How to use this:
KeyboardFocusManager focusManager =
KeyboardFocusManager.getCurrentKeyboardFocusManager();
// instead of registering directly use weak listener
// focusManager.addPropertyChangeListener(focusOwnerListener);
focusManager.addPropertyChangeListener(
new WeakPropertyChangeListener(focusOwnerListener, focusManager));
How does this work:
Instead of registering propertyChangeListener directly to keyboardFocusManager, we wrap it inside
WeakPropertyChangeListener and register this weak listener to keyboardFocusManager. This weak listener acts a
delegate. It receives the propertyChangeEvents from keyboardFocusManager and delegates it the wrapped listener.
The interesting part of this weak listener, it hold a weakReference to the original propertyChangeListener. so this
delegate is eligible for garbage collection which it is no longer reachable via references. When it gets garbage
collection, the weakReference will be pointing to null. On next propertyChangeEvent notification from
keyboardFocusManager, it find that the weakReference is pointing to null, and unregisters itself from
keyboardFocusManager. Thus the weak listener will also become eligible for garbage collection in next gc cycle.
This concept is not something new. If you have a habit of looking into swing sources, you will find that
AbstractButton actually adds a weak listener to its action. The weak listener class used for this is :
javax.swing.AbstractActionPropertyChangeListener; This class is package-private, so you don't find it in javadoc.
The full-fledged, generic implementation of weak listeners is available in Netbeans OpenAPI: WeakListeners.java . It
is worth to have a look at it.