Understanding code of ConcurrentHashMap compute method

by 
casTabAt(tab, i, null, r)

is publishing the reference to r.

static final <K,V> boolean casTabAt(Node<K,V>[] tab, int i,
                                    Node<K,V> c, Node<K,V> v) {
    return U.compareAndSwapObject(tab, ((long)i << ASHIFT) + ABASE, c, v);
}

Because c is being put into tab, it is possible that it is accessed by another thread, e.g. in putVal. As such, this synchronized block is necessary to exclude other threads from doing other synchronized things with that Node.

Leave a Comment