MutexWithData.java

public class MutexWithData<A> {

    private final ReentrantLock lock = new ReentrantLock();
    private final A data;

    public MutexWithData(A data) {
        this.data = data;
    }

    public Guard lock() {
        lock.lock();
        return new Guard();
    }

    public class Guard {
        private Guard() {
        }

        public A getData() {
            return data;
        }

        public void unlock() {
            lock.unlock();
        }
    }

    public <T> T withLock(Function<A, T> op) {
        Guard guard = lock();
        try {
            return op.apply(guard.getData());
        } finally {
            guard.unlock();
        }
    }

    public void withLockNr(Consumer<A> op) {
        withLock(a -> {
            op.accept(a);
            return null;
        });
    }
}