Focus events being received out of order

Question

I have a complicated applet where each component is registered with multiple focus listeners. I have several pairs of JTextFields and JButtons - textfield A, button A, textfield B, button B etc. Clicking on a JButton normally causes a JTable to be popped and focus transferred to it. Clicking on the table header or selecting a table row causes the table popup to close and the focus transferred to the textfield associated with the button.
I'm running into a strange case where the focus is initially on textfield A. Clicking on button B causes a temporary loss of focus on textfield A. The trace shows that the focus owner is null and the permanent focus owner is textfield A. The table still gets launched. Selecting a table row causes it to close but no component has focus any more. Focus owner continues to be null, and permanent owner continues to be textfield A.
After working on other windows in the meantime, and returning to this window I find that the focus in now back to textfield B, as it should. However, the trace shows that B has received permanent focus events. This is followed by receiving permanent focusLost events on A and then permanent focusGained events on A. At the point all in said and done, the focus appears on B.
Does someone know why the focusGained event on B appears before focus events on A? Another strange thing in the trace was that some focus gain/loss method invocations were interrupted, another focus method invoked and the previous method resumed. I could possibly take care of this by using synchronized on all focus gain/loss methods. Also, some methods show only part of execution and not even resumed. This is still a mystery to me. I'll appreciate if someone can shed light on this.

Answer 1

One of the things you'll notice is that when a text field loses focus to a right-click JPopupMenu, it is never just one foucs lost/gained pair, but rather they trade focus back and fourth a few times. This is the same when the popup closes, the focus trades back and fourth a few times.
You are describing a similar situation between a button and a text field.
There never was any guarantee the gained/lost events happen in any order. Also, if there is a chain of focus events, the information in the intermediate events may not be useful.
If you really do need to understand the focus changes at an actionable level, you might want to consider writing a focus helper class that can be used to answer semantic questions about focus changes, such as, did this component lose focus because of a button click? Or was it a Popup? Or was it a Tab-keystroke?
The focus helper can store 'hidden' information in any JComponent using JComponent.setClientProperty(Object key, Object value)...
So instead of creating a focus listener like:
myComponent.addFocusListener(new FocusAdapter() {
  public void focusGained(FocusEvent e) {
    // do stuff here
});You can do it like this:
myComponent.addFocusListener(FocusHelper.createTextFieldToButtonListener());and in that method it creates the listener like this (you might need to pass some parameters):
public static FocusListener createTextFieldToButtonListener() {
FocusAdapter adapter = new FocusAdapter() {
  public void focusGained(FocusEvent e) {
    recordEvent(e);
    // do stuff here
return adapter;
}The helper method "recordEvent" tags those components with additional information about permenant and temporary focus ownership. Also it clears hidden information that is no longer useful.
Then later on you can give the focus helper a FocusEvent, and ask it, is this text field gaining focus because of a Tab-In, or is it because of a popup menu has closed?
The helper constructs answers based on the hidden properties it finds in the e.getOppositeComponent() and e.getSource(). It put those properties there, so it knows what they mean, and it uses them to distinguish between useless information in the focus event chain, and the important things you want to know.