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ArrayList$Itr.checkForComodification throws java.util.ConcurrentModificationException on iterated NOT modified array list [duplicate]

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)

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.

Try using a ConcurrentHashMap instead of a plain HashMap

Modification of a Collection while iterating through that Collection using an Iterator is not permitted by most of the Collection classes. The Java library calls an attempt to modify a Collection while iterating through it a "concurrent modification". That unfortunately suggests the only possible cause is simultaneous modification by multiple threads, but that is not so. Using only one thread it is possible to create an iterator for the Collection (using Collection.iterator(), or an enhanced for loop), start iterating (using Iterator.next(), or equivalently entering the body of the enhanced for loop), modify the Collection, then continue iterating.
To help programmers, some implementations of those Collection classes attempt to detect erroneous concurrent modification, and throw a ConcurrentModificationException if they detect it. However, it is in general not possible and practical to guarantee detection of all concurrent modifications. So erroneous use of the Collection does not always result in a thrown ConcurrentModificationException.
The documentation of ConcurrentModificationException says:
This exception may be thrown by methods that have detected concurrent modification of an object when such modification is not permissible...
Note that this exception does not always indicate that an object has been concurrently modified by a different thread. If a single thread issues a sequence of method invocations that violates the contract of an object, the object may throw this exception...
Note that fail-fast behavior cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast operations throw ConcurrentModificationException on a best-effort basis.
Note that
the exception may be throw, not must be thrown
different threads are not required
throwing the exception cannot be guaranteed
throwing the exception is on a best-effort basis
throwing the exception happens when the concurrent modification is detected, not when it is caused
The documentation of the HashSet, HashMap, TreeSet and ArrayList classes says this:
The iterators returned [directly or indirectly from this class] are fail-fast: if the [collection] is modified at any time after the iterator is created, in any way except through the iterator's own remove method, the Iterator throws a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.
Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.
Note again that the behaviour "cannot be guaranteed" and is only "on a best-effort basis".
The documentation of several methods of the Map interface say this:
Non-concurrent implementations should override this method and, on a best-effort basis, throw a ConcurrentModificationException if it is detected that the mapping function modifies this map during computation. Concurrent implementations should override this method and, on a best-effort basis, throw an IllegalStateException if it is detected that the mapping function modifies this map during computation and as a result computation would never complete.
Note again that only a "best-effort basis" is required for detection, and a ConcurrentModificationException is explicitly suggested only for the non concurrent (non thread-safe) classes.
Debugging ConcurrentModificationException
So, when you see a stack-trace due to a ConcurrentModificationException, you can not immediately assume that the cause is unsafe multi-threaded access to a Collection. You must examine the stack-trace to determine which class of Collection threw the exception (a method of the class will have directly or indirectly thrown it), and for which Collection object. Then you must examine from where that object can be modified.
The most common cause is modification of the Collection within an enhanced for loop over the Collection. Just because you do not see an Iterator object in your source code does not mean there is no Iterator there! Fortunately, one of the statements of the faulty for loop will usually be in the stack-trace, so tracking down the error is usually easy.
A trickier case is when your code passes around references to the Collection object. Note that unmodifiable views of collections (such as produced by Collections.unmodifiableList()) retain a reference to the modifiable collection, so iteration over an "unmodifiable" collection can throw the exception (the modification has been done elsewhere). Other views of your Collection, such as sub lists, Map entry sets and Map key sets also retain references to the original (modifiable) Collection. This can be a problem even for a thread-safe Collection, such as CopyOnWriteList; do not assume that thread-safe (concurrent) collections can never throw the exception.
Which operations can modify a Collection can be unexpected in some cases. For example, LinkedHashMap.get() modifies its collection.
The hardest cases are when the exception is due to concurrent modification by multiple threads.
Programming to prevent concurrent modification errors
When possible, confine all references to a Collection object, so its is easier to prevent concurrent modifications. Make the Collection a private object or a local variable, and do not return references to the Collection or its iterators from methods. It is then much easier to examine all the places where the Collection can be modified. If the Collection is to be used by multiple threads, it is then practical to ensure that the threads access the Collection only with appropriate synchonization and locking.

