Java Object.hashCode()返回的是对象内存地址？

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Java Object.hashCode()返回的是对象内存地址？

#Java Object.hashCode()返回的是对象内存地址？| 来源: 网络整理| 查看: 265

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 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：

/* DO NOT EDIT THIS FILE - it is machine generated */ #include /* 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 #endif 3 . 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。

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 openjdk\hotspot\src\share\vm\prims\jvm.cpp中定义为：

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

openjdk\hotspot\src\share\vm\runtime\synchronizer.cpp 找到其实现方法。

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; }

该方法中

// 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

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(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(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 _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http://hg.openjdk.java.net/jdk8u/jdk8u/hotspot/file/87ee5ee27509/src/share/vm/runtime/globals.hpp#l1127 为：

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

其对应的hashCode计算方案为：

// 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 _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：

// 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。对xorshift算法有兴趣可以参考原论文：https://www.jstatsoft.org/article/view/v008i14/xorshift.pdf 。 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：

unsigned long xor128(){ static unsigned long x=123456789,y=362436069,z=521288629,w=88675123; unsigned long t; t=(xˆ(x19))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 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() ; } 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 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(obj) >> 3 ; value = addrBits ^ (addrBits >> 5) ^ GVars.stwRandom ; } hashCode == 2 此类方案返回固定的1 if (hashCode == 2) { value = 1 ; // for sensitivity testing } hashCode == 3 此类方案返回一个自增序列的当前值 if (hashCode == 3) { value = ++GVars.hcSequence ; } hashCode == 4 此类方案返回当前对象的内存地址 if (hashCode == 4) { value = cast_from_oop(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。

参考资料： https://srvaroa.github.io/jvm/java/openjdk/biased-locking/2017/01/30/hashCode.html https://en.wikipedia.org/wiki/Xorshift http://www.cnblogs.com/mengyou0304/p/4763220.html http://stackoverflow.com/questions/2427631/how-is-hashcode-calculated-in-java http://hllvm.group.iteye.com/group/topic/39183

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Java Object.hashCode()返回的是对象内存地址？

Java Object.hashCode()返回的是对象内存地址？

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