Concurrency - Java 8 Recipes - page 348

stopUpdatingThread();
}
How It Works
Java comes with many concurrent collection options. Which collection to use depends
on how the read operations compare with the write operations within the context of
your application. If writing occurs far and in-between compared with reads, using a
copyOnWriteArrayList instance is most efficient because it doesn't block (stop)
other threads from reading the list and is thread-safe to iterate over (no Concur-
rentModificationException is thrown when iterating through it). If there are
the same number of writes and reads, using a SynchronizedList is the preferred
choice.
In solution 1, the CopyOnWriteArrayList is being updated while you traverse
the list. Because the recipe uses the CopyOnWriteArrayList instance, there is no
need to worry of thread safety when iterating through the collection (as is being done in
this recipe by using the stream ). It is good to note that the CopyOnWriteAr-
rayList offers a snapshot in time when iterating through it. If another thread modi-
fies the list as you're iterating through it, changes are that the modified list will not be
visible when iterating.
Note Locking properly depends on the type of collection being used. Any collec-
tions returned as a result of using Collections.synchronized can be locked via
the collection itself ( synchronized (collectionInstance) ). However, some
more efficient (newer) concurrent collections such as the ConcurrentMap cannot be
used in this fashion because their internal implementations don't lock in the object it-
self.
Solution 2 creates a synchronized list, which is created by using the Collec-
tions helper class. The Collection.synchronizedList() method wraps a
List object (it can be ArrayList , LinkedList , or another List implementer)
into a list that synchronizes the access to the list operations. Each time that you need to
iterate over a list (either by using the stream , a for loop, or an iterator) you must be
aware of the concurrency implications for that list's iterator. The CopyOnWriteAr-
rayList is safe to iterate over (as specified in the Javadoc), but the synchron-
izedList iterator must be synchronized manually (also specified in the Collec-