Java synchronization mechanism allows two threads to hold the same lock?

Question

Dear all,
I am a little bit puzzled since I cannot explain why I get the following behavior.
I have a small Java program to draw components. The drawings are cached in the memory for performance improvements. The cache is access by two different threads. Now the funny thing: even I synchronized every access to the cache, I still get ConcurrentModificationExceptions.
Here is a part of the code:
     @Override public void paint(Graphics g2) {
          // Check if the image is in the cache
          Image drawing;
          synchronized (m_Cache) { drawing = m_Cache.get(m_CurrentOffset); }
          if (drawing == null) {
               // The image is not cached, so draw it
               // Put the image into the cache
               synchronized (m_Cache) { m_Cache.put(m_CurrentOffset, drawing); }
     public void componentResized(ComponentEvent e) {
          // Upon resizing of the component, adjust several pre-calculated values
          // And flush the cache
          synchronized (m_Cache) { m_Cache.clear(); }
     public void elementUpdated(IHexGridElement Element) {
          // Clear cache where the element may be contained at
          synchronized (m_Cache) { for (Point p : m_Cache.keySet()) { if (isElementInScope(Element.getIndex(), p, new Point(p.x + m_MaxColumn, p.y + m_MaxRow))) { m_Cache.remove(p); } } }
     }The paint and componentResized methods are invoked by the AWTEventQueue-0 thread. The elementUpdated method is invoked by "MyThread" thread.
Now my question is why do I get java.util.ConcurrentModificationException, especially in situations where a lot of repaintings have to be done and the window is resized? When I remove the caching stuff, everything works perfect, but only a little bit slow.
Thanks for any help.

Answer 1

In your elementUpdated method, you are using an Iterator to walk over the set of keys in your map. You can't see the Iterator, because you are using the new for-each loop syntax which hides it from you - but it's there under the covers. You are then removing elements from your map directly. This is what causes your ConcurrentModificationException.
You should re-write your loop to explicitly use an iterator, and then do your remove operation through the iterator.
Incidentally, you have a slight race condition in your paint method. Two (or more) threads could simultaneously discover that a given image is not cached, then both draw it, then both cache it. If the drawing operation produces the same drawing every time, then the only problem this causes is the overhead of drawing the image multiple times. To fix this, wrap the get, draw and put operations in a single synchronized block.