Question

As far as I know there are two ways to avoid ConcurrentModificationException while one threading iterates the collection and another thread modifies the collection.

1. client-locking, basically lock the collection during the iteration. Other threads that need to access the collection will block until the iteration is complete.
2. “thread-confined” that clones the collection and iterate the copy.

I am wondering are there any other alternatives ?
because the first way obvious is undesirable and poor performance-wise, if the collection is large that other threads could wait for a long time. second way I am not sure that since we clone the collection, and iterate the copy, so if other threads come in and modify the original one, then the copied one becomes stale right ? does that mean we need to restart over by cloning and iterate it again once it’s modified ?

Answer 1

I am wondering are there any other alternatives ?

Use one of the concurrent collections which doesn’t throw this exception. Instead they provide weak consistency. i.e. an added or delete element may or may not appear while iterating.

http://docs.oracle.com/javase/tutorial/essential/concurrency/collections.html

The java.util.concurrent package includes a number of additions to the Java Collections Framework. These are most easily categorized by the collection interfaces provided:

• BlockingQueue defines a first-in-first-out data structure that blocks or times out when you attempt to add to a full queue, or retrieve from an empty queue.
• ConcurrentMap is a subinterface of java.util.Map that defines useful atomic operations. These operations remove or replace a key-value pair only if the key is present, or add a key-value pair only if the key is absent. Making these operations atomic helps avoid synchronization. The standard general-purpose implementation of ConcurrentMap is ConcurrentHashMap, which is a concurrent analog of HashMap.
• ConcurrentNavigableMap is a subinterface of ConcurrentMap that supports approximate matches. The standard general-purpose implementation of ConcurrentNavigableMap is ConcurrentSkipListMap, which is a concurrent analog of TreeMap.