java本地方法 hashcode是怎样生成的？hashcode与地址有关系吗？

1、java Obeject类中的hashcode函数

/**
返回一个对象的散列码，这个方法有利于哈希表，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类中的方法，当不重写equals 方法时，比较的是 两个对象是否完全相等。

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

我们知道hashcode方法和equals 方法是配套使用的，当我们重写equals 方法时我们需要重写hashcode方法。为什么？因为java中有这样的规范。规范如下

摘自Object规范 （JavaSE6）：

• 在应用程序的执行期间，只要对象的equals方法的比较操作所用到的信息没有被修改，那么对这同一个对象调用多次，hashCode方法都必须始终如一地返回同一个整数。在同一个程序多次执行过程中，每次执行所返回的整数可以不一致；
• 如果两个对象根据equals方法比较是相等的，那么调用者两个对象的hashCode方法必须返回同一个整数；
• 如果两个对象根据equals方法比较是不相等的，那么调用者两个对象的hashCode方法不一定返回不同整数。给不相等的对象产生截然不同的整数结果，有可能提高散列表（hash table）的性能；

 

 

3、tostring方法也与hashcode方法有关，不重写时返回的是hashcode的16进制表达形式

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

 

4、hashcode是本地方法，具体实现是用c++写的，我们可以下载openjdk源码阅读。

下载地址：https://download.csdn.net/download/u014520797/12454866

官网地址：http://openjdk.java.net/

 

5、用c++版本的eclipse来看看 源码

 

 

声明在synchronizer.hpp 实现在这里synchronizer.cpp。

主要函数有两个get_next_hash和FastHashCode，FastHashCode函数里会调用get_next_hash函数，get_next_hash函数中有6种产生hashcode的方式，由于hashCode默认值是5（可在虚拟机启动时配置  -XX:hashCode,  java8以前默认值是0,java8以后是5），所以默认是最后一种产生方式。

static inline intptr_t get_next_hash(Thread * Self, oop obj) {
  intptr_t value = 0 ;
  if (hashCode == 0) {//随机数生成
     // 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) {//这与第5个方式类似，都调用了cast_from_oop函数，只不过此处又增加了位偏移和addrBits ^ (addrBits >> 5) ^ GVars.stwRandom计算
     // 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(obj) >> 3 ;
     value = addrBits ^ (addrBits >> 5) ^ GVars.stwRandom ;
  } else
  if (hashCode == 2) {//灵敏度测试，不知道干嘛的
     value = 1 ;            // for sensitivity testing
  } else
  if (hashCode == 3) {//自增序列
     value = ++GVars.hcSequence ;
  } else
  if (hashCode == 4) {//
     value = cast_from_oop(obj) ;
  } else {//Marsaglia's 异或-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;
}


简单点：
0 - 使用Park-Miller伪随机数生成器（跟地址无关）
1 - 使用地址与一个随机数做异或（地址是输入因素的一部分）
2 - 总是返回常量1作为所有对象的identity hash code（跟地址无关）
3 - 使用全局的递增序列（跟地址无关）
4 - 使用对象地址的“当前”地址来作为它的identity hash code（就是当前地址）
5 - 使用线程局部状态来实现Marsaglia's 异或-位移随机数生成（跟地址无关）

 

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 异或-位移方案是什么？https://www.docin.com/p-1787859407.html

弗罗里达州立大学一位叫做乔治.马尔萨莉亚（George Marsaglia）的数学家发表了一篇使用位移以及亦或运算生成随机数的方法。

7、总结：常规情况下，hashcode与地址无关。

 

 

 

 

解读c++源码请看这里https://www.zhihu.com/question/29976202

参考资料：https://www.docin.com/p-1787859407.html

https://zhuanlan.zhihu.com/p/28270828

https://blog.csdn.net/weixin_30566111/article/details/97126305

http://www.voidcn.com/search/nsepkc

https://zhuanlan.zhihu.com/p/33915892