private boolean containsNullValue() {
        Entry[] tab = table;
        for (int i = 0; i < tab.length; i++)
            for (Entry e = tab[i]; e != null; e = e.next)
                if (e.value == null)
                    return true;
        return false;
    }
Special-case code for containsValue with null argument
src/main/java/com/alibaba/fastjson/util/AntiCollisionHashMap.java