DND from a JList with a single gesture

Question

I am writing an application that allows users to drag an item from a JList in one JInternalFrame to a JList in another. The users are complaining that "sometimes it works and sometimes it doesn't". I'm using JDK 1.4.2_02.
As an example of the required behavior, imagine using the Windows Explorer to move a file from one directory to another. You do not have to click once on a file name to select it and click again to start the drag operation; you just click once and start dragging, whether the file was "selected" or not.
However, when you try to drag from a JList, the gesture works only if the item you click on is already selected. Otherwise you have to click once on the item to select it and click again to start dragging. For most applications (for instance a word processor) you would expect this behavior, but for this application it is confusing and unacceptable. You feel like you're using a vaccuum cleaner that doesn't always pick up the dirt.
The reason for the behavior can be found in the platform-dependent Swing code, in either com/sun/java/swing/plaf/gtk/SynthListUI.java or javax/swing/plaf/basic/BasicTextUI.java:
    private static final ListDragGestureRecognizer defaultDragRecognizer = new ListDragGestureRecognizer();
     * Drag gesture recognizer for JList components
    static class ListDragGestureRecognizer extends BasicDragGestureRecognizer {
      * Determines if the following are true:
      * <ul>
      * <li>the press event is located over a selection
      * <li>the dragEnabled property is true
      * <li>A TranferHandler is installed
      * </ul>
      * <p>
      * This is implemented to perform the superclass behavior
      * followed by a check if the dragEnabled
      * property is set and if the location picked is selected.
        protected boolean isDragPossible(MouseEvent e) {
         if (super.isDragPossible(e)) {
          JList list = (JList) this.getComponent(e);
          if (list.getDragEnabled()) {
              ListUI ui = list.getUI();
              int row = ui.locationToIndex(list, new Point(e.getX(),e.getY()));
              if ((row != -1) && list.isSelectedIndex(row)) {
               return true;
         return false;
    }Note that the method returns true only if the row is already selected.
How can I fix this? The obvious method is to substitute my own version of ListDragGestureRecognizer, but that's an unsafe hack. BasicDragGestureRecognizer is not available to the application programmer and is potentially different for every platform and for different versions of the JVM. It's not "documented" that I know of, so there's no way to guarantee my own version is compatible.
Mark Lutton

Answer 1

First, see http://developer.java.sun.com/developer/bugParade/bugs/4521075.html
There are two approaches there that worked for me: simon's and scmorr's. I prefer Simon's, but here is the code for both:
1) based on scmorr's comments:
   private void fixListMouseListeners()
      MouseListener[] allMouseListeners = getMouseListeners();
      String listenerName = "javax.swing.plaf.basic.BasicListUI$ListDragGestureRecognizer";
      MouseListener listDragGestureRecognizer = null;
      // find the drag gesture listener we are looking for and remove it from
      // mouse listeners
      for (int i = 0; i < allMouseListeners.length; ++i)
         MouseListener currentListener = allMouseListeners[ i ];
         String currentListenerClassName = currentListener.getClass().getName();
         if (currentListenerClassName.equals(listenerName))
            listDragGestureRecognizer = currentListener;
            removeMouseListener(listDragGestureRecognizer);
            break;
      // add drag gesture listener back in at the end -- this allows the
      // mouse input handler (which does selection) to run first
      if (listDragGestureRecognizer != null)
         addMouseListener(listDragGestureRecognizer);
   }  // fixListMouseListeners()2) simon's approach:
   // the cached selection event
   private MouseEvent myCachedEvent;
    * overriding this so we can select and drag at same time
    * @param firstIndex is first interval index
    * @param lastIndex is last interval index
    * @param isAdjusting is whether event is an adjusting event
   protected void fireSelectionValueChanged(int firstIndex,
                                            int lastIndex,
                                            boolean isAdjusting)
      // now continue with update of whoever is listening to list for selection changes
      super.fireSelectionValueChanged(firstIndex, lastIndex, isAdjusting);
      // if the selection occurred and we have cached the event,
      // launch a copy of the cached event so dragging can occur right
      // away, if necessary
      if (myCachedEvent != null)
         super.processMouseEvent(new MouseEvent(               
               (Component) myCachedEvent.getSource(),
               myCachedEvent.getID(),     
               myCachedEvent.getWhen(),     
               myCachedEvent.getModifiersEx(),     
               myCachedEvent.getX(),     
               myCachedEvent.getY(),     
               myCachedEvent.getClickCount(),     
               myCachedEvent.isPopupTrigger()));
         myCachedEvent= null;
   }  // fireSelectionValueChanged()
    * overriding so we can cache the mouse event
    * @param e is the invoking event
   protected void processMouseEvent(MouseEvent e)
      int modifiers = e.getModifiersEx();
      // if clicked with left button, cache the event
      if ((modifiers & e.BUTTON1_DOWN_MASK) != 0)
         myCachedEvent = e;
      // go ahead and do normal processing on event
      super.processMouseEvent(e);
   }  // processMouseEvent()This does the job, but I had further requirements that required a bit more intervention. I am working with dragging an item between 2 JLists (single selection mode), and I am also trying to enforce that only one of my 2 lists can have a selection at any given time, using a ListSelectionListener, paying attention to events only if getValueIsAdjusting() is true.
The problem comes if you abort a drag and then select an item in the other list that you weren't dragging from. When you drag, the list you drag from doesn't get a setValueIsAdjusting(false) since you don't get the mouseReleased() from MouseInputHandler, and leaving the list in "isAdjusting == true"-mode when a drag is aborted makes my selection stuff not work.
My solution was to explictly call comp.setValueIsAdjusting(false) in my exportDone() function of my custom TransferHandler. Now everything is cool.