在java中计算每个键的最佳实践是什么？

Question

In java I need to count items per key so i thought of using ConcurrentMap with key and as value to have AtomicInteger however I wonder if there is a better way. Its not a critical service, its just used for collecting stats, so i don't mind if i'm late 10 seconds with correct stats or something like this. So i wonder if there is a better practice for it in standard java concurrency, or whether i'm left again with dealing by myself with ConcurrentMap and with AtomicInteger (an example which could also solve the problem would be message passing maybe, if yes how could that help me?) (best would be with example...) something which would not have performance issues and would be safe as well, and easy to use, i don't mind about latency.

Answer 1

Guava provides a ConcurrentHashMultiset . This is a concurrent Multiset implementation, which is effectively a set allowing duplicates. You can add(E) and remove(Object) as usual, but also retrieve the count(Object) of an element in the Multiset .

In fact, this is implemented using a backing ConcurrentHashMap<E, AtomicInteger> , so your practice is perfectly fine. However:

• The Multiset interface provides a nicer abstraction, it even extends Collection .
• Properly managing those AtomicIntegers yourself is a tricky business. Quickly glancing over Guava's code should give you an idea of the challenges that concurrent programming imposes: you need lots of compareAndSet s and infinite while loops to make sure the correct results come out.

Answer 2

Yes, I think it is right direction, so I really don't understand why do you think it is not easy. Just use concurrent-map specific functions. Ie to update statistics will be:

AtomicInteger newCounter = new AtomicInteger(1);
AtomicInteger existingCounter = map.putIfAbsent(key, newCounter);
if(existingCounter != null) 
  existingCounter.incrementAndGet();