java本地方法 hashcode是怎樣生成的？hashcode與地址有關係嗎？

1、java Obeject類中的hashcode函數

/**
返回一個對象的散列碼，這個方法有利於哈希表，例如HashMap
Returns a hash code value for the object. This method is supported for the benefit of hash tables such as those provided by java.util.HashMap. 
*/
    public native int hashCode();

2、hashcode方法和equals 方法都是Obeject類中的方法，當不重寫equals 方法時，比較的是兩個對象是否完全相等。

public boolean equals(Object obj) {
        return (this == obj);
    }

我們知道hashcode方法和equals 方法是配套使用的，當我們重寫equals 方法時我們需要重寫hashcode方法。爲什麼？因爲java中有這樣的規範。規範如下

摘自Object規範（JavaSE6）：

在應用程序的執行期間，只要對象的equals方法的比較操作所用到的信息沒有被修改，那麼對這同一個對象調用多次，hashCode方法都必須始終如一地返回同一個整數。在同一個程序多次執行過程中，每次執行所返回的整數可以不一致；
如果兩個對象根據equals方法比較是相等的，那麼調用者兩個對象的hashCode方法必須返回同一個整數；
如果兩個對象根據equals方法比較是不相等的，那麼調用者兩個對象的hashCode方法不一定返回不同整數。給不相等的對象產生截然不同的整數結果，有可能提高散列表（hash table）的性能；

3、tostring方法也與hashcode方法有關，不重寫時返回的是hashcode的16進製表達形式

public String toString() {
        return getClass().getName() + "@" + Integer.toHexString(hashCode());
    }

4、hashcode是本地方法，具體實現是用c++寫的，我們可以下載openjdk源碼閱讀。

下載地址：https://download.csdn.net/download/u014520797/12454866

官網地址：http://openjdk.java.net/

5、用c++版本的eclipse來看看源碼

聲明在synchronizer.hpp 實現在這裏synchronizer.cpp。

主要函數有兩個get_next_hash和FastHashCode，FastHashCode函數裏會調用get_next_hash函數，get_next_hash函數中有6種產生hashcode的方式，由於hashCode默認值是5（可在虛擬機啓動時配置 -XX:hashCode, java8以前默認值是0,java8以後是5），所以默認是最後一種產生方式。


static inline intptr_t get_next_hash(Thread * Self, oop obj) {
  intptr_t value = 0 ;
  if (hashCode == 0) {//隨機數生成
     // 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) {//這與第5個方式類似，都調用了cast_from_oop函數，只不過此處又增加了位偏移和addrBits ^ (addrBits >> 5) ^ GVars.stwRandom計算
     // 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 = cast_from_oop<intptr_t>(obj) >> 3 ;
     value = addrBits ^ (addrBits >> 5) ^ GVars.stwRandom ;
  } else
  if (hashCode == 2) {//靈敏度測試，不知道幹嘛的
     value = 1 ;            // for sensitivity testing
  } else
  if (hashCode == 3) {//自增序列
     value = ++GVars.hcSequence ;
  } else
  if (hashCode == 4) {//
     value = cast_from_oop<intptr_t>(obj) ;
  } else {//Marsaglia's 異或-位移方案
     // 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;
}


簡單點：
0 - 使用Park-Miller僞隨機數生成器（跟地址無關）
1 - 使用地址與一個隨機數做異或（地址是輸入因素的一部分）
2 - 總是返回常量1作爲所有對象的identity hash code（跟地址無關）
3 - 使用全局的遞增序列（跟地址無關）
4 - 使用對象地址的“當前”地址來作爲它的identity hash code（就是當前地址）
5 - 使用線程局部狀態來實現Marsaglia's 異或-位移隨機數生成（跟地址無關）

