Why is a ConcurrentModificationException thrown and how to debug it

java exception collections concurrentmodification
mainstringargs picture mainstringargs · Mar 2, 2009 · Viewed 159.5k times · Source

I am using a Collection (a HashMap used indirectly by the JPA, it so happens), but apparently randomly the code throws a ConcurrentModificationException. What is causing it and how do I fix this problem? By using some synchronization, perhaps?

Here is the full stack-trace:

Exception in thread "pool-1-thread-1" java.util.ConcurrentModificationException
        at java.util.HashMap$HashIterator.nextEntry(Unknown Source)
        at java.util.HashMap$ValueIterator.next(Unknown Source)
        at org.hibernate.collection.AbstractPersistentCollection$IteratorProxy.next(AbstractPersistentCollection.java:555)
        at org.hibernate.engine.Cascade.cascadeCollectionElements(Cascade.java:296)
        at org.hibernate.engine.Cascade.cascadeCollection(Cascade.java:242)
        at org.hibernate.engine.Cascade.cascadeAssociation(Cascade.java:219)
        at org.hibernate.engine.Cascade.cascadeProperty(Cascade.java:169)
        at org.hibernate.engine.Cascade.cascade(Cascade.java:130)

Answer

Robin picture Robin · Mar 2, 2009

This is not a synchronization problem. This will occur if the underlying collection that is being iterated over is modified by anything other than the Iterator itself.

Iterator it = map.entrySet().iterator();
while (it.hasNext())
{
   Entry item = it.next();
   map.remove(item.getKey());
}

This will throw a ConcurrentModificationException when the it.hasNext() is called the second time.

The correct approach would be 

   Iterator it = map.entrySet().iterator();
   while (it.hasNext())
   {
      Entry item = it.next();
      it.remove();
   }

Assuming this iterator supports the remove() operation.

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