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
}
该实现位于hotspot/src/share/vm/runtime/synchronizer.cpp文件下。
java使用哈希值判断通过hash算法比对对象是否修改。根据查询相关公开信息显示，使用string.GetHashCode()方法，将用户对象序列化成字符串，用string.GetHashCode()方法，获取字符串的哈希值，当用户点击保存按钮保存数据时即可判断对象是否修改。
