[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？

      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 JNICALLJava_java_lang_Object_registerNatives(JNIEnv *env, jclass cls){    (*env)->RegisterNatives(env, cls,                            methods, sizeof(methods)/sizeof(methods[0]));}JNIEXPORT jclass JNICALLJava_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 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

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() ;  } 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 = 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 = 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hashcodeintptr_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  ，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。