1基于OpenJDK 8
一直以为Java Object.hashCode()的结果就是通过对象的内存地址做相关运算得到的，但是无意在网上看到有相应的意见争论，故抽时间从源码层面验证了剖析了hashCode的默认计算方法。
先说结论：OpenJDK8 默认hashCode的计算方法是通过和当前线程有关的一个随机数 三个确定值，运用Marsaglia's xorshift scheme随机数算法得到的一个随机数。和对象内存地址无关。
下面通过查找和分析OpenJDK8源码实现来一步步分析。
1. 查找java.lang.Object.hashCode()源码
代码语言：javascript复制public native int hashCode();2. 导出Object的JNI头文件
切换到Object.class文件所在目录，执行 javah -jni java.lang.Object，得到java_lang_Object.h文件，文件内容如下：
代码语言：javascript复制/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class java_lang_Object */
#ifndef _Included_java_lang_Object
#define _Included_java_lang_Object
#ifdef __cplusplus
extern "C" {
#endif
/*
* Class: java_lang_Object
* Method: registerNatives
* Signature: ()V
*/
JNIEXPORT void JNICALL Java_java_lang_Object_registerNatives
(JNIEnv *, jclass);
/*
* Class: java_lang_Object
* Method: getClass
* Signature: ()Ljava/lang/Class;
*/
JNIEXPORT jclass JNICALL Java_java_lang_Object_getClass
(JNIEnv *, jobject);
/*
* Class: java_lang_Object
* Method: hashCode
* Signature: ()I
*/
JNIEXPORT jint JNICALL Java_java_lang_Object_hashCode
(JNIEnv *, jobject);
/*
* Class: java_lang_Object
* Method: clone
* Signature: ()Ljava/lang/Object;
*/
JNIEXPORT jobject JNICALL Java_java_lang_Object_clone
(JNIEnv *, jobject);
/*
* Class: java_lang_Object
* Method: notify
* Signature: ()V
*/
JNIEXPORT void JNICALL Java_java_lang_Object_notify
(JNIEnv *, jobject);
/*
* Class: java_lang_Object
* Method: notifyAll
* Signature: ()V
*/
JNIEXPORT void JNICALL Java_java_lang_Object_notifyAll
(JNIEnv *, jobject);
/*
* Class: java_lang_Object
* Method: wait
* Signature: (J)V
*/
JNIEXPORT void JNICALL Java_java_lang_Object_wait
(JNIEnv *, jobject, jlong);
#ifdef __cplusplus
}
#endif
#endif3 . 查看Object的native方法实现
OpenJDK源码链接：
http://hg.openjdk.java.net/jdk8u/jdk8u/jdk/file/3462d04401ba/src/share/native/java/lang/Object.c
查看Object.c文件，可以看到hashCode()的方法被注册成由JVM_IHashCode方法指针来处理。
代码语言：javascript复制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},
};而JVM_IHashCode方法指针在 openjdkhotspotsrcsharevmprimsjvm.cpp中定义为：
代码语言：javascript复制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从而得知，真正计算获得hashCode的值是ObjectSynchronizer::FastHashCode
4 . ObjectSynchronizer::fashHashCode方法的实现
openjdkhotspotsrcsharevmruntimesynchronizer.cpp 找到其实现方法。
代码语言：javascript复制intptr_t ObjectSynchronizer::FastHashCode (Thread * Self, oop obj) {
if (UseBiasedLocking) {
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;
}
} 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;
}
}
// 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.");
}
}
return hash;
}该方法中
代码语言：javascript复制// 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);
...
}对hash值真正进行了计算，查看get_next_hash方法源码
http://hg.openjdk.java.net/jdk8u/jdk8u/hotspot/file/87ee5ee27509/src/share/vm/runtime/synchronizer.cpp#l555
代码语言：javascript复制static inline intptr_t get_next_hash(Thread * Self, oop obj) {
intptr_t value = 0 ;
if (hashCode == 0) {
value = os::random() ;
} else
if (hashCode == 1) {
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 {
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;
}对于OpenJDK8版本，其默认配置为：
代码语言：javascript复制product(intx, hashCode, 5,
"(Unstable) select hashCode generation algorithm") 其对应的hashCode计算方案为：
代码语言：javascript复制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 ;其中Thread->_hashStateX, Thread->_hashStateY, Thread->_hashStateZ, Thread->_hashStateW在
http://hg.openjdk.java.net/jdk8u/jdk8u/hotspot/file/87ee5ee27509/src/share/vm/runtime/thread.cpp#I263
有定义：
代码语言：javascript复制// thread-specific hashCode stream generator state - Marsaglia shift-xor form
_hashStateX = os::random() ;
_hashStateY = 842502087 ;
_hashStateZ = 0x8767 ; // (int)(3579807591LL & 0xffff) ;
_hashStateW = 273326509 ;所以，JDK8 的默认hashCode的计算方法是通过和当前线程有关的一个随机数 三个确定值，运用Marsaglia's xorshift scheme随机数算法得到的一个随机数。欢迎关注我们，公号终码一生。
xorshift是由George Marsaglia发现的一类伪随机数生成器，其通过移位和与或计算，能够在计算机上以极快的速度生成伪随机数序列。其算法的基本实现如下：
代码语言：javascript复制unsigned long xor128(){
static unsigned long x=123456789,y=362436069,z=521288629,w=88675123;
unsigned long t;
t=(xˆ(x<<11));x=y;y=z;z=w; return( w=(wˆ(w>>19))ˆ(tˆ(t>>8)) );这就和上面计算hashCode的OpenJDK代码对应了起来。
5 . 其他几类hashCode计算方案：
- hashCode == 0
此类方案返回一个Park-Miller伪随机数生成器生成的随机数
OpenJdk 6 &7的默认实现。
http://hg.openjdk.java.net/jdk7u/jdk7u/hotspot/file/5b9a416a5632/src/share/vm/runtime/globals.hpp#l1100
http://hg.openjdk.java.net/jdk6/jdk6/hotspot/file/5cec449cc409/src/share/vm/runtime/globals.hpp#l1128
代码语言：javascript复制if (hashCode == 0) {
value = os::random() ;
}- hashCode == 1
此类方案将对象的内存地址，做移位运算后与一个随机数进行异或得到结果
代码语言：javascript复制if (hashCode == 1) {
intptr_t addrBits = cast_from_oop<intptr_t>(obj) >> 3 ;
value = addrBits ^ (addrBits >> 5) ^ GVars.stwRandom ;
}- hashCode == 2
此类方案返回固定的1
代码语言：javascript复制if (hashCode == 2) {
value = 1 ;
}- hashCode == 3
此类方案返回一个自增序列的当前值
代码语言：javascript复制if (hashCode == 3) {
value = GVars.hcSequence ;
}- hashCode == 4
此类方案返回当前对象的内存地址
代码语言：javascript复制if (hashCode == 4) {
value = cast_from_oop<intptr_t>(obj) ;
}可以通过在JVM启动参数中添加-XX:hashCode=4，改变默认的hashCode计算方式。
—END—
