Atomically incrementing counters stored in ConcurrentHashMap

Question

I would like to collect some metrics from various places in a web app. To keep it simple, all these will be counters and therefore the only modifier operation is to incremen

Answer 1

In Java 8:

ConcurrentHashMap<String, LongAdder> map = new ConcurrentHashMap<>();

map.computeIfAbsent("key", k -> new LongAdder()).increment();

Answer 2

Guava's new AtomicLongMap (in release 11) might address this need.

Answer 3

I did a benchmark to compare the performance of LongAdder and AtomicLong.

LongAdder had a better performance in my benchmark: for 500 iterations using a map with size 100 (10 concurrent threads), the average time for LongAdder was 1270ms while that for AtomicLong was 1315ms.

Answer 4

Other than going with AtomicLong, you can do the usual cas-loop thing:

private final ConcurrentMap<Key,Long> counts =
    new ConcurrentHashMap<Key,Long>();

public void increment(Key key) {
    if (counts.putIfAbsent(key, 1)) == null) {
        return;
    }

    Long old;
    do {
       old = counts.get(key);
    } while (!counts.replace(key, old, old+1)); // Assumes no removal.
}

(I've not written a do-while loop for ages.)

For small values the Long will probably be "cached". For longer values, it may require allocation. But the allocations are actually extremely fast (and you can cache further) - depends upon what you expect, in the worst case.

Answer 5

Got a necessity to do the same. I'm using ConcurrentHashMap + AtomicInteger. Also, ReentrantRW Lock was introduced for atomic flush(very similar behavior).

Tested with 10 Keys and 10 Threads per each Key. Nothing was lost. I just haven't tried several flushing threads yet, but hope it will work.

Massive singleusermode flush is torturing me... I want to remove RWLock and break down flushing into small pieces. Tomorrow.

private ConcurrentHashMap<String,AtomicInteger> counters = new ConcurrentHashMap<String, AtomicInteger>();
private ReadWriteLock rwLock = new ReentrantReadWriteLock();

public void count(String invoker) {

    rwLock.readLock().lock();

    try{
        AtomicInteger currentValue = counters.get(invoker);
        // if entry is absent - initialize it. If other thread has added value before - we will yield and not replace existing value
        if(currentValue == null){
            // value we want to init with
            AtomicInteger newValue = new AtomicInteger(0);
            // try to put and get old
            AtomicInteger oldValue = counters.putIfAbsent(invoker, newValue);
            // if old value not null - our insertion failed, lets use old value as it's in the map
            // if old value is null - our value was inserted - lets use it
            currentValue = oldValue != null ? oldValue : newValue;
        }

        // counter +1
        currentValue.incrementAndGet();
    }finally {
        rwLock.readLock().unlock();
    }

}

/**
 * @return Map with counting results
 */
public Map<String, Integer> getCount() {
    // stop all updates (readlocks)
    rwLock.writeLock().lock();
    try{
        HashMap<String, Integer> resultMap = new HashMap<String, Integer>();
        // read all Integers to a new map
        for(Map.Entry<String,AtomicInteger> entry: counters.entrySet()){
            resultMap.put(entry.getKey(), entry.getValue().intValue());
        }
        // reset ConcurrentMap
        counters.clear();
        return resultMap;

    }finally {
        rwLock.writeLock().unlock();
    }

}

Answer 6

You're pretty close. Why don't you try something like a ConcurrentHashMap<Key, AtomicLong>? If your Keys (metrics) are unchanging, you could even just use a standard HashMap (they are threadsafe if readonly, but you'd be well advised to make this explicit with an ImmutableMap from Google Collections or Collections.unmodifiableMap, etc.).

This way, you can use map.get(myKey).incrementAndGet() to bump statistics.