intptr_t ObjectSynchronizer::FastHashCode (Thread * Self, oop obj) {
  if (UseBiasedLocking) {
    // NOTE: many places throughout the JVM do not expect a safepoint
    // to be taken here, in particular most operations on perm gen
    // objects. However, we only ever bias Java instances and all of
    // the call sites of identity_hash that might revoke biases have
    // been checked to make sure they can handle a safepoint. The
    // added check of the bias pattern is to avoid useless calls to
    // thread-local storage.
    if (obj->mark()->has_bias_pattern()) {
      // Box and unbox the raw reference just in case we cause a STW safepoint.
      Handle hobj (Self, obj) ;
      // Relaxing assertion for bug 6320749.
      assert (Universe::verify_in_progress() ||
              !SafepointSynchronize::is_at_safepoint(),
             "biases should not be seen by VM thread here");
      BiasedLocking::revoke_and_rebias(hobj, false, JavaThread::current());
      obj = hobj() ;
      assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
    }
  }

  // hashCode() is a heap mutator ...
  // Relaxing assertion for bug 6320749.
  assert (Universe::verify_in_progress() ||
          !SafepointSynchronize::is_at_safepoint(), "invariant") ;
  assert (Universe::verify_in_progress() ||
          Self->is_Java_thread() , "invariant") ;
  assert (Universe::verify_in_progress() ||
         ((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant") ;

  ObjectMonitor* monitor = NULL;
  markOop temp, test;
  intptr_t hash;
  markOop mark = ReadStableMark (obj);

  // object should remain ineligible for biased locking
  assert (!mark->has_bias_pattern(), "invariant") ;

  if (mark->is_neutral()) {
    hash = mark->hash();              // this is a normal header
    if (hash) {                       // if it has hash, just return it
      return hash;
    }
    hash = get_next_hash(Self, obj);  // allocate a new hash code
    temp = mark->copy_set_hash(hash); // merge the hash code into header
    // 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;
    }
    // If atomic operation failed, we must inflate the header
    // into heavy weight monitor. We could add more code here
    // for fast path, but it does not worth the complexity.
  } else if (mark->has_monitor()) {
    monitor = mark->monitor();
    temp = monitor->header();
    assert (temp->is_neutral(), "invariant") ;
    hash = temp->hash();
    if (hash) {
      return hash;
    }
    // Skip to the following code to reduce code size
  } else if (Self->is_lock_owned((address)mark->locker())) {
    temp = mark->displaced_mark_helper(); // this is a lightweight monitor owned
    assert (temp->is_neutral(), "invariant") ;
    hash = temp->hash();              // by current thread, check if the displaced
    if (hash) {                       // header contains hash code
      return hash;
    }
    // WARNING:
    //   The displaced header is strictly immutable.
    // It can NOT be changed in ANY cases. So we have
    // to inflate the header into heavyweight monitor
    // even the current thread owns the lock. The reason
    // is the BasicLock (stack slot) will be asynchronously
    // read by other threads during the inflate() function.
    // Any change to stack may not propagate to other threads
    // correctly.
  }

  // Inflate the monitor to set hash code
  monitor = ObjectSynchronizer::inflate(Self, obj);
  // Load displaced header and check it has hash code
  mark = monitor->header();
  assert (mark->is_neutral(), "invariant") ;
  hash = mark->hash();
  if (hash == 0) {
    hash = get_next_hash(Self, obj);
    temp = mark->copy_set_hash(hash); // merge hash code into header
    assert (temp->is_neutral(), "invariant") ;
    test = (markOop) Atomic::cmpxchg_ptr(temp, monitor, mark);
    if (test != mark) {
      // The only update to the header in the monitor (outside GC)
      // is install the hash code. If someone add new usage of
      // displaced header, please update this code
      hash = test->hash();
      assert (test->is_neutral(), "invariant") ;
      assert (hash != 0, "Trivial unexpected object/monitor header usage.");
    }
  }
  // We finally get the hash
  return hash;
}

6、Marsaglia's 異或-位移方案是什麼？https://www.docin.com/p-1787859407.html

弗羅裏達州立大學一位叫做喬治.馬爾薩莉亞（George Marsaglia）的數學家發表了一篇使用位移以及亦或運算生成隨機數的方法。