Java的Object.hashCode()的返回值到底是不是对象内存地址？

刚学Java的时候我也有过这种怀疑，但一直没有验证；最近在OSCHINA上看到有人在回答问题时也这么说，于是萌生了一探究竟的想法——java.lang.Object.hashCode()的返回值到底是不是对象内存地址？

（顺带回顾一下JNI）

hashCode契约

说到这个问题，大家的第一反应一定和我一样——去查Object.hashCode的源码，但翻开源码，看到的却是这样的（Oracle JDK 8）：

view
sourceprint?

01.

<code

class

=

"language-java
hljs "

>

/**

02.

*
Returns a hash code value for the object. This method is

03.

*
supported for the benefit of hash tables such as those provided by

04.

*
{@link java.util.HashMap}.

05.

*
<p>

06.

*
The general contract of {@code hashCode}is:

07.

*
<ul>

08.

*
<li>Whenever it is invoked on the same object more than once during

09.

*
an execution of a Java application,the {@code hashCode}method

10.

*
must consistently return the same integer,provided no information

11.

*
used in {@code equals}comparisons on the object is modified.

12.

*
This integer need not remain consistent from one execution of an

13.

*
application to another execution of the same application.

14.

*
<li>If two objects are equal according to the {@code equals(Object)}

15.

*
method,then calling the {@code hashCode}method on each of

16.

*
the two objects must produce the same integer result.

17.

*
<li>It is <em>not</em> required that if two objects are unequal

18.

*
according to the {@link java.lang.Object#equals(java.lang.Object)}

19.

*
method,then calling the {@code hashCode}method on each of the

20.

*
two objects must produce distinct integer results.  However,the

21.

*
programmer should be aware that producing distinct integer results

22.

*
for unequal objects may improve the performance of hash tables.

23.

*
</ul>

24.

*
<p>

25.

*
As much as is reasonably practical,the hashCode method defined by

26.

*
class {@code Object}does return distinct integers for distinct

27.

*
objects. (This is typically implemented by converting the internal

28.

*
address of the object into an integer,but this implementation

29.

*
technique is not required by the

30.

*
Java™ programming language.)

31.

*

32.

*
@return  a hash code value for this object.

33.

*
@see     java.lang.Object#equals(java.lang.Object)

34.

*
@see     java.lang.System#identityHashCode

35.

*/

36.

public

native

int

hashCode();</code>

Object.hashCode是一个native方法，看不到源码（Java代码，Oracle的JDK是看不到的，OpenJDK或其他开源JRE是可以找到对应的C/C++代码）。

上面这段注释指出了Object.hashCode()在JRE（Java Runtime Library）中应该遵循的一些契约（contract）：

（PS：所谓契约当然是大家一致达成的，各个JVM厂商都会遵循）

一致性（consistent），在程序的一次执行过程中，对同一个对象必须一致地返回同一个整数。

如果两个对象通过equals(Object)比较，结果相等，那么对这两个对象分别调用hashCode方法应该产生相同的整数结果。（PS：这里equals和hashCode说的都是Object类的）

如果两个对象通过java.lang.Object.equals(java.lang.Ojbect)比较，结果不相等，不必保证对这两个对象分别调用hashCode也返回两个不相同的整数。

实际上java.lang包里面的类，都是JRE必须的，属于运行时库（Runtime Library），这也是为什么很多JRE下该类的class文件被打包到rt.jar中的原因（应该是Runtime的简写）。

而这些运行时库一般都是跟JDK/JRE一起发布的；所以，对于不同的JRE环境，问题的答案未必相同。

考虑到具体JVM厂商实现的Object.hashCode相对较复杂，下面先通过另一思路对开头提出的问题进行探究。

最后我们再找一些开源JRE的Object.hashCode的具体实现作简要分析。

Java中如何获得对象内存地址？

看不到Object.hashCode的源码，反过来，我们可以得到对象的内存地址和Object.hashCode比较，也能得出结论。

要验证这个问题自然需要一种得到对象内存地址的方法，但Java本身并没有提供类似的方法；这也是我在初学Java时没有验证这个问题的原因。

“内存地址”在Java里得不到，但在C/C++中却很容易得到。于是，我们想到——通过JNI让Java代码调用一段C/C++代码来得到对象内存地址。

这里可能需要考虑的还有一点——用Java什么类型能放得下C/C++的指针？

在64位机器上，C/C++的指针是8字节；32位是4字节。

嗯(⊙_⊙)~ 不管怎样Java的long都是8字节，足矣~

Think in Java——接口和测试

假设我们已经有了一个NativeUtils.java：

view
sourceprint?

