System.identityHashCode(obj) 与 obj.hashcode()

简介

最近在看dubbo源码时,经常看到System.identityHashCode(obj) 的使用,想了解一下这个跟我们平常的hashcode方法又有啥异同,所以本篇简单的探讨一下。

概念

1、hashCode是 java.lang.Object.hashCode() 或者 java.lang.System.identityHashCode(obj) 会返回的值。他是一个对象的身份标识。官方称呼为:标识哈希码( identity hash code)。

2、哪些特点?
(1)一个对象在其生命期中 identity hash code 必定保持不变;
(2)如果a == b,那么他们的System.identityHashCode() 必须相等;
如果他们的System.identityHashCode() 不相等,那他们必定不是同一个对象(逆否命题与原命题真实性总是相同);
(3)如果System.identityHashCode() 相等的话,并不能保证 a == b(毕竟这只是一个散列值,是允许冲突的)。
3、有什么作用?
加速对象去重:由特征2可知,只要判断出两个对象的hashCode不一致,就知道这两个对象不是同一个;又因为hashCode()的性能比 " == "性能高得多,所以多数时候,用它来判断重复。

扩展:为啥hashCode()性能高?
因为hashCode()的结果算出来后缓存起来,下次调用直接用不需要重新计算,提高性能

identityHashCode

看官方提供的API , 对System.identityHashCode()的解释为 :
public static int identityHashCode([Object] x)

返回给定对象的哈希码,该代码与默认的方法 hashCode() 返回的代码一样,无论给定对象的类是否重写 hashCode()。null 引用的哈希码为 0。

obj.hashcode()

hashCode()方法是顶级类Object类的提供的一个方法,所有的类都可以进行对hashCode方法重写。这时hash值计算根据重写后的hashCode方法计算

异同

从上面的概念可以看出identityHashCode是根据Object类hashCode()方法来计算hash值,无论子类是否重写了hashCode()方法。而obj.hashcode()是根据obj的hashcode()方法计算hash值

验证

    @Test
    public void testHashCode() {
        TestExample example = new TestExample();
        int exampleCode = System.identityHashCode(example);
        int exampleCode2 = System.identityHashCode(example);
        int exampleHashcode = example.hashCode();
        System.out.println("example identityHashCode:" + exampleCode);
        System.out.println("example2 identityHashCode:" + exampleCode2);
        System.out.println("example Hashcode:" + exampleHashcode);
        String str = "dd";
        String str2 = "dd";
        int strCode = System.identityHashCode(str);
        int strHashCode = str.hashCode();
        int str2HashCode = str.hashCode();
        System.out.println("str identityHashCode:" + strCode);
        System.out.println("str hashcode:" + strHashCode);
        System.out.println("str2 hashcode:" + str2HashCode);
    }
输出:
example identityHashCode:1144748369
example2 identityHashCode:1144748369
example Hashcode:1144748369
str identityHashCode:340870931
str hashcode:3200
str2 hashcode:3200

结果分析:
(1)从上面样例可以看出,TestExample 对象没重写HashCode()方法,所以两个都是调用Object的HashCode()方法,自然结果一样。
而String重写了HashCode()方法,identityHashCode和HashCode结果自然不一样。
(2)str和str2的hashCode是相同的,是因为String类重写了hashCode方法,它根据String的值来确定hashCode的值,所以只要值一样,hashCode就会一样。

hashCode如何计算的?

JDK8 hashCode( )源码:
进入openjdk\jdk\src\share\classes\java\lang目录下,可以看到 Object.java源码

public native int hashCode();

即该方法是一个本地方法,Java将调用本地方法库对此方法的实现。由于Object类中有JNI方法调用,按照JNI的规则,应当生成JNI 的头文件,在此目录下执行javah -jni java.lang.Object 指令,将生成一个java_lang_Object.h头文件,

java_lang_Object.h头文件关于hashcode方法的信息如下所示:

/*
 * Class:     java_lang_Object
 * Method:    hashCode
 * Signature: ()I
 */
JNIEXPORT jint JNICALL Java_java_lang_Object_hashCode
  (JNIEnv *, jobject);
1. Object对象的hashCode()方法在C语言文件Object.c中实现

打开openjdk\jdk\src\share\native\java\lang目录,查看Object.c文件,可以看到hashCode()的方法被注册成有JVM_IHashCode方法指针来处理:

/*-
 *      Implementation of class Object
 *
 *      former threadruntime.c, Sun Sep 22 12:09:39 1991
 */

