Concurrent updates on Collection

Question

I am developing a System and it has ArrayList that access in several places(inserting, removing and updates the values). Due to access of ArrayList in several places when i run the program it gives Concurrent update error.

Instead of ArrayList I can use Vector because Vector is synchronized. But if i use Vector will It be cause to decrease the performance of the system? Give me Ideas. How I can solve this issue?

This is part of the exception I get:

].[localhost].[/uckt].[Faces Servlet]] (http-127.0.0.1-8080-144) 
Servlet.service() for servlet Faces Servlet threw exception: java.util.ConcurrentModificationException
at java.util.ArrayList$Itr.checkForComodification(Unknown Source) [:1.7.0_02]
at java.util.ArrayList$Itr.next(Unknown Source) [:1.7.0_02]

Do you access your arraylist from several threads? You can also get a ConcurrentModificationException if you remove items from a list while iterating for example (unless you use iterator.remove()). — assylias, Aug 13 '12 at 11:07
In that case you probably don't need a thread safe variant of the an ArrayList - see [this post for an example](http://stackoverflow.com/questions/5113016/concurrentmodificationexception-when-remove-element-from-a-java-util-list-while). — assylias, Aug 13 '12 at 11:14
@PradeeGamage You should post the part of the code that throws the exception with a stacktrace to get better and more specific answers. — assylias, Aug 13 '12 at 11:19
This is part of exception ].[localhost].[/uckt].[Faces Servlet]] (http-127.0.0.1-8080-144) Servlet.service() for servlet Faces Servlet threw exception: java.util.ConcurrentModificationException at java.util.ArrayList$Itr.checkForComodification(Unknown Source) [:1.7.0_02] at java.util.ArrayList$Itr.next(Unknown Source) [:1.7.0_02] — Pradeep Gamage, Aug 13 '12 at 11:26

score 2 · Accepted Answer · answered Aug 13 '12 at 11:35

There is one more thing: Your ConcurrentModificationException might not spawn from an actual concurrent modification through two Threads. There is another possible reason:

While iterating over the ArrayList, you might delete an element. If you try this

Object o = iterator.next()
if(someCondition)
    arrayList.remove(o)

in a single Thread, you will get a ConcurrentModificationException. In that case you will have to use ListIterator and it's remove method.

I think this will be cause. I will look into it. Thank You very much — Pradeep Gamage, Aug 13 '12 at 12:32

score 1 · Answer 2 · answered Aug 13 '12 at 11:06

Have you considered 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.

As ever, I would not worry too much about efficiency until you know it's a problem.

score 0 · Answer 3 · answered Aug 13 '12 at 11:10

0

See Collections#synchronizedCollection(Collection). This will return a synchronized ArrayList that you can use to safely add/remove/update elements in the list.

answered Aug 13 '12 at 11:10

Radu Murzea

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score 0 · Answer 4 · answered Aug 13 '12 at 11:15

0

You can also synchronize your ArrayList on each access:

synchronize(myList)
{
  myList.add(object);
}

answered Aug 13 '12 at 11:15

Peter Ilfrich

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Concurrent updates on Collection

4 Answers4