Java 8 ConcurrentHashMap

Question

I've observed that ConcurrentHashMap has been entirely rewritten in Java 8 to be more "lock-free". I've browsed the code of the get() method and see that there is no explicit lock mechanism:

public V get(Object key) {
    Node<K,V>[] tab; Node<K,V> e, p; int n, eh; K ek;
    int h = spread(key.hashCode());
    if ((tab = table) != null && (n = tab.length) > 0 &&
        (e = tabAt(tab, (n - 1) & h)) != null) {
        if ((eh = e.hash) == h) {
            if ((ek = e.key) == key || (ek != null && key.equals(ek)))
                return e.val;
        }
        else if (eh < 0)
            return (p = e.find(h, key)) != null ? p.val : null;
        while ((e = e.next) != null) {
            if (e.hash == h &&
                ((ek = e.key) == key || (ek != null && key.equals(ek))))
                return e.val;
        }
    }
    return null;
}

Question:

How it is possible to see from one thread, modifications done to this hashmap from other threads, since the code isn't under a synchronize umbrella (which would enforce a happens-before relation)?

Note: The entire ConcurrentHashMap is a wrapper of a table: transient volatile Node<K,V>[] table;

So table is a volatile reference to an array, not a reference to an array of volatile elements! Which means that if someone is updating an element inside this array, the modification won't be seen in other threads.

Answer 1

Short answer

The Node#val is volatile which establishes your happens before ordering.

Longer answer

synchronized isn't a requirement for thread safety, it's one tool in a toolbox to make a system thread safe. You'll have to consider an entire set of actions on this ConcurrentHashMap to reason about thread safety.

It's useful to know the original ConcurrentHashMap too is non-blocking. Notice pre-Java 8 CHM get

V get(Object key, int hash) {
    if (count != 0) { // read-volatile
        HashEntry<K,V> e = getFirst(hash);
        while (e != null) {
            if (e.hash == hash && key.equals(e.key)) {
                V v = e.value;
                if (v != null)
                    return v;
                return readValueUnderLock(e); // ignore this
            }
            e = e.next;
        }
    }
    return null;
}

In this case, there is no blocking, so how does it work? The HashEntry#value is volatile. That is the synchronization point for thread safety.

The Node class for CHM-8 is the same.

static class Node<K,V> implements Map.Entry<K,V> {
    final int hash;
    final K key;
    volatile V val;
    volatile Node<K,V> next;

So a non-null val in this case should ensure thee happens-before relationship with respect to actions prior to a put.