[Java基础要义] Java语言中Object对象的hashCode()取值的底层算法是怎样实现的？

本文主要是介绍[Java基础要义] Java语言中Object对象的hashCode()取值的底层算法是怎样实现的？，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

Java语言中，Object对象有个特殊的方法：hashcode(), hashcode()表示的是JVM虚拟机为这个Object对象分配的一个int类型的数值，JVM会使用对象的hashcode值来提高对HashMap、Hashtable哈希表存取对象的使用效率。

关于Object对象的hashCode()返回值，网上对它就是一个简单的描述：“JVM根据某种策略生成的”，那么这种策略到底是什么呢？我有一个毛病，遇到这种含糊其辞的东西，就想探个究竟，所以，本文就将hashCode()本地方法的实现给扒出来，也给大家在了解hashCode()的过程中提供一点点帮助吧。

本文将根据openJDK 7源码，向展示Java语言中的Object对象的hashCode() 生成的神秘面纱，我将一步一步地向读者介绍Java Object 的hashcode()方法到底底层调用了什么函数。为了更好地了解这个过程，你可以自己下载openJDK 7 源码，亲自查看和跟踪源码，了解hashCode()的生成过程：

openJDK 7 下载地址1：http://download.java.net/openjdk/jdk7 （官网，下载速度较慢）

openJDK 7 下载地址2 ：openjdk-7-fcs-src-b147-27_jun_2011.zip (csdn 网友提供的资源，很不错)

1.查看openJDK 关于 java.lang.Object类及其hashcode()方法的定义：

进入openjdk\jdk\src\share\classes\java\lang目录下，可以看到 Object.java源码，打开，查看hashCode()的定义如下所示：

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，该头文件将在后面用到它

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);

2. 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：

#include <stdio.h>
#include <signal.h>
#include <limits.h>#include "jni.h"
#include "jni_util.h"
#include "jvm.h"#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},
};JNIEXPORT void JNICALL
Java_java_lang_Object_registerNatives(JNIEnv *env, jclass cls)
{(*env)->RegisterNatives(env, cls,methods, sizeof(methods)/sizeof(methods[0]));
}JNIEXPORT jclass JNICALL
Java_java_lang_Object_getClass(JNIEnv *env, jobject this)
{if (this == NULL) {JNU_ThrowNullPointerException(env, NULL);return 0;} else {return (*env)->GetObjectClass(env, this);}
}

3.JVM_IHashCode方法指针在 openjdk\hotspot\src\share\vm\prims\jvm.cpp中定义，如下：

JVM_ENTRY(jint, JVM_IHashCode(JNIEnv* env, jobject handle))JVMWrapper("JVM_IHashCode");// as implemented in the classic virtual machine; return 0 if object is NULLreturn 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

4. ObjectSynchronizer::fashHashCode方法的实现：

ObjectSynchronizer::fashHashCode()方法在 openjdk\hotspot\src\share\vm\runtime\synchronizer.cpp 文件中实现，其核心代码实现如下所示：

// 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:
//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() ;} elseif (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 = intptr_t(obj) >> 3 ;value = addrBits ^ (addrBits >> 5) ^ GVars.stwRandom ;} elseif (hashCode == 2) {value = 1 ;            // for sensitivity testing} elseif (hashCode == 3) {value = ++GVars.hcSequence ;} elseif (hashCode == 4) {value = 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，该方法最终会返回我们期望已久的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 lockingassert (!mark->has_bias_pattern(), "invariant") ;if (mark->is_neutral()) {hash = mark->hash();              // this is a normal headerif (hash) {                       // if it has hash, just return itreturn hash;}hash = get_next_hash(Self, obj);  // allocate a new hash codetemp = mark->copy_set_hash(hash); // merge the hash code into header// use (machine word version) atomic operation to install the hashtest = (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 ownedassert (temp->is_neutral(), "invariant") ;hash = temp->hash();              // by current thread, check if the displacedif (hash) {                       // header contains hash codereturn 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 codemonitor = ObjectSynchronizer::inflate(Self, obj);// Load displaced header and check it has hash codemark = 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 headerassert (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 codehash = test->hash();assert (test->is_neutral(), "invariant") ;assert (hash != 0, "Trivial unexpected object/monitor header usage.");}}// We finally get the hash  ，看到这句话，就特别兴奋，WE FINALLY GET THE HASH!!!!return hash;
}

好了，经过上述如此复杂步骤，终于生成了我们的hashcode了，上述的代码是使用的C++实现的，我是看不懂啦，不过有一点可以确定：

Java 中Object对象的hashcode()返回值一定不会是Object对象的内存地址这么简单！

即hashcode()返回的不是对象在内存中的地址。