1.

<code

class

=

"language-java
hljs "

>

class

NativeUtils
{

2.

public

native

static

long

getNativePointer(Object
o);

3.

}</code>

并且已经实现了getNativePointer方法。

那么验证开头提出的问题就变得异常简单了：

view
sourceprint?

01.

<code

class

=

"language-java
hljs "

>

class

TestNativeUtils
{

02.

public

static

void

main(String
args[]) {

03.

Object
o =

new

Object();

04.

05.

long

nptr
= NativeUtils.getNativePointer(o);

06.

long

hash
= o.hashCode();

07.

08.

System.out.println(String.format(

"hash:
%x,nptr: %x"

,
hash,nptr));

09.

}

10.

}</code>

Think in C++——实现native方法

好了说干就干，现在就差那个native的getNativePointer了。

用javah生成对应的.h文件：

$ javah NativeUtils

该命令执行后生成了NativeUtils.h：

view
sourceprint?

01.

<code

class

=

"language-c
hljs "

>

/*
DO NOT EDIT THIS FILE - it is machine generated */

02.

#include
<jni.h>

03.

/*
Header for class NativeUtils */

04.

05.

#ifndef
_Included_NativeUtils

06.

#define
_Included_NativeUtils

07.

#ifdef
__cplusplus

08.

extern

"C"

{

09.

#endif

10.

/*

11.

*
Class:     NativeUtils

12.

*
Method:    getNativePointer

13.

*
Signature: (Ljava/lang/Object;)J

14.

*/

15.

JNIEXPORT
jlong JNICALL Java_NativeUtils_getNativePointer

16.

(JNIEnv
*,jclass,jobject);

17.

18.

#ifdef
__cplusplus

19.

}

20.

#endif

21.

#endif</code>

接着实现这个Java_NativeUtils_getNativePointer，文件命名为NativeUtils.cc：

view
sourceprint?

1.

<code

class

=

"language-c
hljs "

>#include

"NativeUtils.h"

2.

3.

JNIEXPORT
jlong JNICALL

4.

Java_NativeUtils_getNativePointer(JNIEnv
*env,jclass clazz,jobject o)

5.

{

6.

return

reinterpret_cast<jlong>(o);

7.

}</code>

编译为动态库：

$ g++-shared -o libnative-utils.so NativeUtils.cc

可能因为找不到jni.h，报错：

NativeUtils.h:2:17: fatal error: jni.h: No such file or directory

在JDK安装目录下查找jni.h:

user@host:/usr/lib/jvm/java-7-openjdk-amd64$ find . -name jni.h

./include/jni.h

知道jni.h路径后，用-I选项加到编译命令上再次编译：

$ g++-shared -I/usr/lib/jvm/java-7-openjdk-amd64/include/ -o libnative-utils.so NativeUtils.cc

OK， 编译成功，生成了 libnative-utils.so 文件。

Run in shell——在shell环境运行

下面就让TestNativeUtils在shell环境执行起来。

首先，编译NativeUtils和TestNativeUtils:

$ javac NativeUtils.java

$ javac TestNativeUtils.java

分别生成了NativeUtils.class和TestNativeUtils.class

好了，就差临门一脚了——执行class文件：

$ java TestNativeUtils

居然出错了：

view
sourceprint?

1.

