CODEWITHSUNDEEP
javamultithreadingthread-safetyhashtablehashmap
rajesh
rajesh

Reputation:

Hashmap and hashtable in multithreaded environment

I am really confused on how these 2 collections behave in multithreaded environment.

Hash table is synchronized that means no 2 threads will be updating its value simultaneously right?

Upvotes: 8

Views: 16159

Answers (5)

Miserable Variable
Miserable Variable

Reputation: 28752

Also note that Hashtable and Collections.synchronizedMap are safe only for individual operations. Any operations involving multiple keys or check-then-act that need to be atomic will not be so and additional client side locking will be required.

For example, you cannot write any of the following methods without additional locking:

  • swap the values at two different keys: swapValues(Map, Object k1, Object k2)

  • append the parameter to value at a key: appendToValue(Map, Object k1, String suffix)

And yes, all of this is covered in JCIP :-)

Upvotes: 3

Paul Whelan
Paul Whelan

Reputation: 16809

Look at ConcurrentHashMaps for Thread safe Maps.

They offer all the features of HashTable with a performance very close to a HashMap.

Performance is gained by instead of using a map wide lock, the collection maintains a list of 16 locks by default, each of which is used to lock a single bucket of the map. You can even configure the number of buckets :) Tweaking this can help performance depending on your data.

I can't recommend enough Java Concurrency in Practice by Brian Goetz http://jcip.net/

I still learn something new every time I read it.

Upvotes: 14

Adrian Pronk
Adrian Pronk

Reputation: 13906

Hashtables are synchronized but they're an old implementation that you could almost say is deprecated. Also, they don't allow null keys (maybe not null values either? not sure).

One problem is that although every method call is synchronized, most interesting actions require more than one call so you have to synchronize around the several calls.

A similar level of synchronization can be obtained for HashMaps by calling:

Map m = Collections.synchronizedMap(new HashMap());

which wraps a map in synchronized method calls. But this has the same concurrency drawbacks as Hashtable.

As Paul says, ConcurrentHashMaps provide thread safe maps with additional useful methods for atomic updates.

Upvotes: 0

ATorras
ATorras

Reputation: 4293

Yes, all the methods are done atomically, but values() method not (see docs).

Paul was faster than me recommending you the java.util.concurrent package, which gives you a very fine control and data structures for multithreade environments.

Upvotes: 0

Nettogrof
Nettogrof

Reputation: 2136

Exactly, HashTable is synchronized that mean that it's safe to use it in multi-thread environment (many thread access the same HashTable) If two thread try to update the hashtable at the sametime, one of them will have to wait that the other thread finish his update.

HashMap is not synchronized, so it's faster, but you can have problem in a multi-thread environment.

Upvotes: 3

Related Questions