这显然是特定于实现的。
下面，我包括
Object.hashCode()OpenJDK 7中使用的实现。
该函数支持六种不同的计算方法，其中只有两种可以注意到对象的地址（“地址”是C ++
oop强制转换为
intptr_t）。两种方法之一按原样使用地址，而另一种方法则进行一些打乱，然后使用不经常更新的随机数将结果混搭。
在其余方法中，一个返回一个常量（大概是为了测试），一个返回序号，其余的则基于伪随机序列。
似乎可以在运行时选择该方法，默认似乎是方法0，即
os::random()。后者是线性同余生成器，带有所谓的竞争条件。:-)竞争条件是可以接受的，因为在最坏的情况下，它将导致两个对象共享相同的哈希码。这不会破坏任何不变性。
第一次需要哈希码时执行计算。为了保持一致性，结果将存储在对象的标头中，并在随后的调用中返回
hashCode()。缓存是在此功能之外完成的。
总而言之，Object.hashCode()
基于对象地址的概念在很大程度上是一个历史性的手工艺品，已被现代垃圾收集器的特性所淘汰。
// hotspot/src/share/vm/runtime/synchronizer.hpp// hashCode() generation ://// Possibilities:// * MD5Digest of {obj,stwRandom}// * CRC32 of {obj,stwRandom} or any linear-feedback shift register function.// * A DES- or AES-style SBox[] mechanism// * One of the Phi-based schemes, such as:// 2654435761 = 2^32 * Phi (golden ratio)// HashCodevalue = ((uintptr_t(obj) >> 3) * 2654435761) ^ GVars.stwRandom ;// * A variation of Marsaglia's shift-xor RNG scheme.// * (obj ^ stwRandom) is appealing, but can result// in undesirable regularity in the hashCode values of adjacent objects// (objects allocated back-to-back, in particular). This could potentially// result in hashtable collisions and reduced hashtable efficiency.// There are simple ways to "diffuse" the middle address bits over the// generated hashCode values://static inline intptr_t get_next_hash(Thread * Self, oop obj) { intptr_t value = 0 ; if (hashCode == 0) { // This form uses an unguarded global Park-Miller RNG, // so it's possible for two threads to race and generate the same RNG. // On MP system we'll have lots of RW access to a global, so the // mechanism induces lots of coherency traffic. value = os::random() ; } else if (hashCode == 1) { // This variation has the property of being stable (idempotent) // between STW operations. This can be useful in some of the 1-0 // synchronization schemes. intptr_t addrBits = intptr_t(obj) >> 3 ; value = addrBits ^ (addrBits >> 5) ^ GVars.stwRandom ; } else if (hashCode == 2) { value = 1 ; // for sensitivity testing } else if (hashCode == 3) { value = ++GVars.hcSequence ; } else if (hashCode == 4) { value = intptr_t(obj) ; } else { // Marsaglia's xor-shift scheme with thread-specific state // This is probably the best overall implementation -- we'll // likely make this the default in future releases. unsigned t = Self->_hashStateX ; t ^= (t << 11) ; Self->_hashStateX = Self->_hashStateY ; Self->_hashStateY = Self->_hashStateZ ; Self->_hashStateZ = Self->_hashStateW ; unsigned v = Self->_hashStateW ; v = (v ^ (v >> 19)) ^ (t ^ (t >> 8)) ; Self->_hashStateW = v ; value = v ; } value &= markOopDesc::hash_mask; if (value == 0) value = 0xBAD ; assert (value != markOopDesc::no_hash, "invariant") ; TEVENT (hashCode: GENERATE) ; return value;}
