Object.hashCode()有31位的原因吗？

Question

如果在HotSpot Java 7 64位版本上运行以下命令.

int countTopBit = 0, countLowestBit = 0;
for (int i = 0; i < 100000000; i++) {
    int h = new Object().hashCode();
    if (h < 0)
        countTopBit++;
    if ((h & 1) == 1)
        countLowestBit++;
}
System.out.println("The count of negative hashCodes was " + countTopBit + ", the count of odd hashCodes was " + countLowestBit);

你可以得到像这样的结果

The count of negative hashCodes was 0, the count of odd hashCodes was 49994232

我想知道这是否意味着Object.hashCode()它只是真正的31位,为什么会这样呢？

---

不是不使用顶部位的情况.来自HashMap的源代码

257   /**
258    * Applies a supplemental hash function to a given hashCode, which
259    * defends against poor quality hash functions.  This is critical
260    * because HashMap uses power-of-two length hash tables, that
261    * otherwise encounter collisions for hashCodes that do not differ
262    * in lower bits. Note: Null keys always map to hash 0, thus index 0.
263    */
264   static int hash(int h) {
265       // This function ensures that hashCodes that differ only by
266       // constant multiples at each bit position have a bounded
267       // number of collisions (approximately 8 at default load factor).
268       h ^= (h >>> 20) ^ (h >>> 12);
269       return h ^ (h >>> 7) ^ (h >>> 4);
270   }

Answer 1

HotSpot支持各种哈希算法Object.正如您在经验中发现的那样,在返回结果之前总是屏蔽掉最高位:

// src/share/vm/runtime/synchronizer.cpp
static inline intptr_t get_next_hash(Thread * Self, oop obj) {
   ...
   value &= markOopDesc::hash_mask;
   ...
   return value;
}

markOopDesc::hash_mask 计算如下:

  enum { age_bits                 = 4,
         lock_bits                = 2,
         biased_lock_bits         = 1,
         max_hash_bits            = BitsPerWord - age_bits - lock_bits - biased_lock_bits,
         hash_bits                = max_hash_bits > 31 ? 31 : max_hash_bits,
         ...
         hash_mask               = right_n_bits(hash_bits),

如您所见,markOopDesc::hash_mask始终将第31位设置为零.

至于为什么这样做,你的猜测和我的一样好.可能是原始开发人员认为只处理正整数会简化事情.就我们所知,它甚至可能是hash_bits计算中的一个错误.;-)