusOwner() method merely returns the JFrame's JRootPane, which is utterly unhelpful. Both the JScrollPane and the JPanel in question are focusable according to their isFocusable() methods. This happens even if I actually enter text into one of the JTextComponents before clicking the menu item. The cursor still blinks in the text field while I open the menu, and everything. For what it's worth, getMostRecentFocusOwner() also simply returns the JRootPane.

Answer 1

If you use TextActions, the action knows which JTextComponent has the focus. I've modified some code that I found here to show that even if the TextActions come from one JTextArea, they still will automatically work on any and all text components that have focus:

import java.awt.GridLayout;
import javax.swing.*;

public class TextActionExample {
  public static void main(String[] args) {

    // Create a text area.
    JTextArea ta = new JTextArea(15, 30);
    ta.setLineWrap(true);

    // Add all actions to the menu (split into two menus to make it more
    // usable).
    Action[] actions = ta.getActions();
    JMenuBar menubar = new JMenuBar();
    JMenu actionmenu = new JMenu("Actions");
    menubar.add(actionmenu);

    JMenu firstHalf = new JMenu("1st Half");
    JMenu secondHalf = new JMenu("2nd Half");
    actionmenu.add(firstHalf);
    actionmenu.add(secondHalf);

    int mid = actions.length / 2;
    for (int i = 0; i < mid; i++) {
      firstHalf.add(actions[i]);
    }
    for (int i = mid; i < actions.length; i++) {
      secondHalf.add(actions[i]);
    }

    JTextField textField = new JTextField(20);
    JPanel textFieldPanel = new JPanel();
    textFieldPanel.add(textField);

    JPanel mainPanel = new JPanel(new GridLayout(1, 0, 5, 5));
    mainPanel.add(new JScrollPane(ta));
    mainPanel.add(new JScrollPane(new JTextArea(15, 30)));
    mainPanel.add(textFieldPanel);


    // Show it . . .
    JFrame f = new JFrame();
    f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    f.getContentPane().add(mainPanel);
    f.setJMenuBar(menubar);
    f.pack();
    f.setVisible(true);
  }
}

This is very interesting stuff that I have to learn more about.

Answer 2

I think I have solved this, because of the fact that you lose focus when you click on a menu item, we simply have to wait for focus to return to the component before we check who has focus, I have done this using a swing timer that waits 100ms and then executes its method to check which component has focus:

import java.awt.EventQueue;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.TimerTask;
import javax.swing.*;

public class JavaApplication180 extends JFrame {

    private JTextField[] JTextFields;
    private JMenuBar menuBar;
    private JMenu menu;
    private JMenuItem item;

    public JavaApplication180() {
        initComponents();

        createAndShowUI();
    }

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

            @Override
            public void run() {
                new JavaApplication180();
            }
        });
    }

    private void createAndShowUI() {
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        getContentPane().setLayout(new GridLayout(2, 2, 10, 10));
        setJMenuBar(menuBar);
        addComponentsToPane();

        pack();
        setVisible(true);
    }

    private void initComponents() {
        JTextFields = new JTextField[4];

        menuBar = new JMenuBar();
        item = new JMenuItem("Who has focus?");
        item.addActionListener(new ActionListener() {

            @Override
            public void actionPerformed(ActionEvent ae) {
                TimerTask tt = new TimerTask() {

                    @Override
                    public void run() {
                        JOptionPane.showMessageDialog(null, getMostRecentFocusOwner().getName());
                    }
                };
                new java.util.Timer().schedule(tt, 100);
            }
        });

        menu = new JMenu("File");
        menu.add(item);
        menuBar.add(menu);
    }

    private void addComponentsToPane() {
        for (int i = 0; i < JTextFields.length; i++) {
            JTextFields[i] = new JTextField();
            JTextFields[i].setText(String.valueOf(i));
            JTextFields[i].setName(String.valueOf(i));
            getContentPane().add(JTextFields[i]);
        }
    }
}