<code

class

=

"
hljs avrasm"

>Exception
in thread

"main"

java.lang.UnsatisfiedLinkError:
NativeUtils.getNativePointer(Ljava/lang/Object;)J

2.

at
NativeUtils.getNativePointer(Native Method)

3.

at
TestNativeUtils.main(TestNativeUtils.java:

5

)</code>

加载动态库到我们的程序中

到目前为止，我们的Java代码并没有实现NativeUtils.getNativePointer；所以，会有上面的错误。

必须在调用NativeUtils.getNativePointer前，将我们编译好的动态库加载上。可以用static代码块，以保证在调用前完成加载；修改后的NativeUtils：

view
sourceprint?

1.

<code

class

=

"language-java
hljs "

>

class

NativeUtils
{

2.

static

{

3.

System.loadLibrary(

"native-utils"

);

4.

}

5.

6.

public

native

static

long

getNativePointer(Object
o);

7.

}</code>

让JVM能找到动态库

再次编译、执行：

$ javac NativeUtils.java

$ java TestNativeUtils

又有错误，但已经和刚才不一样了：

view
sourceprint?

1.

<code

class

=

"
hljs avrasm"

>Exception
in thread

"main"

java.lang.UnsatisfiedLinkError:
no

native

-utils
in java.library.path

2.

at
java.lang.ClassLoader.loadLibrary(ClassLoader.java:

1886

)

3.

at
java.lang.Runtime.loadLibrary0(Runtime.java:

849

)

4.

at
java.lang.System.loadLibrary(System.java:

1088

)

5.

at
NativeUtils.<clinit>(NativeUtils.java:

3

)

6.

at
TestNativeUtils.main(TestNativeUtils.java:

5

)</code>

这次错误是：没有在java.library.path中找到native-utils，可以在javac命令-D参数上将当期目录加到java.library.path上，让其能够找到native-utils：

$ java -Djava.library.path=. TestNativeUtils

hash: 4f5f1ace,nptr: 7f223a5fb958

All in one —— Makefile

上面的多个命令可以写到一个Makefile里，可以实现“一键执行”：

view
sourceprint?

01.

<code

class

=

"language-Makefile
hljs http"

>test:
runtest

02.

03.

all:
libnative-utils.so

04.

05.

JNI_INCLUDE=/usr/lib/jvm/java-

7

-openjdk-amd64/include

06.

07.

NativeUtils.

class

:
NativeUtils.java

08.

javac
NativeUtils.java

09.

10.

TestNativeUtils.

class

:
TestNativeUtils.java

11.

javac
TestNativeUtils.java

12.

13.

NativeUtils.h:
NativeUtils.java

14.

javah
-jni NativeUtils

15.

16.

libnative-utils.so:
NativeUtils.cc NativeUtils.h

17.

g++
-shared -I${JNI_INCLUDE}-o libnative-utils.so NativeUtils.cc

18.

19.

runtest:
TestNativeUtils.

class

libnative-utils.so

20.

@echo

"run
test:"

21.

java
-Djava.library.path=. TestNativeUtils

22.

23.

clean:

24.

rm
-v *.

class

*.so
*.h</code>

几个JRE的具体实现

说道开源的Java运行环境，首先能想到的自然是OpenJDK和Android，下面分别简要分析。

hashCode on Android

Android的Object.hashCode和Oracle JDK的略有不同：

view
sourceprint?

01.

<code

class

=

"language-java
hljs "

>

private

transient

int

shadow$_monitor_;

02.

03.

public

int

hashCode()
{

04.

int

lockWord
= shadow$_monitor_;

05.

final

int

lockWordMask
=

0xC0000000

;

//
Top 2 bits.

06.

final

int

lockWordStateHash
=

0x80000000

;

//
Top 2 bits are value 2 (kStateHash).

07.

if

((lockWord
& lockWordMask) == lockWordStateHash) {

08.

return

lockWord
& ~lockWordMask;

//

09.

}

10.

return

System.identityHashCode(

this

);

11.

}</code>

（PS：估计shadow$_monitor_应该是hash值的一个cache，第一次需要计算一下，以后都不用计算了）

第一次执行时，shadow$_monitor_的值为0，将会调用System.identityHashCode：

view
sourceprint?

1.

