public V computeIfAbsent(K key, Function<? super K,? extends V> mappingFunction)

public V computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction) {if (key == null || mappingFunction == null)throw new NullPointerException();int h = spread(key.hashCode());V val = null;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) & h)) == null) {Node<K,V> r = new ReservationNode<K,V>();synchronized (r) {if (casTabAt(tab, i, null, r)) {binCount = 1;Node<K,V> node = null;try {if ((val = mappingFunction.apply(key)) != null)node = new Node<K,V>(h, key, val, null);} finally {setTabAt(tab, i, node);}}}if (binCount != 0)break;}else if ((fh = f.hash) == MOVED)tab = helpTransfer(tab, f);else {boolean added = false;synchronized (f) {if (tabAt(tab, i) == f) {if (fh >= 0) {binCount = 1;for (Node<K,V> e = f;; ++binCount) {K ek; V ev;if (e.hash == h &&((ek = e.key) == key ||(ek != null && key.equals(ek)))) {val = e.val;break;}Node<K,V> pred = e;if ((e = e.next) == null) {if ((val = mappingFunction.apply(key)) != null) {added = true;pred.next = new Node<K,V>(h, key, val, null);}break;}}}else if (f instanceof TreeBin) {binCount = 2;TreeBin<K,V> t = (TreeBin<K,V>)f;TreeNode<K,V> r, p;if ((r = t.root) != null &&(p = r.findTreeNode(h, key, null)) != null)val = p.val;else if ((val = mappingFunction.apply(key)) != null) {added = true;t.putTreeVal(h, key, val);}}}}if (binCount != 0) {if (binCount >= TREEIFY_THRESHOLD)treeifyBin(tab, i);if (!added)return val;break;}}}if (val != null)addCount(1L, binCount);return val;
}

ConcurrentHashMap<String, String> map = new ConcurrentHashMap<>();
Client esClient = map.get("key");
if (esClient == null) {//防止启动时高并发导致创建多个clientsynchronized (ClientUtil.class) {esClient = map.get("key");if (esClient == null) {//初始化操作esClient = new Client();}}
}

ConcurrentHashMap<String, String> map = new ConcurrentHashMap<>();
map.computeIfAbsent("key",()->{//初始化操作return new Client();
});

public static void main(String[] args) {Map<String, Integer> map = new ConcurrentHashMap<>(16);map.computeIfAbsent("AaAa", key -> {return map.computeIfAbsent("BBBB", key2 -> 42);});System.out.println(map.size());}