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copied!<p>Using your <a href="http://sscce.org/" rel="nofollow">sscce</a>, I am unable to reproduce the effect you describe on Mac OS X, Java 6, which supports @CatalinaIsland's observation. In particular, focus never leaves an empty text field using either <kbd>tab</kbd> or <kbd>shift-tab</kbd>; focus becomes <code>null</code> only when the frame is deactivated.</p> <p>I see two threads accessing multiple fields with no synchronization at all. At a minimum, you should use <code>EventQueue.invokeLater()</code> in <code>t</code> to update the GUI, as described in <a href="http://docs.oracle.com/javase/tutorial/uiswing/concurrency/index.html" rel="nofollow"><em>Concurrency in Swing</em></a> and show below.</p> <p>The broader question is this: What focus problem are you trying to solve using <code>t</code>?</p> <pre><code>import java.awt.Component; import java.awt.DefaultKeyboardFocusManager; import java.awt.Dimension; import java.awt.EventQueue; import java.awt.GridLayout; import java.awt.KeyboardFocusManager; import javax.swing.InputVerifier; import javax.swing.JComponent; import javax.swing.JFrame; import javax.swing.JLabel; import javax.swing.JTextField; public class FocusDemo { private static final JTextField txtOne = new JTextField("Ignore this control"); private static final JTextField txtTwo = new JTextField("Delete this text, then press shift-tab"); private static final JLabel lblFocus = new JLabel(""); public static void main(String[] args) { new FocusDemo(); Thread t = new Thread() { @Override public void run() { while (true) { EventQueue.invokeLater(new Runnable() { KeyboardFocusManager kfm = new DefaultKeyboardFocusManager(); @Override public void run() { Component c = kfm.getFocusOwner(); String focusInfo = "elsewhere"; if (c == null) { focusInfo = "null"; } else if (c == txtOne) { focusInfo = "txtOne"; } else if (c == txtTwo) { focusInfo = "txtTwo"; } lblFocus.setText(System.currentTimeMillis() + " - Focus owner " + focusInfo); } }); try { Thread.sleep(1000); } catch (InterruptedException e) { e.printStackTrace(System.err); } } } }; t.start(); } private FocusDemo() { EventQueue.invokeLater(new Runnable() { @Override public void run() { JFrame f = new JFrame("Focus bug demo"); f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); f.setPreferredSize(new Dimension(300, 100)); f.setLayout(new GridLayout(3, 1)); NotEmpty validator = new NotEmpty(); txtOne.setInputVerifier(validator); txtTwo.setInputVerifier(validator); f.add(txtOne); f.add(txtTwo); f.add(lblFocus); f.pack(); f.setVisible(true); } }); } private class NotEmpty extends InputVerifier { @Override public boolean verify(JComponent input) { JTextField txtField = (JTextField) input; return (txtField.getText().length() > 0); } } } </code></pre>

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