<code

class

=

"language-java
hljs "

>

public

static

native

int

identityHashCode(Object
anObject);</code>

而它是一个native的方法，对应代码（java_lang_System.cc）：

view
sourceprint?

1.

<code

class

=

"language-c
hljs "

>

static

jint
System_identityHashCode(JNIEnv* env,jclass,jobject javaObject) {

2.

if

(UNLIKELY(javaObject
== nullptr)) {

3.

return

0

;

4.

}

5.

ScopedFastNativeObjectAccess
soa(env);

6.

mirror::Object*
o = soa.Decode<mirror::Object*>(javaObject);

7.

return

static_cast<jint>(o->IdentityHashCode());

8.

}</code>

很显然，这里的关键在于mirror::Object::IdentityHashCode：

view
sourceprint?

01.

<code

class

=

"
hljs cpp"

>int32_t
Object::IdentityHashCode()

const

{

02.

mirror::Object*
current_this = const_cast<mirror::Object*>(

this

);

03.

while

(

true

)
{

04.

LockWord
lw = current_this->GetLockWord(

false

);

05.

switch

(lw.GetState())
{

06.

case

LockWord::kUnlocked:
{

//
kUnlocked 是 LockWord的默认State值

07.

//
Try to compare and swap in a new hash,if we succeed we will return the hash on the next

08.

//
loop iteration.

09.

LockWord
hash_<a href=

"http://www.it165.net/edu/ebg/"

target=

"_blank"

class

=

"keylink"

>word</a>(LockWord::FromHashCode(GenerateIdentityHashCode()));

10.

DCHECK_EQ(hash_<a
href=

"http://www.it165.net/edu/ebg/"

target=

"_blank"

class

=

"keylink"

>word</a>.GetState(),
LockWord::kHashCode);

11.

if

(const_cast<Object*>(

this

)->CasLockWordWeakRelaxed(lw,
hash_word)) {

12.

return

hash_word.GetHashCode();

13.

}

14.

break

;

15.

}

16.

case

LockWord::kThinLocked:
{

17.

//
Inflate the thin lock to a monitor and stick the hash code inside of the monitor. May

18.

//
fail spuriously.

19.

Thread*
self = Thread::Current();

20.

StackHandleScope<

1

>
hs(self);

21.

Handle<mirror::Object>
h_this(hs.NewHandle(current_this));

22.

Monitor::InflateThinLocked(self,
h_this,lw,GenerateIdentityHashCode());

23.

//
A GC may have occurred when we switched to kBlocked.

24.

current_this
= h_this.Get();

25.

break

;

26.

}

27.

case

LockWord::kFatLocked:
{

28.

//
Already inflated,return the has stored in the monitor.

29.

Monitor*
monitor = lw.FatLockMonitor();

30.

DCHECK(monitor
!= nullptr);

31.

return

monitor->GetHashCode();

32.

}

33.

case

LockWord::kHashCode:
{

//
以后调用

34.

return

lw.GetHashCode();

35.

}

36.

default

:
{

37.

LOG(FATAL)
<<

"Invalid
state during hashcode "

<<
lw.GetState();

38.

break

;

39.

}

40.

}

41.

}

42.

LOG(FATAL)
<<

"Unreachable"

;

43.

return

0

;

44.

}</code>

这段代码可以看出——ART的Object.hashCode确实是有cache的，对于同一个Ojbect，第一次调用Object.hashCode将会执行实际的计算并记入cache，以后直接从cache中取出。

真正计算hashcode的是GenerateIdentityHashCode：

view
sourceprint?

01.

<code

class

=

"
hljs cpp"

>int32_t
Object::GenerateIdentityHashCode() {

02.

static

AtomicInteger
seed(

987654321

+
std::time(nullptr));

03.

int32_t
expected_value,new_value;

04.

do

{

05.

expected_value
= static_cast<uint32_t>(seed.LoadRelaxed());

06.

new_value
= expected_value *

1103515245

+

12345

;

07.

}

while

((expected_value
& LockWord::kHashMask) ==

0

||

08.

!seed.CompareExchangeWeakRelaxed(expected_value,
new_value));

09.

return

expected_value
& LockWord::kHashMask;

10.

