Building Blocks - Java Concurrency in Practice - page 99

Because it has so many advantages and so few disadvantages compared to Hashtable
or synchronizedMap , replacing synchronized Map implementations with Concur-
rentHashMap in most cases results only in better scalability. Only if your application needs
to lock the map for exclusive access [3] is ConcurrentHashMap not an appropriate drop-
in replacement.
5.2.2. Additional Atomic Map Operations
Since a ConcurrentHashMap cannot be locked for exclusive access, we cannot use
client-side locking to create new atomic operations such as put-if-absent, as we did for
Vector in Section 4.4.1 . Instead, a number of common compound operations such as put-if-
absent, remove-if-equal, and replace-if-equal are implemented as atomic operations and spe-
cified by the ConcurrentMap interface, shown in Listing 5.7 . If you find yourself adding
such functionality to an existing synchronized Map implementation, it is probably a sign that
you should consider using a ConcurrentMap instead.
5.2.3. CopyOnWriteArrayList
CopyOnWriteArrayList is a concurrent replacement for a synchronized List that of-
fers better concurrency in some common situations and eliminates the need to lock or copy
the collection during iteration. (Similarly, CopyOnWriteArraySet is a concurrent re-
placement for a synchronized Set .)
The copy-on-write collections derive their thread safety from the fact that as long as an ef-
fectively immutable object is properly published, no further synchronization is required when
accessing it. They implement mutability by creating and republishing a new copy of the col-
lection every time it is modified. Iterators for the copy-on-write collections retain a reference
to the backing array that was current at the start of iteration, and since this will never change,
they need to synchronize only briefly to ensure visibility of the array contents. As a res-
ult, multiple threads can iterate the collection without interference from one another or from
threads wanting to modify the collection. The iterators returned by the copy-on-write col-
lections do not throw ConcurrentModificationException and return the elements
exactly as they were at the time the iterator was created, regardless of subsequent modifica-
tions.
Listing 5.7. ConcurrentMap Interface.