Thursday, 9 February 2012

Concurrent collections

As I already mentioned, situation that requires concurrent access to a collection is pretty often. Let's say that we have a map that is accessed by many threads.
Synchronizing it like this is not the best choice, as only one thread at a time will be able to access it.
Another approach is useful only in a few cases. The factory method creates synchronized wrapper for our map that makes it thread-safe. However if we want to iterate over it, we still need a synchronization, otherwise it will throw ConcurrentModificationException when some other thread will modify it while we're iterating.

The better solution is to use ConcurrentMap  It not only gives us a concurrent access, but we can also iterate over it without synchronization.

Let's imagine a situation that we need to put some value to the map, on a condition that it isn't there already.
Doing it that way is a wrong solution: After we check that the key is not there, another thread could put some entry to the map and we will override existing value. Basically our operation in this implementation is not atomic.
So should we add the synchronization? No, the ConcurrentMap provides a method that will make this operation atomic:

ConcurrentMap provides also other methods that are able to perform atomic replacement or removal of an entry, only if it is mapped to a specified value.
There are also a concurrent implementations of List and Set - CopyOnWriteArrayList and CopyOnWriteArraySet. Every mutative operation on them creates a new copy, so they should be used with a special care. 

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