In Java 8, you can use lambda expression:
map.keySet().removeIf(key -> key condition);
removeIf is a convenient default method in Collection which uses Iterator internally to iterate over the elements of the calling collection.
The extraction of the removal condition is expressed by allowing the caller to provide a Predicate<? super E>.
"I'll perform the iteration for you and test your Predicate on each one of the elements in the collection. If an element causes the test method of the Predicate to return true, I'll remove it."

It sounds less like a Java synchronization issue and more like a database locking problem.
I don't know if adding a version to all your persistent classes will sort it out, but that's one way that Hibernate can provide exclusive access to rows in a table.
Could be that isolation level needs to be higher. If you allow "dirty reads", maybe you need to bump up to serializable.

Note that the selected answer cannot be applied to your context directly before some modification, if you are trying to remove some entries from the map while iterating the map just like me.
I just give my working example here for newbies to save their time:
HashMap<Character,Integer> map=new HashMap();
//adding some entries to the map
...
int threshold;
//initialize the threshold
...
Iterator it=map.entrySet().iterator();
while(it.hasNext()){
Map.Entry<Character,Integer> item=(Map.Entry<Character,Integer>)it.next();
//it.remove() will delete the item from the map
if((Integer)item.getValue()<threshold){
it.remove();
}

Try either CopyOnWriteArrayList or CopyOnWriteArraySet depending on what you are trying to do.

I ran into this exception when try to remove x last items from list.
myList.subList(lastIndex, myList.size()).clear(); was the only solution that worked for me.

Is Hashtable.entrySet() thread safe?

Is it safe to iterate over the set returned by Hashtable.entrySet()? What about Hashtable.values() and Hashtable.keySet()?
What I intent to do: I want update the entries of a Hastable while the table is used by different other threads. For this I have to iterate over all entries currently in the map. It's not important if any entries added/removed from other threads during the iteration are handled or not. Synchronizing on the Hashtable is not an option because the update may take to long.

From the JavaDoc:
Unlike the new collection implementations, Hashtable is synchronized. If a thread-safe implementation is not needed, it is recommended to use HashMap in place of Hashtable. If a thread-safe highly-concurrent implementation is desired, then it is recommended to use ConcurrentHashMap in place of Hashtable.
Better use a ConcurrentHashMap.

No, iterating over the views of a Hashtable is not safe in the face of concurrent modification. From the javadoc (emphasis mine):
The iterators returned by the iterator method of the collections returned by all of this class's "collection view methods" are fail-fast: if the Hashtable is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove method, the iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future. The Enumerations returned by Hashtable's keys and elements methods are not fail-fast.
jabu.10245 is correct that a ConcurrentHashMap is more appropriate, and does meet your requirement of allowing iteration while concurrent modification is occurring.

I think we're just going to quote the documentation of Hashtable at you.
Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.
I think the docs say it all: You can't depend on the iterator telling you when you've modified a Hashtable incorrectly. Either synchronize on the Hashtable, or use a different class.

Java ConcurrentModificationException: Is it possible to add elements to a hashtable while iterating through it?

I am iterating through a Hashtable and at one point, I add something in to the Hashtable which is clearly giving me a ConcurrentModificationException. I understand why I am getting the error, but is there a way around this such that I could still iterate through the Hashtable and add values simultaneously?

