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Java CopyOnWriteArrayList

The CopyOnWriteArrayList class is intended as a replacement for ArrayList in concurrent applications where traversals greatly outnumber insertions or removals. This is quite common when ArrayList is used to store a list of listeners, such as in AWT or Swing applications, or in JavaBean classes in general. (The related CopyOnWriteArraySet uses aCopyOnWriteArrayList to implement the Set interface.)

If you are using an ordinary ArrayList to store a list of listeners, as long as the list remains mutable and may be accessed by multiple threads, you must either lock the entire list during iteration or clone it before iteration, both of which have a significant cost. CopyOnWriteArrayList instead creates a fresh copy of the list whenever a mutative operation is performed, and its iterators are guaranteed to return the state of the list at the time the iterator was constructed and not throw aConcurrentModificationException. It is not necessary to clone the list before iteration or lock it during iteration because the copy of the list that the iterator sees will not change. In other words, CopyOnWriteArrayList contains a mutable reference to an immutable array, so as long as that reference is held fixed, you get all the thread-safety benefits of immutability without the need for locking.

For more information please refer “Java theory and practice: Concurrent collections classes”.