ConcurrentHashMap vs Hashtable

1.ConcurrentHashMap

1.7 ReentrantLock + Segment + HashEntry。

1.8 CAS + synchronized + HashEntry + 红黑树。

    public V put(K key, V value) {return putVal(key, value, false);}final V putVal(K key, V value, boolean onlyIfAbsent) {if (key == null || value == null) throw new NullPointerException();int hash = spread(key.hashCode());int binCount = 0;for (Node<K,V>[] tab = table;;) {Node<K,V> f; int n, i, fh;if (tab == null || (n = tab.length) == 0)tab = initTable();else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {if (casTabAt(tab, i, null,new Node<K,V>(hash, key, value, null)))break;                   // no lock when adding to empty bin}else if ((fh = f.hash) == MOVED)tab = helpTransfer(tab, f);else {V oldVal = null;synchronized (f) {if (tabAt(tab, i) == f) {if (fh >= 0) {binCount = 1;for (Node<K,V> e = f;; ++binCount) {K ek;if (e.hash == hash &&((ek = e.key) == key ||(ek != null && key.equals(ek)))) {oldVal = e.val;if (!onlyIfAbsent)e.val = value;break;}Node<K,V> pred = e;if ((e = e.next) == null) {pred.next = new Node<K,V>(hash, key,value, null);break;}}}else if (f instanceof TreeBin) {Node<K,V> p;binCount = 2;if ((p = ((TreeBin<K,V>)f).putTreeVal(hash, key,value)) != null) {oldVal = p.val;if (!onlyIfAbsent)p.val = value;}}}}if (binCount != 0) {if (binCount >= TREEIFY_THRESHOLD)treeifyBin(tab, i);if (oldVal != null)return oldVal;break;}}}addCount(1L, binCount);return null;}

2.Hashtable

直接使用 synchronized。

    public synchronized V put(K key, V value) {// Make sure the value is not nullif (value == null) {throw new NullPointerException();}// Makes sure the key is not already in the hashtable.Entry<?,?> tab[] = table;int hash = key.hashCode();int index = (hash & 0x7FFFFFFF) % tab.length;@SuppressWarnings("unchecked")Entry<K,V> entry = (Entry<K,V>)tab[index];for(; entry != null ; entry = entry.next) {if ((entry.hash == hash) && entry.key.equals(key)) {V old = entry.value;entry.value = value;return old;}}addEntry(hash, key, value, index);return null;}