Question

It’s plenty of questions regarding ConcurrentModificationException for ArrayList objects, but I could not find yet an answer to my problem.

In my servlet I have an ArrayList as a member object:

List myList<Object> = new ArrayList<Object> (...);

The list must be shared among users and sessions.

In one of the servlet’s methods, method1, I need to iterate over the ArrayList items, and eventually add clear the list after the iteration. Here a snippet:

for (Object o : myList) {
    // read item o
}
myList.clear();

In another method, method2, I simply add a new Item to the list.

Most of the times the method ends its job without errors. Sometimes, probably due to the concurrent invocation of this method by different users, I get the famous java util.ConcurrentModificationException exception.
Should I define my List as:

List myList = Collections.synchronizedList(new ArrayList(...));

Would this be enough or am I missing something? What’s behind the scenes? When there is a possible concurrency, is the second thread held in standby by the container?

EDIT: I have added the answers to some comments.

Answer 1

Just using synchronizedList makes all methods thread safe EXCEPT Iterators.

I would use CopyOnWriteArrayList. It is thread safe and doesn’t produce ConcurrentModificationException.