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POJava LostFocus and InputVerifier, moving in reverse-tab-order
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copied!<p>I have a GUI application that uses an InputVerifier to check the content of text fields before yielding the focus. This is all very normal. Yesterday, however, discovered a problem - it seems to be a bug, but I cannot find any mention of it anywhere. Before I report this as a bug, I thought I would ask: am I missing something obvious here?</p> <p>Situation:</p> <ul> <li>A set of text fields with InputVerifiers.</li> <li>Listeners for FocusLost and FocusGained on all controls, so I can see what is happening.</li> <li>A separate thread uses a DefaultKeyboardFocusManager to report (every 2 seconds) which control has the focus.</li> <li>I place invalid data in a JTextField in the middle of the form, and try to leave the control.</li> </ul> <p>If I try to move away from this control using the mouse, or using the tab-key, I cannot. The FocusLost event does not fire and the control properly retains the focus.</p> <p>However, if I try to move away from the control <i>in reverse tab order, using Shift-Tab</i>, <b>sometimes</b> the FocusLost event fires. If this happens, the separate thread reports that <b>no control has the focus</b>, i.e., getFocusOwner() returns null. <hr> Edit: below is a small sample program that shows the problem. The problem has nothing to do with the extra thread - the thread is just there to make the problem more obvious. If there is a race-condition, it is somewhere in Swing.</p> <p>To see the problem, go to the second text box and empty it. The control should retain the focus, and does so <em>unless</em> you leave it by pressing shift-tab. Unlike the full application, the error seems to occur here 100% of the time. This is true both under OpenJDK 6 and under Oracle Java 7.</p> <p>This is almost too obvious to be a bug, plus it happens in multiple Java environments. Hence, my suspicion that I am missing something obvious. Anyone?</p> <pre><code>public class FocusBugDemo extends JFrame { static JTextField txtOne = new JTextField("Ignore this control"); static JTextField txtTwo = new JTextField("Delete this text, then press shift-tab"); static JLabel lblFocus = new JLabel(""); static KeyboardFocusManager kfm = new DefaultKeyboardFocusManager(); public static void main(String[] args) { new FocusBugDemo(); Thread t = new Thread() { @Override public void run() { while(true) { 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) { } } } }; t.start(); } private FocusBugDemo() { super("Focus bug demo"); setDefaultCloseOperation(EXIT_ON_CLOSE); setPreferredSize(new Dimension(300,100)); setLayout(new GridLayout(3,1)); NotEmpty validator = new NotEmpty(); txtOne.setInputVerifier(validator); txtTwo.setInputVerifier(validator); add(txtOne); add(txtTwo); add(lblFocus); pack(); 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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