为什么Hashtable ConcurrentHashmap不支持key或者value为null

妖狐艹你老母 2023-10-06 16:38 61阅读 0赞

### 1：ConcurrentHashmap、Hashtable不支持key或者value为null ###

在很多java资料中，都有提到 ConcurrentHashmap HashMap和Hashtable都是key-value存储结构，但他们有一个不同点是 ConcurrentHashmap、Hashtable不支持key或者value为null，而HashMap是支持的。为什么会有这个区别？在设计上的目的是什么？

在网上找到了这样的解答: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.

理解如下：ConcurrentHashmap和Hashtable都是支持并发的，这样会有一个问题，当你通过get(k)获取对应的value时，如果获取到的是null时，你无法判断，它是put（k,v）的时候value为null，还是这个key从来没有做过映射。HashMap是非并发的，可以通过contains(key)来做这个判断。而支持并发的Map在调用m.contains（key）和m.get(key),m可能已经不同了。

个人觉得这个解答还是很有道理的，也是解决了心头的一个疑惑，大牛们在设计时确实考虑的很多，在这里分享给大家。

类似的解答还有这个：  
down vote  
I believe it is, at least in part, to allow you to combine containsKey and get into a single call. If the map can hold nulls, there is no way to tell if get is returning a null because there was no key for that value, or just because the value was null.

Why is that a problem? Because there is no safe way to do that yourself. Take the following code:

if (m.containsKey(k)) {
       return m.get(k);
    } else {
       throw new KeyNotPresentException();
    }

Since m is a concurrent map, key k may be deleted between the containsKey and get calls, causing this snippet to return a null that was never in the table, rather than the desired KeyNotPresentException.

Normally you would solve that by synchronizing, but with a concurrent map that of course won’t work. Hence the signature for get had to change, and the only way to do that in a backwards-compatible way was to prevent the user inserting null values in the first place, and continue using that as a placeholder for “key not found”.

### 2：ConcurrentHashmap、Hashtable存null会发生什么问题 ###

先看下HashTable的源码：

public synchronized V put(K key, V value) {
            // Make sure the value is not null
           if (value == null) {
                throw new NullPointerException();
            }
     
            // Makes sure the key is not already in the hashtable.
            Entry tab[] = table;
           int hash = hash(key);
            int index = (hash & 0x7FFFFFFF) % tab.length;
            for (Entry<K,V> e = tab[index] ; e != null ; e = e.next) {
                if ((e.hash == hash) && e.key.equals(key)) {
                    V old = e.value;
                    e.value = value;
                    return old;
                }
            }
     
            modCount++;
            if (count >= threshold) {
                // Rehash the table if the threshold is exceeded
                rehash();
     
                tab = table;
                hash = hash(key);
                index = (hash & 0x7FFFFFFF) % tab.length;
            }
     
            // Creates the new entry.
            Entry<K,V> e = tab[index];
            tab[index] = new Entry<>(hash, key, value, e);
            count++;
            return null;
        }

源码中判断了null value的情况，null value就抛出空指针异常异常

但是我们并没有看到判断null key然后抛出异常的语句，那么继续看看hash方法的源码就明白了：

private int hash(Object k) {
            // hashSeed will be zero if alternative hashing is disabled.
            return hashSeed ^ k.hashCode();
        }

k.hashCode()，当k出入为null key 就会报错 空指针异常。

ConcurrentHashMap同上