//ConcurrentHashMap源碼：
/** Implementation for put and putIfAbsent */ final V putVal(K key, V value, boolean onlyIfAbsent) { if (key == null || value == null) throw new NullPointerException(); //......
關於為什么這么設計（stackoverflow上的回答）：
The main reason that nulls aren't allowed in ConcurrentMaps (ConcurrentHashMaps, ConcurrentSkipListMaps) is that ambiguities that may be just barely tolerable in non-concurrent maps can't be accommodated. The main one is that if map.get(key) returns null, you can't detect whether the key explicitly maps to null vs the key isn't mapped. In a non-concurrent map, you can check this via map.contains(key), but in a concurrent one, the map might have changed between calls.
也就是說對於能夠支持並發修改的ConcurrentMaps，如果取值的時候返回null，你很難判斷是本來那個值就是被映射為的null，還是根本就沒設置這個key-value對。