HashMap synchronization for Map with value-incrementation

Question

I have a questions regarding synchronization of HashMap. The background is that I am trying to implement a simple way of Brute-Force-Detection. I will use a map which has username as key and is used to save the amount of failed login attempts of the user. If a login fails, I want to do something like this:

    Integer failedAmount = myMap.get("username");
    if (failedAmount == null) {
        myMap.put("username", 1);
    } else {
        failedAmount++;
        if (failedAmount >= THRESHOLD) {
            // possible brute force detected! alert admin / slow down login
            // / or whatever
        }
        myMap.put("username", failedAmount);
    }

The mechanism I have in mind at the moment is pretty simple: I would just track this for the whole day and clear() the HashMap at midnight or something like that.

so my question is: what is the best/fastest Map implementation I can use for this? Do I need a fully schronized Map (Collections.sychronizedMap()) or is a ConcurrentHashMap sufficient? Or maybe even just a normal HashMap? I guess it's not that much of a problem if a few increments slipped through?

Answer 1

I would use a combination of ConcurrentHashMap and AtomicInteger http://docs.oracle.com/javase/6/docs/api/java/util/concurrent/atomic/AtomicInteger.html.

Using AtomicInteger will not help you with the comparison, but it will help you with keeping numbers accurate - no need to doing the ++ and the put in two steps.

On the ConcurrentHashMap, I would use the putIfAbsent method, which will eliminate your first if condition.

AtomicInteger failedAmount = new AtomicInteger(0);

failedAmount = myMap.putIfAbsent("username", failedAmount);

if (failedAmount.incrementAndGet() >= THRESHOLD) {
    // possible brute force detected! alert admin / slow down login
    // / or whatever
}