}</code>

从GenerateIdentityHashCode可以看出，ART的Object.hashCode的返回值和对象的地址并没有直接的关系。

hashCode on OpenJDK

OpenJDK项目首页：openjdk.java.net

TODO: OpenJDK上的hashCode具体实现和简要分析

Object.c

view
sourceprint?

01.

<code

class

=

"
hljs coffeescript"

>

static

JNINativeMethod
methods[] = {

02.

{

"hashCode"

,

"()I"

,
(

void

*)&JVM_IHashCode},

03.

{

"wait"

,

"(J)V"

,
(

void

*)&JVM_MonitorWait},

04.

{

"notify"

, 

"()V"

,
(

void

*)&JVM_MonitorNotify},

05.

{

"notifyAll"

,

"()V"

,
(

void

*)&JVM_MonitorNotifyAll},

06.

{

"clone"

,

"()Ljava/lang/Object;"

,
(

void

*)&JVM_Clone},

07.

};

08.

09.

JNIEXPORT

void

JNICALL

10.

Java_java_lang_Object_registerNatives(JNIEnv
*env,jclass cls)

11.

{

12.

(*env)->RegisterNatives(env,
cls,

13.

methods,
sizeof(methods)/sizeof(methods[

0

]));

14.

}

15.

16.

JNIEXPORT
jclass JNICALL

17.

Java_java_lang_Object_getClass(JNIEnv
*env,jobject

this

)

18.

{

19.

if

(

this

==
NULL) {

20.

JNU_ThrowNullPointerException(env,
NULL);

21.

return

0

;

22.

}

else

{

23.

return

(*env)->GetObjectClass(env,

this

);

24.

}

25.

}</code>

这段代码指出了Object.hashCode对应的C函数为JVM_IHashCode，下面需要找到JVM_IHashCode的代码jvm.cpp：

view
sourceprint?

1.

<code

class

=

"
hljs lasso"

>JVM_ENTRY(jint,
JVM_IHashCode(JNIEnv* env,jobject handle))

2.

JVMWrapper(

"JVM_IHashCode"

);

3.

//
as implemented in the classic virtual machine; return 0 if object is NULL

4.

return

handle
== NULL ?

0

:
ObjectSynchronizer::FastHashCode (THREAD,JNIHandles::resolve_non_null(handle)) ;

5.

JVM_END</code>

这里只是一个包装，实际计算hashCode的是ObjectSynchronizer::FastHashCode，位于synchronizer.cpp：

view
sourceprint?

001.

<code

class

=

"
hljs lasso"

>intptr_t
ObjectSynchronizer::FastHashCode (Thread * Self,oop obj) {

002.

if

(UseBiasedLocking)
{

003.

//
NOTE: many places throughout the JVM do not expect a safepoint

004.

//
to be taken here,in particular most operations on perm gen

005.

//
objects. However,we only ever bias Java instances and all of

006.

//
the call sites of identity_hash that might revoke biases have

007.

//
been checked to make sure they can handle a safepoint. The

008.

//
added check of the bias pattern is to avoid useless calls to

009.

//
thread-local storage.

010.

if

(obj->mark()->has_bias_pattern())
{

011.

//
Box and unbox the raw reference just in case we cause a STW safepoint.

012.

Handle
hobj (Self,obj) ;

013.

//
Relaxing assertion for bug 6320749.

014.

assert

(Universe::verify_in_progress()
||

015.

!SafepointSynchronize::is_at_safepoint(),

016.

"biases
should not be seen by VM thread here"

);

017.

BiasedLocking::revoke_and_rebias(hobj,

false

,
JavaThread::current());

018.

obj
= hobj() ;

019.

assert

(!obj->mark()->has_bias_pattern(),

"biases
should be revoked by now"

);

020.

}

021.

}

022.

023.

//
hashCode() is a heap mutator ...

024.

//
Relaxing assertion for bug 6320749.

025.

assert

(Universe::verify_in_progress()
||

026.

!SafepointSynchronize::is_at_safepoint(),

"invariant"

)
;

027.

assert

(Universe::verify_in_progress()
||

028.

