Question

I have this snippet of code

private Templates retrieveFromCache(String name) {
        TemplatesWrapper t = xlCache.get(name);
        synchronized(t){
            if (!t.isValid()) {
                xlCache.remove(name);
                return null;
            }
        }
        return t.getTemplate();
    }

xlCache is a ConcurrentHashMap; my reason for synchronizing on t is that 2 threads could interleave where by the time Thread 1 verifies the predicate Thread 2 has already removed the object from the map and then a NullPointerException would be thrown. Is my assumption correct as I know concurrency is one of the more difficult things to reason about. And then to my original question, can I lock on t even if it’s local?

And this is private method as well which gets called from a public method, does it make a diff?

EDIT: MY original premise that a NullPointerException is thrown was incorrect as remove() returns boolean making synchronization moot; however, my question was of locking on a local object was answered.

Answer 1

ConcurrentHashMap (and Map/ConcurrentMap in general) won’t throw an exception if the specified key doesn’t exist. That’s why the remove method returns a boolean, to indicate whether or not anything was actually removed.

But yes, you can lock on the local variable. After all, you’re really locking via a reference (and the monitor associated with the referenced object), not a variable – and the other concurrently running method would have the same reference.