From the docs
The iterators returned by the iterator
method of the collections returned by
all of this class's "collection view
methods" are fail-fast: if the
Hashtable is structurally modified at
any time after the iterator is
created, in any way except through the
iterator's own remove method, the
iterator will throw a
ConcurrentModificationException. Thus,
in the face of concurrent
modification, the iterator fails
quickly and cleanly, rather than
risking arbitrary, non-deterministic
behavior at an undetermined time in
the future. The Enumerations returned
by Hashtable's keys and elements
methods are not fail-fast.
Note that the fail-fast behavior of an
iterator cannot be guaranteed as it
is, generally speaking, impossible to
make any hard guarantees in the
presence of unsynchronized concurrent
modification. Fail-fast iterators
throw ConcurrentModificationException
on a best-effort basis. Therefore, it
would be wrong to write a program that
depended on this exception for its
correctness: the fail-fast behavior of
iterators should be used only to
detect bugs.
If you need this kind of behavior you can safely copy the set of keys and iterate through the copy. Another option if the hashtable is large and copying the keyset is likely to be expensive is to add to a separate collection during the iteration and add the elements of the separate collection post iteration.

You may also want to know about CopyOnWriteSet, which is specifically designed for safe iteration while set is modified. Note that iterator sees only the original set. Any additions will not be visible until next iteration.
http://download.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/CopyOnWriteArraySet.html
This is most useful in many readers / few writers scenario. It is unlikely to be the most efficient solution if reading and writing happens in the same code path.

Make a new Hashtable that you add new entries to; then when you are done iterating, add in the entries from the first table.
Optionally, if you need to, you can skip keys that exist in the original table.

Another alternative would be to use a ConcurrentHashMap instead of a HashMap. However:
The iterators for a ConcurrentHashMap are defined to return objects reflecting a state some time at or after the creation of the iterator. A more precise statement of the behaviour is in the javadocs for the relevant methods.
A ConcurrentHashMap is probably slower than a regular HashMap.

FailSafe Feature

Before asking this question I have tried understanding (here on SOF and on some other websites) fail-safe feature. I understand Java Collection Iterators are fail-fast, which basically means they fail gracefully as soon as underlying Collection is being structurally modified (even by the same thread). My question is does fail-safe property have anything to do with Iterators' remove() or add() features? In my understanding because through Iterators you can add or remove (safely) while iterating over a Collection and you wont get a concurrent exception (that you do without using their remove and add features), so that makes Iterators fail-safe. Or I have got it completely wrong?
Thanks!

Not exactly. In my understanding fail-safe iterators work on snapshots of data and guarantee consistent view of the represented collection on the moment when the iterator has been created. (please see this blog post for more detailed treatment of this question). This property is guaranteed f.i. by iterators of the CopyOnWriteArrayList. It's iterators do not support collection modification operations and its javadoc clarifies their behavior further:
This array never changes during the
lifetime of the iterator, so
interference is impossible and the
iterator is guaranteed not to throw
ConcurrentModificationException. The
iterator will not reflect additions,
removals, or changes to the list since
the iterator was created.
Element-changing operations on
iterators themselves (remove, set, and
add) are not supported. These methods
throw UnsupportedOperationException.
UPDATE:
When talking about fail-safeness and fail-fastness it is important to separate "the failure". In case of iterator there are different cases and hazards. As for the linked article I would say that there author implements fail-safe and fail-fast iteration at the first place by implementing iterators.
The failure in that case can be defined as concurrent modification of the iterated collection. When the collection is modified then fail-fast approach will be to stop iteration and to make the caller aware of the changed conditions (via CME or by some other means).
When dealing with the same use-case and hazard we can move forward and think about fail-safe iteration. Fail-safeness property means that iteration should comply to its contract as long as it possible (and authors of the COWAS succeed by copying underlying data).

The Iterator or ListIterator work on the underlying collection is a way which doesn't not invalidate that Iterator. Any modification via any other iterator or the collection itself, invalidates the iterator for some collections. Some collections are designed for concurrent access and don't have this restriction.

Why do we need synchronized ArrayLists when we already have Vectors?

When do we use synchronized ArrayList? We already have Vector which is synchronized.

I think that you've got this wrong. ArrayList is unsynchronized, Vector is.
Being synchronized means that every operation is thread safe - if you use the same vector from two threads at the same time, they can't corrupt the state. However, this makes it slower.
If you are working in a single threaded environment (or the list is limited to a thread and never shared), use ArrayList. If you are working with multiple threads that share the same collection, either use Vector, or use ArrayList but synchronize in some other way (e.g., manually or via a wrapper).