#include <stdio.h>
#include <signal.h>
#include <limits.h>

#include "jni.h"
#include "jni_util.h"
#include "jvm.h"
//使用了上面生成的java_lang_Object.h文件
#include "java_lang_Object.h"

static JNINativeMethod methods[] = {
    {"hashCode",    "()I",                    (void *)&JVM_IHashCode},//hashcode的方法指针JVM_IHashCode
    {"wait",        "(J)V",                   (void *)&JVM_MonitorWait},
    {"notify",      "()V",                    (void *)&JVM_MonitorNotify},
    {"notifyAll",   "()V",                    (void *)&JVM_MonitorNotifyAll},
    {"clone",       "()Ljava/lang/Object;",   (void *)&JVM_Clone},
};
2.JVM_IHashCode方法指针

在 openjdk\hotspot\src\share\vm\prims\jvm.cpp中定义,如下:

// 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)) ;
JVM_END

如上可以看出,JVM_IHashCode方法中调用了ObjectSynchronizer::FastHashCode方法

3. ObjectSynchronizer::fashHashCode方法的实现:

ObjectSynchronizer::fashHashCode()方法在hotspot\src\share\vm\runtime\synchronizer.cpp

// hashCode() generation :
//
// Possibilities:
// * MD5Digest of {obj,stwRandom}
// * CRC32 of {obj,stwRandom} or any linear-feedback shift register function.
// * A DES- or AES-style SBox[] mechanism
// * One of the Phi-based schemes, such as:
//   2654435761 = 2^32 * Phi (golden ratio)
//   HashCodeValue = ((uintptr_t(obj) >> 3) * 2654435761) ^ GVars.stwRandom ;
// * A variation of Marsaglia's shift-xor RNG scheme.
// * (obj ^ stwRandom) is appealing, but can result
//   in undesirable regularity in the hashCode values of adjacent objects
//   (objects allocated back-to-back, in particular).  This could potentially
//   result in hashtable collisions and reduced hashtable efficiency.
//   There are simple ways to "diffuse" the middle address bits over the
//   generated hashCode values:
//
//最终生成hash的方法
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 = cast_from_oop<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 = cast_from_oop<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;
}
//ObjectSynchronizer::FastHashCode方法的实现,该方法最终会返回hashcode
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;
    }
  //调用get_next_hash生成hashcode
    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;
}

从上面代码可以看出生成hash最终由get_next_hash函数生成,该函数提供了基于某个hashCode 变量值的六种方法。怎么生成最终值取决于hashCode这个变量值。

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 - 使用全局的递增数列(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 xor-shift随机数生成(跟地址无关),从注释中可以看到:这可能是最好的hashcode生成实现—在未来的版本中可能会将此设置为默认值
扩展补充:Xorshift 算法介绍

Xorshift 随机数生成器是 George Marsaglia 发明的一类伪随机数生成器。它们通过和自己逻辑移位后的数进行异或操作来生成序列中的下一个数。这在现代计算机体系结构非常快。它们是线性反馈移位寄存器的一个子类,其简单的实现使它们速度更快且使用更少的空间。然而,必须仔细选择合适参数以达到长周期。
Xorshift生成器是非密码安全的随机数生成器中最快的一种,只需要非常短的代码和状态。虽然它们没有进一步改进以通过统计检验,这个缺点非常著名且容易修改(Marsaglia 在原来的论文中指出),用复合一个非线性函数的方式,可以得到比如像 xorshift+ 或 xorshift* 生成器。一个简单的C语言实现的 xorshift+ 生成器通过了所有的 BigCrush 的测试(比 Mersenne Twister 算法和 WELL 算法的失败次数减少了一个数量级),而且在 x86 上产生一个随机数通常只需要不到十个时钟周期,多亏了指令流水线。

VM到底用的是哪种方法?
在openjdk\hotspot\src\share\vm\runtime\globals.hpp定义
JDK 8 和 JDK 9 默认值:

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

JDK 8 以前默认值:

product(intx, hashCode, 0,"(Unstable) select hashCode generation algorithm") ;
不同的JDK,生成方式不一样。

注意:
虽然方式不一样,但有个共同点:java生成的hashCode和对象内存地址没什么关系。

修改生成方法

HotSpot提供了一个VM参数来让用户选择identity hash code的生成方式:

-XX:hashCode

什么时候计算出来的

在VM里,Java对象会在首次真正使用到它的identity hash code(例如通过Object.hashCode() / System.identityHashCode())时调用VM里的函数来计算出值,然后会保存在对象里,后面对同一对象查询其identity hash code时总是会返回最初记录的值。
因此,不是对象创建时。

参考

hashCode,一个实验引发的思考

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