面试必问的HashCode技术内幕( 三 ) _HashCode

问：
为什么要先撤销偏向锁到无锁状态，再来生成hashcode呢？这跟锁有什么关系？
答：
mark word里， hashcode存储的字节位置被偏向锁给占了！偏向锁存储了锁持有者的线程id
（参考上面的markword图）
扩展：关于hashCode的生成算法（了解）
// hashCode() generation :// 涉及到c++算法领域，感兴趣的同学自行研究// 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 = https://www.isolves.com/it/qt/2022-08-03/((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() ;//返回随机数} 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.//和地址相关，但不是地址；右移+异或算法intptr_t addrBits = cast_from_oop(obj) >> 3 ;value = addrBits ^ (addrBits >> 5) ^ GVars.stwRandom ;//随机数位移异或计算} elseif (hashCode == 2) {value = 1 ;// 返回1} elseif (hashCode == 3) {value = ++GVars.hcSequence ;//返回一个Sequence序列号} elseif (hashCode == 4) {value = cast_from_oop(obj) ;//也不是地址} else {//常用// 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.//马萨利亚教授写的xor-shift 随机数算法（异或随机算法)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 ;}5）总结通过分析虚拟机源码我们证明了hashCode不是直接用的内存地址，而是采取一定的算法来生成
hashcode值的存储在mark word里，与锁共用一段bit位，这就造成了跟锁状态相关性