Self->is_Java_thread()
,

"invariant"

)
;

029.

assert

(Universe::verify_in_progress()
||

030.

((JavaThread
*)Self)->thread_state() != _thread_blocked,

"invariant"

)
;

031.

032.

ObjectMonitor*
monitor = NULL;

033.

markOop
temp,test;

034.

intptr_t
hash;

035.

markOop
mark = ReadStableMark (obj);

036.

037.

//
object should remain ineligible for biased locking

038.

assert

(!mark->has_bias_pattern(),

"invariant"

)
;

039.

040.

if

(mark->is_neutral())
{

041.

hash
= mark->hash();         

//
this is a normal header

042.

if

(hash)
{                       

//
if it has hash,just return it

043.

return

hash;

044.

}

045.

hash
= get_next_hash(Self,obj);

//
allocate a new hash code

046.

temp
= mark->copy_set_hash(hash);

//
merge the hash code into header

047.

//
use (machine word version) atomic operation to install the hash

048.

test
= (markOop) Atomic::cmpxchg_ptr(temp,obj->mark_addr(),mark);

049.

if

(test
== mark) {

050.

return

hash;

051.

}

052.

//
If atomic operation failed,we must inflate the header

053.

//
into heavy weight monitor. We could add more code here

054.

//
for fast path,but it does not worth the complexity.

055.

}

else

if

(mark->has_monitor())
{

056.

monitor
= mark->monitor();

057.

temp
= monitor->header();

058.

assert

(temp->is_neutral(),

"invariant"

)
;

059.

hash
= temp->hash();

060.

if

(hash)
{

061.

return

hash;

062.

}

063.

//
Skip to the following code to reduce code size

064.

}

else

if

(Self->is_lock_owned((address)mark->locker()))
{

065.

temp
= mark->displaced_mark_helper();

//
this is a lightweight monitor owned

066.

assert

(temp->is_neutral(),

"invariant"

)
;

067.

hash
= temp->hash();          

//
by current thread,check if the displaced

068.

if

(hash)
{                       

//
header contains hash code

069.

return

hash;

070.

}

071.

//
WARNING:

072.

//
The displaced header is strictly immutable.

073.

//
It can NOT be changed in ANY cases. So we have

074.

//
to inflate the header into heavyweight monitor

075.

//
even the current thread owns the lock. The reason

076.

//
is the BasicLock (stack slot) will be asynchronously

077.

//
read by other threads during the inflate() function.

078.

//
Any change to stack may not propagate to other threads

079.

//
correctly.

080.

}

081.

082.

//
Inflate the monitor to set hash code

083.

monitor
= ObjectSynchronizer::inflate(Self,obj);

084.

//
Load displaced header and check it has hash code

085.

mark
= monitor->header();

086.

assert

(mark->is_neutral(),

"invariant"

)
;

087.

hash
= mark->hash();

//
取出缓存

088.

if

(hash
==

0

)
{

089.

hash
= get_next_hash(Self,obj);

//
实际计算

090.

temp
= mark->copy_set_hash(hash);

//
merge hash code into header

091.

assert

(temp->is_neutral(),

"invariant"

)
;

092.

test
= (markOop) Atomic::cmpxchg_ptr(temp,monitor,mark);

093.

if

(test
!= mark) {

094.

//
The only update to the header in the monitor (outside GC)

095.

//
is install the hash code. If someone add new usage of

096.

//
displaced header,please update this code

097.

hash
= test->hash();

098.

assert

(test->is_neutral(),

"invariant"

)
;

099.

assert

(hash
!=

0

,

"Trivial
unexpected object/monitor header usage."

);

100.

}

101.

}

102.

//
We finally get the hash

103.

return

hash;

104.

}</code>

又是假牙，实际计算hashCode的是get_next_hash，代码和ObjectSynchronizer::FastHashCode相邻：

view
sourceprint?

01.

<code

class

=

"
hljs lasso"

>

//
hashCode() generation :

02.

//

03.

//
Possibilities:

04.

