The Collections framework introduced iterators for traversing a list or other collection, which optimizes the process of iterating through the elements in a collection. However, the iterators implemented in the java.util Collections classes are fail-fast, which means that if one thread changes a collection while another thread is traversing it through an Iterator, the next Iterator.hasNext() or Iterator.next() call will throw ConcurrentModificationException.
Look at this,
[Only good for single thread]
List list = new ArrayList();
Iterator iterator = list.iterator();
while(iterator.hasNext()) {
Item item = iterator.next();
if(isOld(item)) {
iterator.remove();
}
}
if this is single-threaded code and need to modify a collection while iterating through it. That’s fine.
But you run in multiple-threaded, how to avoid ConcurrentModificationException?
First you may want to use synchronizedList like this,
[Scalability problems]
List originalList = new ArrayList();
List list = Collections.synchronizedList(originalList);
synchronized(list) {
Iterator iterator = list.iterator(); // Must be in synchronized block
while (iterator.hasNext()) {
Item item = iterator.next();
if(isOld(item)) {
iterator.remove();
}
}
}
The bigger problem with the synchronized Collections wrappers(synchronizedList, synchronizedMap), and the earlier Hashtable and Vector classes, is that they synchronize on a single lock. This means that only one thread may access the collection at once, and if one thread is in the process of reading from a Map, all other threads that want to either read from it or write to it must wait.
So what’s perfect way to do?
[Concurrent Collections]
List originalList = new ArrayList();
List list = new CopyOnWriteArrayList(originalList);
Iterator iterator = list.iterator();
while (iterator.hasNext()) {
Item item = iterator.next();
if(isOld(item)) {
iterator.remove();
}
}
The same story for Map, you should use ConcurrentHashMap instead synchronizedMap.
Reference:
1. Concurrent collections classes
2. Building a better HashMap
3. Java Concurrency Bugs #4: ConcurrentModificationException
4. Taming Tiger: Concurrent collections
5. Introduction to nonblocking algorithms
6. Concurrency: Blocking Queues and You
