Per-key blocking Map in Java

Question

I\'m dealing with some third-party library code that involves creating expensive objects and caching them in a Map. The existing implementation is something like

Answer 1

public class Cache {

    private static final Set lockedKeys = new HashSet<>();

    private void lock(String key) {
        synchronized (lockedKeys) {
            while (!lockedKeys.add(key)) {
                try {
                    lockedKeys.wait();
                } catch (InterruptedException e) {
                    log.error("...");
                    throw new RuntimeException(e);
                }
            }
        }
    }

    private void unlock(String key) {
        synchronized (lockedKeys) {
            lockedKeys.remove(key);
            lockedKeys.notifyAll();
        }
    }

    public Foo getFromCache(String key) {
        try {
            lock(key);

            Foo result = cache.get(key);
            if (result == null) {
                result = createFooExpensively(key);
                cache.put(key, result);
            }
            return result;
            //For different keys it is executed in parallel.
            //For the same key it is executed synchronously.

        } finally {
            unlock(key);
        }
    }

}

• key can be not only a 'String' but any class with correctly overridden 'equals' and 'hashCode' methods.
• try-finally - is very important - you must guarantee to unlock waiting threads after your operation even if your operation threw exception.
• It will not work if your back-end is distributed across multiple servers/JVMs.