//
* MD5Digest of {obj,stwRandom}

05.

//
* CRC32 of {obj,stwRandom}or any linear-feedback shift register function.

06.

//
* A DES- or AES-style SBox[] mechanism

07.

//
* One of the Phi-based schemes,such as:

08.

//
2654435761 = 2^32 * Phi (golden ratio)

09.

//
HashCodeValue = ((uintptr_t(obj) >> 3) * 2654435761) ^ GVars.stwRandom ;

10.

//
* A variation of Marsaglia's shift-xor RNG scheme.

11.

//
* (obj ^ stwRandom) is appealing,but can result

12.

//
in undesirable regularity in the hashCode values of adjacent objects

13.

//
(objects allocated back-to-back,in particular).  This could potentially

14.

//
result in hashtable collisions and reduced hashtable efficiency.

15.

//
There are simple ways to "diffuse" the middle address bits over the

16.

//
generated hashCode values:

17.

//

18.

19.

static

inline
intptr_t get_next_hash(Thread * Self,oop obj) {

20.

intptr_t
value =

0

;

21.

if

(hashCode
==

0

)
{

22.

//
This form uses an unguarded global Park-Miller RNG,

23.

//
so it's possible for two threads to race and generate the same RNG.

24.

//
On MP system we'll have lots of RW access to a global,so the

25.

//
mechanism induces lots of coherency traffic.

26.

value
= os::random() ;

//
随机数

27.

}

else

28.

if

(hashCode
==

1

)
{

29.

//
This variation has the property of being stable (idempotent)

30.

//
between STW operations.  This can be useful in some of the 1-0

31.

//
synchronization schemes.

32.

//
地址基础上hack

33.

intptr_t
addrBits = intptr_t(obj) >>

3

;

34.

value
= addrBits ^ (addrBits >>

5

)
^ GVars.stwRandom ;

35.

}

else

36.

if

(hashCode
==

2

)
{

37.

value
=

1

;        

//
for sensitivity testing， 实际不会使用

38.

}

else

39.

if

(hashCode
==

3

)
{

40.

value
=++GVars.hcSequence ;

41.

}

else

42.

if

(hashCode
==

4

)
{

43.

value
=intptr_t(obj) ; 

//
直接用地址

44.

}

else

{

45.

//
Marsaglia's xor-shift scheme with thread-specific state

46.

//
This is probably the best overall implementation -- we'll

47.

//
likely make this the default in future releases.

48.

unsigned
t = Self->_hashStateX ;

49.

t
^= (t <<

11

)
;

50.

Self->_hashStateX
= Self->_hashStateY ;

51.

Self->_hashStateY
= Self->_hashStateZ ;

52.

Self->_hashStateZ
= Self->_hashStateW ;

53.

unsigned
v = Self->_hashStateW ;

54.

v
= (v ^ (v >>

19

))
^ (t ^ (t >>

8

))
;

55.

Self->_hashStateW
= v ;

56.

value
=v ;

57.

}

58.

59.

value
&= markOopDesc::hash_mask;

60.

if

(value
==

0

)
value =

0xBAD

;

61.

assert

(value
!= markOopDesc::no_hash,

"invariant"

)
;

62.

TEVENT
(hashCode: GENERATE) ;

63.

return

value;

64.

}</code>

这段代码可以看出OpenJDK一共实现了5中不同的计算hash值的方法，通过

这段代码中hashCode进行切换。其中hashCode == 4的是直接使用地址的（前面的实验说明OpenJDK默认情况下并没有使用这种方式，或许可以通过运行/编译时参数进行选择）。

结论

前面通过JNI验证已经能够得到很显然的结论，hashCode返回的并不一定是对象的（虚拟）内存地址，具体取决于运行时库和JVM的具体实现。

我的运行环境：

OS：

Ubuntu 12.04 64bit Desktop |

JDK：

java version “1.7.0_55”

OpenJDK Runtime Environment (IcedTea 2.4.7) (7u55-2.4.7-1ubuntu1~0.12.04.2) \

OpenJDK 64-Bit Server VM (build 24.51-b03,mixed mode)