ArrayList is not synchronized via http://java.sun.com/javase/6/docs/api/java/util/ArrayList.html

ArrayList is not synchronized out of the box.
Resizable-array implementation of the
List interface. Implements all
optional list operations, and permits
all elements, including null. In
addition to implementing the List
interface, this class provides methods
to manipulate the size of the array
that is used internally to store the
list. (This class is roughly
equivalent to Vector, except that it
is unsynchronized.)
This avoids some performance issues in situations where you know that you won't need thread safety (e.g., entirely encapsulated private data). However both ArrayList and Vector have issues when using iterators over them: when iterating through either type of collection, if data is added or removed, you will throw a ConcurrentModificationException:
Note that this implementation is not
synchronized. If multiple threads
access an ArrayList instance
concurrently, and at least one of the
threads modifies the list
structurally, it must be synchronized
externally. (A structural modification
is any operation that adds or deletes
one or more elements, or explicitly
resizes the backing array; merely
setting the value of an element is not
a structural modification.) This is
typically accomplished by
synchronizing on some object that
naturally encapsulates the list. If no
such object exists, the list should be
"wrapped" using the
Collections.synchronizedList method.
This is best done at creation time, to
prevent accidental unsynchronized
access to the list:
List list =
Collections.synchronizedList(new
ArrayList(...));
The iterators returned by this class's
iterator and listIterator methods are
fail-fast: if the list is structurally
modified at any time after the
iterator is created, in any way except
through the iterator's own remove or
add methods, the iterator will throw a
ConcurrentModificationException. Thus,
in the face of concurrent
modification, the iterator fails
quickly and cleanly, rather than
risking arbitrary, non-deterministic
behavior at an undetermined time in
the future.
Note that the fail-fast behavior of an
iterator cannot be guaranteed as it
is, generally speaking, impossible to
make any hard guarantees in the
presence of unsynchronized concurrent
modification. Fail-fast iterators
throw ConcurrentModificationException
on a best-effort basis. Therefore, it
would be wrong to write a program that
depended on this exception for its
correctness: the fail-fast behavior of
iterators should be used only to
detect bugs.
ArrayList comes in a variety of useful flavors, however, while Vector does not. My personal favorite is the CopyOnWriteArrayList:
A thread-safe variant of ArrayList in which all mutative operations (add, set, and so on) are implemented by making a fresh copy of the underlying array.
This is ordinarily too costly, but may be more efficient than alternatives when traversal operations vastly outnumber mutations, and is useful when you cannot or don't want to synchronize traversals, yet need to preclude interference among concurrent threads. The "snapshot" style iterator method uses a reference to the state of the array at the point that the iterator was created. This array never changes during the lifetime of the iterator, so interference is impossible and the iterator is guaranteed not to throw ConcurrentModificationException. The iterator will not reflect additions, removals, or changes to the list since the iterator was created. Element-changing operations on iterators themselves (remove, set, and add) are not supported. These methods throw UnsupportedOperationException.
CopyOnWriteArrayLists are tremendously useful in GUI work, especially in situations where you are displaying an updating set of data (e.g., moving icons on a screen). If you can tolerate having your displayed list of data be one frame out of date (because your producer thread is slightly behind your graphical update thread), CopyOnWriteArrayLists are the perfect data structure.

What does it mean array list is synchronized in java?
It means it is thread-safe.
Vectors are synchronized. Any method that touches the Vector's contents is thread safe.
ArrayList, on the other hand, is unsynchronized, making them, therefore, not thread safe.

I read ArrayList is synchronized in
java..
But the array list api says that
Note that this implementation is not
synchronized
So you use an ArrayList when you are sure that you wont be dealing with concurrency.
Using Vector might be an overkill, and may result in performance issues.