public native int hashCode();

// 文件路徑: jdk\src\share\native\java\lang\Object.c
static JNINativeMethod methods[] = {
    {"hashCode",    "()I",                    (void *)&JVM_IHashCode}, // 這個(gè)方法就是我們想看的hashCode方法
    {"wait",        "(J)V",                   (void *)&JVM_MonitorWait},
    {"notify",      "()V",                    (void *)&JVM_MonitorNotify},
    {"notifyAll",   "()V",                    (void *)&JVM_MonitorNotifyAll},
    {"clone",       "()Ljava/lang/Object;",   (void *)&JVM_Clone},
};

// hotspot\src\share\vm\prims\jvm.h
/*
 * java.lang.Object
 */
JNIEXPORT jint JNICALL
JVM_IHashCode(JNIEnv *env, jobject obj); // 此時(shí)定了已hashCode方法的接口, 具體實(shí)現(xiàn)在jvm.cpp中

// hotspot\src\share\vm\prims\jvm.cpp
// java.lang.Object ///
JVM_ENTRY(jint, JVM_IHashCode(JNIEnv* env, jobject handle))
  JVMWrapper("JVM_IHashCode");
  // as implemented in the classic virtual machine; return 0 if object is NULL
  return handle == NULL ? 0 : ObjectSynchronizer::FastHashCode (THREAD, JNIHandles::resolve_non_null(handle)) ; // 如果object為null, 就返回0; 否則就調(diào)用ObjectSynchronizer::FastHashCode
JVM_END

// hotspot\src\share\vm\runtime\synchronizer.cpp
intptr_t ObjectSynchronizer::FastHashCode (Thread * Self, oop obj) {
// ....
 // 在FastHashCode方法中有一段關(guān)鍵代碼:
 if (mark->is_neutral()) {
     hash = mark->hash();              // 首先通過(guò)對(duì)象的markword中取出hashCode
     if (hash) {                       // 如果取調(diào)到了, 就直接返回
       return hash;
     }
     hash = get_next_hash(Self, obj);  // 如果markword中沒(méi)有設(shè)置hashCode, 則調(diào)用get_next_hash生成hashCode
     temp = mark->copy_set_hash(hash); // 生成的hashCode設(shè)置到markword中
     // use (machine word version) atomic operation to install the hash
     test = (markOop) Atomic::cmpxchg_ptr(temp, obj->mark_addr(), mark);
     if (test == mark) {
       return hash;
     }
 }
// ....
}

// hotspot\src\share\vm\runtime\synchronizer.cpp
static inline intptr_t get_next_hash(Thread * Self, oop obj) {
  intptr_t value = 0 ;
  // 一共支持6中生成hashCode策略, 默認(rèn)策略值是5
  if (hashCode == 0) {
  // 策略1: 直接通過(guò)隨機(jī)數(shù)生成
     value = os::random() ;
  } else if (hashCode == 1) {
     // 策略2: 通過(guò)object地址和隨機(jī)數(shù)運(yùn)算生成
     intptr_t addrBits = cast_from_oop<intptr_t>(obj) >> 3 ;
     value = addrBits ^ (addrBits >> 5) ^ GVars.stwRandom ;
  } else if (hashCode == 2) {
  // 策略3: 永遠(yuǎn)返回1, 用于測(cè)試
     value = 1 ;            // for sensitivity testing
  } else if (hashCode == 3) {
  // 策略4: 返回一個(gè)全局遞增的序列數(shù)
     value = ++GVars.hcSequence ;
  } else if (hashCode == 4) {
  // 策略5: 直接采用object的地址值
     value = cast_from_oop<intptr_t>(obj) ;
  } else {
     // 策略6: 通過(guò)在每個(gè)線程中的四個(gè)變量: _hashStateX, _hashStateY, _hashStateZ, _hashStateW
     // 組合運(yùn)算出hashCode值, 根據(jù)計(jì)算結(jié)果同步修改這個(gè)四個(gè)值
     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; // 通過(guò)hashCode的mask獲得最終的hashCode值
  if (value == 0) value = 0xBAD ;
  assert (value != markOopDesc::no_hash, "invariant") ;
  TEVENT (hashCode: GENERATE) ;
  return value;
}

// hotspot\src\share\vm\oops\markOop.hpp
class markOopDesc: public oopDesc {
 private:
  // Conversion
  uintptr_t value() const { return (uintptr_t) this; }
 public:
  // Constants
  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, // 通過(guò)這個(gè)定義可知, hashcode可占用31位bit. 在32位jvm中,  hashCode占用25位
         cms_bits                 = LP64_ONLY(1) NOT_LP64(0),
         epoch_bits               = 2
  };
  
}

package test;
/***
 * 可以通過(guò)系列參數(shù)指定hashCode生成策略
 * -XX:hashCode=2
 */
public class TestHashCode {
    public static void main(String[] args) {
        Object obj1 = new Object();
        Object obj2 = new Object();
        System.out.println(obj1.hashCode());
        System.out.println(obj2.hashCode());
    }
}

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