hashCode

根据一定的规则将与对象相关的信息（比如对象的存储地址，对象的字段等）映射成一个数值，这个数值称作为散列值。

    public static void main(String[] args) {Object o = new Object();System.out.println(o);int hashCode = o.hashCode();//hashCode 16进制System.out.println(Integer.toHexString(hashCode));System.out.println(hashCode);//获取hashCodeSystem.out.println(System.identityHashCode(o));}

@后面的为16进制的hashCode

hashCode 的生成逻辑

openjdk 源码里生成 hashCode 的核心方法

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;  
}  

从源码里可以发现，生成策略是由一个 hashCode 的全局变量控制的，默认为5；而这个变量的定义在另一个头文件里：

product(intx, hashCode, 5,                                              "(Unstable) select hashCode generation algorithm" )   

第 0 种算法

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();  }  

这种生成算法，使用的一种Park-Miller RNG的随机数生成策略。不过需要注意的是……这个随机算法在高并发的时候会出现自旋等待

第 1 种算法

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 ;  
}  

这个算法，真的是对象的内存地址了，直接获取对象的 intptr_t 类型指针只不过封装了一下数字。

第 2 种算法

if (hashCode == 2) {  value = 1 ;            // for sensitivity testing  
}  

value = 1

第 3 种算法

if (hashCode == 3) {  value = ++GVars.hcSequence ;  
}  

递增

第 4 种算法

{value = intptr_t(obj) ;  
}

真实的指针地址

第 5 种算法

     // 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 ;  }