Using lock-free algorithm to put values into a Map of custom objects

Question

I have a Map. In order to update the key, I need to check if it already exists. Else, I need to create a new Object and put it.

Map<K,Foo> map = new ConcurrentHashMap<K, Foo>();

My function is this

put(Object value)  {
   if(!map.containsKey(K key)) {
       map.put(key, new Foo());
   } else {
       Foo modifiedFoo = modifyFoo(map.get(key));
       map.put(key, modifiedFoo));
   }
}

I do not want to use synchronization. I am guessing it can probably be transformed into Map<K,AtomicReference<Foo>> map = new ConcurrentHashMap<K, AtomicReference<Foo>>() and furthermore do value.compareAndSet(old, new) where value is type AtomicReference<Foo>. However, two things. How does the case of a non-existing key be handled? And how is the comparison done between the old and new Foo objects?

[`putIfAbsent`](http://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ConcurrentHashMap.html#putIfAbsent-K-V-) — Sotirios Delimanolis, Feb 17 '15 at 17:03

score 1 · Accepted Answer · answered Feb 17 '15 at 17:08

A common pattern in lock-free algorithms is to use an optimistic algorithm wrapped in a retry loop.

insertOrUpdate(key) {
    while(true) {
        var foo = map.get(key);
        if (foo == null) {
            if (map.putIfAbsent(key, new Foo())) break;
        } else {
            if (map.replace(key, foo, modifyFoo(foo))) break;
        }
    }
}

Note that this algorithm doesn't protect against the A-B-A problem, which can in principle be relevant if you allow deletions, depending on your desired semantics in that case.

Yes. replace does the trick! Regarding the ABA problem, isn't it taken care of by the ConcurrentHashMap? — user592748, Feb 17 '15 at 17:18

Using lock-free algorithm to put values into a Map of custom objects

1 Answers1