I'm looking for code equivalent to the following:

ConcurrentHashMap<int, Object> map = new ConcurrentHashMap<>();
map.computeIfAbsent(key, n -> f(n));

Where f(n) is HTTP network call and blocking for the result

Bur referring to single element held in AtomicReference<Object> where I need to ensure f is called only once upon even if multiple threads do the access concurrently. I tried using compareAndSet but this doesn't allow lambda passing. Does updateAndGet achieve that? Its documentation mentions

The function should be side-effect-free, since it may be re-applied when attempted updates fail due to contention among threads.

Which doesn't seem to fill the need of invoking f only once.

CodePudding user response：

From your question, I think you want to avoid doing the HTTP request multiple times.

You could have a map of FutureTask(s) that asynchronously performs the HTTP request for you. In this way, if a thread tries to computeIfAbsent it will see the FutureTask created by another thread even if the HTTP operation is not done yet.

CodePudding user response：

You could use an AtomicBoolean with an initial value of true and allow each thread should call AtomicBoolean::getAndSet with the value false. If the return value is true then you execute your function.

This will ensure that the call is only made once since only the first thread will succeed.