Synchronization in a HashMap cache

Question

I\'ve got a web application where people ask for resources. This resources are cached using a synchronized hash map for efficiency. The problem here is when two different re

Answer 1

One possible problem is that you create unnecessary contention by executing veryCostlyOperation() inside a synchronized block, so that many threads cannot retrieve their (independent) resources at the same time. This can be solved by using Future as values of the map:

Map> map = new ConcurrentHashMap>();    
...
Future r = map.get(name);
if (r == null) {
    FutureTask task = null;
    synchronized (lock) {
        r = map.get(name);
        if (r == null) {
            task = new FutureTask(new Callable() {
                public Resource call() {
                    return veryCostlyOperation(name);
                }
            });
            r = task;
            map.put(name, r);
        }
    }
    if (task != null) task.run(); // Retrieve the resource
}

return r.get(); // Wait while other thread is retrieving the resource if necessary