I used synchronized list, and i still get ConcurrentModificationException [duplicate]

Question

I'm using Vector instead of ArrayList to make a list safe in multi-threaded enviroment. But I keep getting ConcurrentModificationException when I trying to add items to the Vector while iterating it. Why is that and how can I prevent it?

Quennie

concurrent != parallel. You can get a ConcurrentModificationException with a single thread. – Melvamelvena

Answer 1

You cannot modify a Vector while iterating over it. Store the items to add in a separate vector, and move them to the Vector when the loop is finished or loop over a copy of the original Vector.

ADDED: To get a mutex around the Vector in java, do this in both functions:

synchronized (list) {
  // modifying list
}

and:

synchronized (list) {
  // iterating over list
}

Of course I've assumed that the list is named list

Lightship

Side note: this holds true even in single-threaded programs. – Paco

what do you mean it holds true in single-threaded programs? – Quennie

You can never modify the list while iterating over it. Even in single threaded programs. Modifying the underlying list invalidates the iterator. – Lightship

so what the purpose of synchronized collections in this case? – Quennie

None. It only synchronizes direct modifications and reads of the list. Which, in at least Java, does not include Iterators of the list. You need to use a mutex instead - locked for the entire duration of the loop, and which is locked while any other operation is performed on the list. – Lightship

To prevent another thread from modifying the current thread's view of the list. Note that this does not imply the list is immutable. – Trant

"Store the items to add in a separate vector, and move them to the Vector when the loop is finished or loop over a copy of the original Vector." => or use an existing class of the standard JDK which does exactly that: a CopyOnWriteArrayList – Ketch

@Ketch That is not what that class does. It copies the list, throwing away the old one. What I propose is to wait with appending new items until the iterator finished. – Lightship

I have one method that adds items to the Vector, and another method itareting on the Vector. I synchronized both, is that good idea? – Quennie

Synchronizing methods will not help - as it only prevents that function from being run concurrently. It does not help with two different functions, they can still be executed concurrently – Lightship

There is also recommended to declare synchronizing object as final, to sure that it's reference should be changed. – Dido

Answer 2

if you want to add items as you iterate, you'll want to use a ListIterator. by using Vector, you're not bypassing this rule (obviously), so I would recommend using the ArrayList instead.

Thorlay

I don't think it is safe to add while iterating with a ListIterator if more than one thread is involved. – Ketch

@assylias, I misunderstood the question, I suppose. I was just addressing the ConcurrentModificationException. – Thorlay

Answer 3

If you need to iterate and add concurrently to your list, you should use a concurrent list, such as CopyOnWriteArrayList. Note that if you write a lot to the list it will not be very efficient.

Otherwise, if you use a Vector or a synchronizedList, you need to hold the list's lock while iterating. That will prevent the exception but it will also prevent concurrency...

Ketch