How to debug ConcurrentModificationException? [duplicate]

Question

I encountered ConcurrentModificationException and by looking at it I can't see the reason why it's happening; the area throwing the exception and all the places modifying the collection are surrounded by

synchronized (this.locks.get(id)) {
  ...
} // locks is a HashMap<String, Object>;

I tried to catch the the pesky thread but all I could nail (by setting a breakpoint in the exception) is that the throwing thread owns the monitor while the other thread (there are two threads in the program) sleeps.

How should I proceed? What do you usually do when you encounter similar threading issues?

Answer 1

It may have nothing to do with the synchronization block. ConcurrentModificationExceptions often occur when you're modifying a collection while you are iterating over its elements.

List<String> messages = ...;
for (String message : messages) {
    // Prone to ConcurrentModificationException
    messages.add("A COMPLETELY NEW MESSAGE");
}

Answer 2

Similar to a previous post, you can get the same issue if you delete an entry. e.g.

for(String message : messages) {
  if (condition(message))
     messages.remove(message);
}

Another common example is cleaning up a Map.

This particular problem can be resolved using an Iterator explicitly.

for(Iterator<String> iter = messages.iterator(); iter.hasNext();) {
   String message = iter.next();
   if (condition(message))
       iter.remove(); // doesn't cause a ConcurrentModificationException 
}